Cartesian Therapeutics (RNAC) 10-K 15 Mar 19 2018 Q4 Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 08, 2019	Jun. 29, 2018
Document and Entity Information
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q4
Entity Registrant Name	SELECTA BIOSCIENCES INC
Entity Central Index Key	0001453687
Entity Voluntary Filers	No
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Emerging Growth Company	true
Entity Smaller Reporting Company	true
Entity Extended Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		44,733,437
Entity Public Float			$ 197,700,000
Entity Current Reporting Status	Yes

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash, cash equivalents, and restricted cash	$ 37,403	$ 70,698
Short-term deposits and investments	0	25,940
Prepaid expenses and other current assets	4,673	2,042
Total current assets	42,076	98,680
Property and equipment, net	2,127	2,091
Restricted cash and other assets	279	329
Total assets	44,482	101,100
Current liabilities:
Accounts payable	1,100	1,606
Accrued expenses	11,700	8,580
Loan payable, current portion	21,385	0
Deferred revenue, current portion	959	787
Total current liabilities	35,144	10,973
Non‑current liabilities:
Deferred rent and lease incentive	34	151
Loan payable, net of current portion	0	21,042
Deferred revenue, net of current portion	13,818	15,919
Other long‑term liabilities	904	1,201
Total liabilities	49,900	49,286
Commitments and contingencies (Note 8)
Stockholders’ (deficit) equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 22,471,776 and 22,343,254 shares issued and outstanding as of December 31, 2018 and December 31, 2017, respectively	3	3
Additional paid-in capital	279,539	273,128
Accumulated deficit	(280,403)	(216,897)
Accumulated other comprehensive loss	(4,557)	(4,420)
Total stockholders’ (deficit) equity	(5,418)	51,814
Total liabilities and stockholders’ equity	$ 44,482	$ 101,100

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	22,471,776	22,343,254
Common stock, shares outstanding	22,471,776	22,343,254

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 903	$ 207	$ 8,083
Operating expenses:
Research and development	47,687	45,165	29,702
General and administrative	18,238	18,826	13,051
Total operating expenses	65,925	63,991	42,753
Loss from operations	(65,022)	(63,784)	(34,670)
Investment income	1,050	617	234
Loss on extinguishment of debt	0	(673)	0
Foreign currency transaction gain (loss), net	120	(123)	(525)
Interest expense	(1,494)	(1,206)	(1,253)
Other income (expense), net	10	(152)	4
Net loss	(65,336)	(65,321)	(36,210)
Other comprehensive loss:
Foreign currency translation adjustment	(153)	78	504
Unrealized gain (loss) on securities	16	20	(36)
Total comprehensive loss	(65,473)	(65,223)	(35,742)
Accretion of redeemable convertible preferred stock	0	0	(4,566)
Net loss attributable to common stockholders	$ (65,336)	$ (65,321)	$ (40,776)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (2.92)	$ (3.20)	$ (3.89)
Weighted average common shares outstanding:
Basic and diluted (in shares)	22,389,286	20,425,050	10,493,939

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common stock	Additional paid-In Capital	Stock option receivable	Accumulated deficit	Accumulated other comprehensive loss	Series A Redeemable Convertible Preferred StockPreferred Stock	Series B Redeemable Convertible Preferred StockPreferred Stock	Series C Redeemable Convertible Preferred StockPreferred Stock	Series D Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred StockPreferred Stock	Series D Redeemable Convertible Preferred StockAdditional paid-In Capital	Series SRN Redeemable Convertible Preferred StockPreferred Stock	Series E Redeemable Convertible Preferred Stock	Series E Redeemable Convertible Preferred StockPreferred Stock	Series E Redeemable Convertible Preferred StockAdditional paid-In Capital
Temporary equity, shares outstanding at the beginning of the period (in shares) at Dec. 31, 2015							2,589,868	7,437,325	5,000,002		8,099,994		2,111,109		8,888,888
Temporary equity, outstanding at the beginning of the period at Dec. 31, 2015							$ 3,644	$ 21,448	$ 20,178		$ 42,902		$ 12,082		$ 37,228
Beginning balance (in shares) at Dec. 31, 2015		2,173,399
Beginning balance at Dec. 31, 2015	$ (116,493)	$ 0	$ 1	$ 0	$ (111,508)	$ (4,986)
Increase (Decrease) in Stockholders' Equity
Vesting of restricted common stock (in shares)		8,535
Vesting of restricted common stock	21		21
Issuance of common stock upon exercise of options (in shares)		71,816
Issuance of common stock upon exercise of options	149		224	(75)
Stock‑based compensation expense	2,030		2,030
Accretion of preferred stock to redemption value	4,566						$ 75	$ 449	$ 446		$ 1,131		$ 913		$ 1,552
Accretion of preferred stock to redemption value	(4,566)		(708)		(3,858)
Currency translation adjustment	504					504
Net loss	(36,210)				(36,210)
Exercise of common warrants (in shares)		572,003
Temporary equity, conversion of convertible stock upon listing (in shares)							(2,589,868)	(7,437,325)	(5,000,002)		(8,099,994)		(2,111,109)		(8,888,888)
Common stock, conversion of convertible stock upon listing (in shares)		10,126,118
Conversion of convertible stock upon listing	142,048	$ 1	142,047				$ (3,719)	$ (21,897)	$ (20,624)		$ (44,033)		$ (12,995)		$ (38,780)
Issuance of common stock, initial public offering net of issuance costs (in shares)		5,289,633
Issuance of common stock, Initial public offering net of issuance costs	64,465		64,465
Issuance of common stock, license agreement (in shares)		197,238
Issuance of common stock, license agreement	2,743		2,743
Issuance of common stock, Private placement net of issuance costs										$ 189		$ 189		$ 113		$ 113
Unrealized gains (losses) on securities	(36)					(36)
Temporary equity, shares outstanding at the end of the period (in shares) at Dec. 31, 2016							0	0	0		0		0		0
Temporary equity, outstanding at the end of the period at Dec. 31, 2016							$ 0	$ 0	$ 0		$ 0		$ 0		$ 0
Ending balance (in shares) at Dec. 31, 2016		18,438,742
Ending balance at Dec. 31, 2016	54,957	$ 1	211,125	(75)	(151,576)	(4,518)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)		269,842
Issuance of common stock upon exercise of options	705		630	75
Stock‑based compensation expense	4,080		4,080
Accretion of preferred stock to redemption value	0
Currency translation adjustment	78					78
Net loss	(65,321)				(65,321)
Issuance of common stock, license agreement (in shares)		529,616
Issuance of common stock, license agreement	10,000		10,000
Issuance of common stock, Private placement net of issuance costs (in shares)		3,088,791
Issuance of common stock, Private placement net of issuance costs	47,115	$ 2	47,113
Issuance of common stock under Employee Stock Purchase Plan (in shares)		16,263
Issuance of common stock under Employee Stock Purchase Plan	180		180
Unrealized gains (losses) on securities	$ 20					20
Temporary equity, shares outstanding at the end of the period (in shares) at Dec. 31, 2017							0	0	0		0		0		0
Temporary equity, outstanding at the end of the period at Dec. 31, 2017							$ 0	$ 0	$ 0		$ 0		$ 0		$ 0
Ending balance (in shares) at Dec. 31, 2017	22,343,254	22,343,254
Ending balance at Dec. 31, 2017	$ 51,814	$ 3	273,128	0	(216,897)	(4,420)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)		103,784
Issuance of common stock upon exercise of options	501		501
Stock‑based compensation expense	5,714		5,714
Accretion of preferred stock to redemption value	0
Currency translation adjustment	(153)					(153)
Net loss	(65,336)				(65,336)
Issuance of common stock under Employee Stock Purchase Plan (in shares)		24,738
Issuance of common stock under Employee Stock Purchase Plan	196		196
Unrealized gains (losses) on securities	$ 16					16
Temporary equity, shares outstanding at the end of the period (in shares) at Dec. 31, 2018							0	0	0		0		0		0
Temporary equity, outstanding at the end of the period at Dec. 31, 2018							$ 0	$ 0	$ 0		$ 0		$ 0		$ 0
Ending balance (in shares) at Dec. 31, 2018	22,471,776	22,471,776
Ending balance at Dec. 31, 2018	$ (5,418)	$ 3	$ 279,539	$ 0	$ (280,403)	$ (4,557)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (65,336,000)	$ (65,321,000)	$ (36,210,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	975,000	750,000	802,000
Amortization of premiums (accretion of discounts) on investments	(101,000)	233,000	193,000
(Gain) loss on disposal of property and equipment	(81,000)	36,000	3,000
Stock‑based compensation expense	5,714,000	4,081,000	2,051,000
Non‑cash interest expense	449,000	390,000	263,000
Loss on extinguishment of debt	0	673,000	0
Net unrealized losses on investments	0	0	12,000
Changes in operating assets and liabilities:
Accounts receivable	0	152,000	610,000
Prepaid expenses, deposits and other current assets	(2,631,000)	1,225,000	(831,000)
Accounts payable	(516,000)	(2,327,000)	2,686,000
Deferred revenue	(101,000)	2,432,000	10,409,000
Contingently repayable grant funding	0	0	(461,000)
Accrued expenses and other liabilities	2,467,000	5,650,000	716,000
Net cash used in operating activities	(59,161,000)	(52,026,000)	(19,757,000)
Cash flows from investing activities
Receipts from the maturity of short-term investments	41,655,000	60,158,000	6,900,000
Purchases of short-term investments	(15,598,000)	(61,527,000)	(28,416,000)
Purchases of property and equipment	(884,000)	(733,000)	(586,000)
Proceeds from the sale of property and equipment	99,000	4,000	2,000
Net cash provided by (used in) investing activities	25,272,000	(2,098,000)	(22,100,000)
Cash flows from financing activities
Proceeds from issuance of long-term debt, net of expenses	0	20,957,000	0
Repayments of long-term debt	0	(12,934,000)	0
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions	0	0	68,870,000
Deferred IPO costs paid	0	0	(4,103,000)
Proceeds from Private Placement, net of issuance costs ($2.9 million)	0	47,114,000	0
Proceeds from issuance of common stock	0	10,000,000	2,743,000
Proceeds from exercise of stock options	501,000	706,000	149,000
Proceeds from issuance of common stock under Employee Stock Purchase Plan	196,000	180,000	0
Net cash provided by financing activities	697,000	66,023,000	67,659,000
Effect of exchange rate changes on cash	(153,000)	78,000	463,000
Net change in cash, cash equivalents, and restricted cash	(33,345,000)	11,977,000	26,265,000
Cash, cash equivalents, and restricted cash at beginning of period	71,027,000	59,050,000	32,785,000
Cash, cash equivalents, and restricted cash at end of period	37,682,000	71,027,000	59,050,000
Supplement cash flow information
Cash paid for interest	1,134,000	905,000	972,000
Noncash investing activities
Purchase of property and equipment not yet paid	145,000	103,000	147,000
Unrealized gain (loss) on marketable securities	16,000	20,000	(36,000)
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital	0	0	4,404,000
Debt issuance costs included in accounts payable	0	0	122,000
Accrued dividends and accretion of preferred stock to redemption value	$ 0	$ 0	$ 4,566,000

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a biopharmaceutical company dedicated to developing the first generation of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. The accompanying financial statements have been prepared on a basis that assumes the Company is a going concern, and does not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from any uncertainty related to its ability to continue as a going concern. Liquidity and Management's Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital which may not be available to the Company on favorable terms. To date, the Company has financed its operations primarily through the initial public offering of its common stock, a private placement of its common stock, issuances of preferred stock, debt, research grants and research collaborations. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. All of its revenue to date has been collaboration and grant revenue. The Company has devoted substantially all of its financial resources and efforts to developing its ImmTOR technology, identifying potential product candidates and conducting preclinical studies and its clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any ImmTOR-enabled therapies. As of December 31, 2018 , the Company’s cash and cash equivalents were $37.4 million , of which $0.4 million was held by its Russian subsidiary designated solely for use in its operations. In addition, approximately $31.3 million was raised from an equity offering that closed in January 2019. The Company has incurred losses and negative cash flows from operating activities since inception. As of December 31, 2018 , and December 31, 2017 , the Company had an accumulated deficit of $280.4 million and $216.9 million , respectively. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research, development of its product candidates, conducting preclinical studies and clinical trials, and its administrative organization. The Company will require substantial additional financing to fund its operations and to continue to execute its strategy, and the Company will pursue a range of options to secure additional capital. These conditions raise substantial doubt about its ability to continue as a going concern within one year after the date that the financial statements are issued. Management is exploring various sources of funding such as strategic collaborations and the issuance of equity to fund its operations. If the Company raises additional funds through strategic collaborations and alliances, which may include existing collaboration partners, the Company may have to relinquish valuable rights to its technologies or product candidates, or grant licenses on terms that are not favorable to the Company. To the extent that the Company raises additional capital through the sale of equity, the ownership interest of its existing shareholders will be diluted and other preferences may be necessary that adversely affect the rights of existing shareholders. The current operating plan includes the cost to prepare for the Phase 3 clinical program for SEL-212. However, prior to beginning the Phase 3 clinical program, the Company expects to conduct the planned head-to-head Phase 2 trial against Krystexxa. The Company will require additional external sources of capital to conduct the Phase 3 clinical program. If the Company is unable to raise sufficient capital through the transactions discussed above, it intends to curtail expenses contemplated by the current operating plan, and the Company may be required to delay, limit, reduce or terminate its product development efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself. Because of the uncertainty in securing additional capital and the insufficient amount of cash resources at December 31, 2018 and through the issuance of additional equity in January 2019, management has concluded that substantial doubt exists with respect to our ability to continue as a going concern within one year after the date of the filing of this Annual Report on Form 10-K. All amounts due under the 2017 Term Loan (Note 9) have been classified as a current liability as of December 31, 2018 due to the considerations discussed above and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by the Lender as of the date of the filing of this Annual Report on Form 10-K. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2018

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated. Foreign Currency The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases. Cash Equivalents, Short-term Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the reporting periods, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value. As of December 31, 2018 , the Company has restricted cash balances relating to a secured letter of credit in connection with its Headquarters Lease (as defined in Note 8). The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows: Year Ended December 31, 2018 2017 2016 Cash and cash equivalents $ 37,403 $ 70,622 $ 58,656 Restricted cash — 76 78 Restricted cash included in other assets 279 329 316 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 37,682 $ 71,027 $ 59,050 Concentrations of Credit Risk and Off‑Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2018 , the Company maintained approximately $0.4 million in Russian bank accounts, of which $0.4 million was held in U.S. dollars. The Company did not have any off-balance sheet arrangements as of December 31, 2018 and December 31, 2017 . Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, short-term investments, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2018 , the carrying amount of the Company's loan payable approximates its estimated fair value due to the short-term nature of the instrument. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 3 inputs. Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the twelve months ended December 31, 2018 or the year ended December 31, 2017 . Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use. Impairment of Long‑Lived Assets The Company reviews long‑lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level ("asset group"). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Based on management's evaluation, the fair value of the asset group, measured as the market capitalization of the Company exceeds its carrying value, and for this reason the Company did no t recognize an impairment loss during the twelve months ended December 31, 2018 and 2017 . Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred. Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on available-for-sale securities. The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands): Foreign currency translation adjustment Unrealized gains (losses) on available-for-sale securities Accumulated other comprehensive income (loss) Balance at December 31, 2015 $ (4,986 ) $ — $ (4,986 ) Other comprehensive income (loss) during the year 504 (36 ) 468 Balance at December 31, 2016 $ (4,482 ) $ (36 ) $ (4,518 ) Other comprehensive income during the year 78 20 98 Balance at December 31, 2017 $ (4,404 ) $ (16 ) $ (4,420 ) Other comprehensive income (loss) during the period (153 ) 16 (137 ) Balance at December 31, 2018 $ (4,557 ) $ — $ (4,557 ) Revenue Recognition The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Prior to January 1, 2018, the Company recognized revenue in accordance with ASC Topic 605, Revenue Recognition ("ASC 605"). Under ASC 605, the Company recognized revenue when all of the following criteria were met: - Persuasive evidence of an arrangement exists; - Delivery has occurred or services have been rendered; - The seller’s price to the buyer is fixed or determinable; and - Collectability is reasonably assured. The Company enters into agreements that contain multiple deliverables. Prior to the adoption of ASC 606, when evaluating multiple element arrangements, the Company considered whether the deliverables under the arrangement represent separate units of accounting. In determining the units of accounting, management evaluated certain criteria, including whether the deliverables had standalone value. The consideration received was allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria was applied to each of the separate units. If deliverables had no standalone value, they were grouped together and revenue was recognized over the estimated performance period. Prior to the adoption of ASC 606, the Company determined the estimated selling price for deliverables within each agreement using vendor‑specific objective evidence (“VSOE”) of selling price, if available, third‑party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price if neither VSOE nor TPE was available. Since the Company is a clinical stage company without a marketable product the Company used its best estimate of selling price to estimate the selling price for licenses and deliverables related to the Company’s proprietary technology. Prior to the adoption of ASC 606, options for future deliverables were considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. When an option was considered substantive and there is no significant incremental discount, the option is not considered a deliverable in the arrangement and no consideration is allocated to it. Conversely, when an option was not considered substantive or it was considered substantive but was priced at an incremental discount, it would have been considered a deliverable at the inception of the arrangement and a corresponding amount would be included in the initial consideration. The Company may receive upfront payments when licensing its intellectual property in conjunction with a manufacturing or a research and development agreement. Prior to the adoption of ASC 606, when management believed the license to its intellectual property did not have stand‑alone value from the other deliverables to be provided in the arrangement, the Company generally recognized revenue attributed to the license over the Company’s contractual or estimated performance period. When management believed the license to its intellectual property had stand‑alone value, the Company generally recognized revenue attributed to the license upon delivery. Prior to the adoption of ASC 606, at the inception of each agreement that included milestone payments, the Company evaluated whether each milestone is substantive and at risk to both parties based on the contingent nature of the milestone. Revenues from milestones, if they were nonrefundable and deemed substantive, were recognized upon successful accomplishment of the milestones. Milestones that were not considered substantive were accounted for as license payments and recognized over the remaining period of performance. Under ASC 605, amounts received prior to satisfying the revenue recognition criteria were recognized as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date were classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date were classified as deferred revenue, net of current portion. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The Company adopted ASC 606, Revenue from Contracts with Customers , on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, which is also referred to herein as "legacy GAAP" or the "previous guidance". Refer to Note 13 to its consolidated financial statements included elsewhere in this Annual Report on Form 10-K for the impact of adoption of ASU 2014-09 on the Company’s consolidated financial position, results of operations, equity and cash flows as of the adoption date and for the year ended December 31, 2018. Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark (see Note 13 to the Company's consolidated financial statements included elsewhere in this Annual Report on Form 10-K) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. The grants, collaboration and license agreements are within the scope of ASC 606. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and we outsource a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. We also record accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, we analyze progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions. Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity , and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock . Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date. Stock‑Based Compensation The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation awarded to employees is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Prior to the adoption of ASU 2018-07 during the second quarter of 2018 described below in "Recent Accounting Pronouncements", stock‑based compensation awarded to non‑employees was subject to revaluation over its vesting terms. Subsequent to the adoption of ASU 2018-07, non-employee share-based payment awards are measured on the date of grant, similar to share-based payment awards granted to employees. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest. Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same. Deferred Rent Rent expense and lease incentives from operating leases are recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the accompanying consolidated balance sheets. Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies . A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of December 31, 2018 and December 31, 2017 , the Company was not a party to any litigation that could have a material adverse effect on the Compa

Available-for-Sale Marketable S

Available-for-Sale Marketable Securities	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Marketable Securities	Available-for-Sale Marketable Securities As of December 31, 2018 , the Company did not have available-for-sale marketable securities. As of December 31, 2017 , the Company’s available-for-sale marketable securities consisted of U.S. government and agency securities and corporate bonds. The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2017 (in thousands): December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. government and agency securities $ 12,798 $ — $ (10 ) $ 12,788 Corporate bonds 13,158 — (6 ) 13,152 Total available-for-sale marketable securities $ 25,956 $ — $ (16 ) $ 25,940 All available-for-sale marketable securities are classified in the Company’s consolidated balance sheets as short-term deposits and investments. The Company classifies its marketable debt securities based on their contractual maturity dates. The fair values and amortized cost of marketable debt securities by contractual maturity were as follows (in thousands): December 31, 2017 Fair Value Amortized Cost Less than one year $ 25,940 $ 25,956

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company has reported a net loss for the years ended December 31, 2018 , and 2017 , and 2016 , and for this reason basic and diluted net loss per share are the same for all periods presented. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data): Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (65,336 ) $ (65,321 ) $ (36,210 ) Less: accretion on preferred stock — — (4,566 ) Net loss $ (65,336 ) $ (65,321 ) $ (40,776 ) Denominator: Weighted‑average common shares outstanding—basic and diluted 22,389,286 20,425,050 10,493,939 Net loss per share —basic and diluted $ (2.92 ) $ (3.20 ) $ (3.89 ) All potential dilutive common shares have been excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive. Potential dilutive common share equivalents consist of the following: Year Ended December 31, 2018 2017 2016 Stock options to purchase common stock 4,093,979 2,657,187 2,128,346 Unvested restricted stock units 175,000 — — Stock warrants to purchase common stock 95,619 176,432 97,302 Total 4,364,598 2,833,619 2,225,648

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The tables below present information about the Company’s financial assets that are measured and carried at fair value as of December 31, 2018 and December 31, 2017 , and indicate the level within the fair value hierarchy where each measurement is classified. Below is a summary of assets measured at fair value on a recurring basis (in thousands): December 31, 2018 (Level 1) (Level 2) (Level 3) Total Cash equivalents: Money market funds $ 10,123 $ — $ — $ 10,123 Total cash equivalents $ 10,123 $ — $ — $ 10,123 December 31, 2017 (Level 1) (Level 2) (Level 3) Total Cash equivalents: Money market funds $ 39,478 $ — $ — $ 39,478 U.S. government and agency securities — 1,000 — 1,000 Total cash equivalents $ 39,478 $ 1,000 $ — $ 40,478 Short-term investments: Corporate bonds $ 13,152 $ — $ — $ 13,152 U.S. government and agency securities — 12,788 — 12,788 Total short-term investments $ 13,152 $ 12,788 $ — $ 25,940 At December 31, 2018 , cash and cash equivalent investments were held in money market funds maturing within 90 days from the date of purchase. At December 31, 2017 , cash and cash equivalent investments were held in money market funds and U.S. government and agency securities maturing within 90 days from the date of purchase. The average maturity date for U.S. government and agency securities, included in investments at December 31, 2017 , was 179 days. Fair value of U.S. government and agency securities approximated amortized value at the reporting date. The average maturity date for corporate bonds, included in investments at December 31, 2017 was 168

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): December 31, December 31, 2018 2017 Laboratory equipment $ 5,379 $ 5,138 Computer equipment and software 561 480 Leasehold improvements 278 278 Furniture and fixtures 247 235 Office equipment 135 118 Construction in process 79 — Total property and equipment 6,679 6,249 Less accumulated depreciation (4,552 ) (4,158 ) Property and equipment, net $ 2,127 $ 2,091 Depreciation expense was $1.0 million , $0.8 million and $0.8 million for the years ended December 31, 2018 , 2017 and 2016

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): December 31, December 31, 2018 2017 Payroll and employee related expenses $ 2,497 $ 1,902 Current portion of deferred rent and lease incentive 117 72 Collaboration and licensing 1,222 1,020 Accrued patent fees 736 268 Accrued external research and development costs 5,344 3,578 Accrued professional and consulting services 994 859 Accrued grant refund 175 175 Accrued interest 106 89 Other 509 617 Accrued expenses $ 11,700 $ 8,580

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Operating Leases The Company has a non‑cancellable operating lease for its laboratory and office space located at 480 Arsenal Way, Watertown, Massachusetts (" Headquarters Lease "). As part of the Headquarters Lease agreement, the landlord provided the Company a tenant improvement allowance of up to $0.7 million , which the Company fully utilized during 2012. The tenant improvement allowance is accounted for as a lease incentive obligation and is being amortized as a reduction to rent expense over the lease term. The leasehold improvements are capitalized as a component of property and equipment. In connection with the Headquarters Lease , the Company secured a letter of credit for $0.3 million which renews automatically each year and is classified in restricted cash and other deposits in the accompanying consolidated balance sheets. In August 2016, the Company signed an amendment to the Headquarters Lease , which extends the term through March 31, 2020. The Headquarters Lease includes a rent escalation clause, and accordingly, rent expense is being recognized on a straight-line basis over the lease term. In October 2017, the Company entered into a lease for approximately 5,100 square feet of additional office space located in Watertown, Massachusetts. The lease includes a rent escalation clause, and accordingly, rent expense is being recognized on a straight-line basis over the lease term. In addition, the Company decided to vacate certain office space in Watertown, MA on January 11, 2019 and consolidated all employees at its corporate headquarters also in Watertown, MA. The lease term expires in March 2020. As of both December 31, 2018 and December 31, 2017 , deferred rent and lease incentive liability totaled $0.2 million . The current portion of deferred rent and lease incentive liability is presented within accrued expenses, and was $0.1 million and less than $0.1 million at December 31, 2018 and December 31, 2017 , respectively. The Company has a month‑to‑month facility agreement for its Moscow, Russia facility. Rent expense is recognized as incurred. Rent expense for the years ended December 31, 2018 , 2017 and 2016 was $2.0 million , $1.9 million and $1.6 million , respectively. As of December 31, 2018 , the aggregate future minimum lease payments related to these leases are as follows (in thousands): Year ending December 31, 2019 1,482 2020 375 Total minimum lease payments $ 1,857 Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non‑performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding.

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt	Debt 2015 Term Loan On August 9, 2013, the Company entered into a loan and security agreement with Oxford Finance LLC (“Oxford”) and Square 1 Bank (“Square 1”) to borrow up to $7.5 million . The Company initially borrowed $3.0 million in August 2013 and subsequently borrowed an additional $4.5 million in July 2014. In December 2015, the Company refinanced this debt facility to increase the amount of the borrowing to $12.0 million and to extend the repayment term. The amounts borrowed are collectively referred to as the “ 2015 Term Loan ”. The lenders for the refinanced debt facility are Oxford and Pacific Western Bank (“Pacific Western”). Pacific Western had acquired Square 1 since the time of the original loan. Such a change in lender does not constitute third party financing on its own and does not require extinguishment accounting. As a result of the refinancing, the stated interest rate was also adjusted to reflect the current market borrowing rate. Prior to the extinguishment of the 2015 Term Loan debt in September 2017 that is discussed in greater detail below, the 2015 Term Loan was collateralized by substantially all of the assets of the Company and bore interest at 8.1% per annum. The monthly payments for the 2015 Term Loan were initially interest only through January 2017. Principal repayments for the 2015 Term Loan were due over 30 monthly installments beginning on February 1, 2017. The 2015 Term Loan could be prepaid at the Company’s option at any time prior to maturity subject to a prepayment fee of 2% if prepaid after the first anniversary but before the second anniversaries, and 1% if prepaid after the second anniversary. The 2015 Term Loan did not include any financial covenants. The 2015 Term Loan required a final payment fee of 6.0% on the aggregate principal amounts borrowed upon repayment at maturity, on a prepayment date, or upon default. The final payment fee totaling $0.7 million was recorded as a loan discount. In connection with the 2015 Term Loan , the Company granted the lenders warrants in August 2013 to purchase up to 26,668 shares of the Company’s Series D Preferred and additional warrants in July 2014 to purchase up to 40,000 shares of the Company’s Series D Preferred. As of the IPO, the warrants to purchase up to 66,668 shares of the Company’s Series D Preferred were converted to warrants to purchase up to 17,888 shares of the Company’s common stock at an exercise price of $16.77 per share. These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance. Additionally, with the refinancing of the 2015 Term Loan at December 31, 2015, the Company granted the lenders 37,978 shares of the Company’s Series E Preferred which also was converted at the IPO to warrants to purchase up to 15,094 shares of Company’s common stock at an exercise price of $11.32 per share. These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance. The initial grant date fair value of the Series D Preferred and Series E Preferred warrants was $0.1 million and $0.1 million for each issuance, respectively and was recorded as a loan discount. In December 2016, a total of 16,493 warrants to purchase common stock were exercised under a cashless exercise, resulting in a net issuance of 4,697 shares of common stock. The warrant exercise price had been established at the time the warrants were converted. 2017 Term Loan On September 12, 2017, the Company entered into a term loan facility of up to $21.0 million (the “ 2017 Term Loan ”) with Silicon Valley Bank, a California corporation (“SVB”), the proceeds of which were used to repay the 2015 Term Loan and for general corporate and working capital purposes. The Company refers to the 2015 Term Loan and the 2017 Term Loan, collectively, as the “Term Loans.” The 2017 Term Loan is governed by a loan and security agreement, dated September 12, 2017, between the Company and SVB (the “Loan Agreement”). The 2017 Term Loan was funded in full on September 13, 2017 (the “Funding Date”). On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which SVB utilized $10.0 million of the 2017 Term Loan to pay off all outstanding obligations under the 2015 Term Loan. The Company recognized a loss on extinguishment of debt in the amount of $0.7 million during the three months ended September 30, 2017. The Company incurred less than $0.1 million in debt issuance costs in connection with the closing of the 2017 Term Loan. Debt issuance costs are presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt. The 2017 Term Loan will mature on February 1, 2022. Each advance under the 2017 Term Loan accrues interest at a floating per annum rate equal to one-half of one percent above the prime rate (as published in the money rates section of The Wall Street Journal). The 2017 Term Loan provides for interest-only payments monthly until August 31, 2019. Thereafter, amortization payments will be payable monthly in equal installments of principal and variable interest to fully amortize the outstanding principal over the remaining term of the loan. The monthly interest is subject to recalculation upon a change in the prime rate. The Company may prepay the 2017 Term Loan in full but not in part provided that the Company (i) provides five business days’ prior written notice to SVB, (ii) pays on the date of such prepayment (A) all outstanding principal plus accrued and unpaid interest, and (B) a prepayment premium of 3% if prepaid before the first anniversary, 2% if prepaid after the first anniversary but before the second anniversaries, and 1% if prepaid after the second anniversary. Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 4.0% per annum of the past due amount outstanding. The events of default under the Loan Agreement include, but are not limited to, the Company’s failure to make any payments of principal or interest under the Loan Agreement or other transaction documents, the Company’s breach or default in the performance of any covenant under the Loan Agreement or other transaction documents, the occurrence of a material adverse effect, the Company making a false or misleading representation or warranty in any material respect under the Loan Agreement, the Company’s insolvency or bankruptcy, any attachment or judgment on the Company’s assets in excess of approximately $0.3 million , or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of approximately $0.3 million . If an event of default occurs, SVB is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement. The 2017 Term Loan is secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets includes any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company has also granted SVB a negative pledge with respect to its intellectual property. The 2017 Term Loan does not include any financial covenants. The 2017 Term Loan requires a final payment fee of 5% on the aggregate principal amounts borrowed upon repayment at maturity, on a prepayment date, or upon default. The final payment fee totaling $1.1 million is recorded as a loan discount. Under the 2017 Term Loan , the Company is not required to maintain a minimum cash balance. In addition, the 2017 Term Loan contains a subjective acceleration clause whereby in an event of default, an immediate acceleration of repayment occurs if there is a material impairment of the lenders’ lien or the value of the collateral, a material adverse change in the business condition or operations, or a material uncertainty exists that any portion of the loan may not be repaid. The Company assessed all terms and features of the 2017 Term Loan in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the 2017 Term Loan , including any put and call features. The Company determined that all features of the 2017 Term Loan were clearly and closely associated with the debt host and did not require bifurcation as a derivative liability, or the fair value of the embedded feature was immaterial to the Company's consolidated financial statements. The Company reassesses the identified features on a quarterly basis to determine if they require bifurcation. As of December 31, 2018 and December 31, 2017 , the outstanding principal balance under the 2017 Term Loan was $21.0 million . Future minimum principal and interest payments on the 2017 Term Loan as of December 31, 2018 are as follows (in thousands): 2019 4,742 2020 9,232 2021 8,718 2022 1,754 Total minimum debt payments $ 24,446 Less: Amount representing interest (2,396 ) Less: Debt discount and deferred charges (665 ) Less: Current portion of loan payable (21,385 ) Loan payable, net of current portion $ — All amounts due under the 2017 Term Loan have been classified as a current liability as of December 31, 2018 due to the considerations discussed in Note 1 and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by SVB as of the date of the filing of this Annual Report on Form 10-K. During the years ended December 31, 2018 , 2017 and 2016 , the Company recognized $1.5 million , $1.2 million and $1.3 million

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Preferred Stock	Preferred Stock Prior to the completion of its IPO, the Company had 37,835,623 authorized shares of preferred stock, par value $0.0001 per share (the "Preferred Stock") as of December 31, 2015. The Company issued Preferred Stock to investors for cash or as settlement for outstanding debt under convertible notes. As of December 31, 2015, the Company had issued and outstanding Preferred Stock of (i) 2,589,868 shares of Series A redeemable convertible preferred stock (“Series A Preferred”), (ii) 7,437,325 shares of Series B redeemable convertible preferred stock (“Series B Preferred”), (iii) 5,000,002 shares of Series C redeemable convertible preferred stock (“Series C Preferred”), (iv) 8,099,994 shares of Series D Preferred, (v) 2,111,109 shares of Series SRN Redeemable Convertible Preferred Stock (“Series SRN Preferred”) and (vi) 8,888,888 shares of Series E Preferred. All outstanding shares of the Company’s convertible preferred stock automatically converted into 10,126,118

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Common Stock	Common Stock 2017 Shelf Registration Statement On August 10, 2017 , the Company filed a universal shelf registration statement on Form S-3 with the SEC to sell up to $200 million of equity and/or debt securities and entered into a sales agreement with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50 million in an at-the-market, or ATM, offering. The shelf registration statement was declared effective by the SEC on August 28, 2017 . As of December 31, 2018 , no securities have been issued from this shelf registration statement. PIPE Financing On June 26, 2017, the Company entered into a securities purchase agreement (the "Institutional Purchase Agreement") with a select group of institutional investors (the “Institutional Investors”) and a securities purchase agreement with Timothy Springer, Ph.D., a member of the board of directors (the "Springer Purchase Agreement") for a private placement of the Company's securities (the "2017 PIPE"). The closing of the 2017 PIPE occurred on June 27, 2017. Pursuant to the Institutional Purchase Agreement, the Company agreed to sell an aggregate of 2,750,000 shares of its common stock, par value $0.0001 per share, at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company agreed to sell to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years . The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. Under the terms of the Common Stock Purchase Warrant, the warrants can be settled in unregistered shares. The Warrant Shares qualify for equity classification. The fair value of the allocated proceeds was determined on the relative fair value basis. After deducting for placement agent fees and offering expenses, the aggregate net proceeds from the 2017 PIPE was approximately $47.1 million . The Company expects to use the proceeds from the 2017 PIPE towards working capital requirements and general corporate purposes. On June 27, 2017, in connection with the 2017 PIPE, the Company entered into a Registration Rights Agreement (the “Registration Rights Agreement”) with the Institutional Investors and Dr. Springer (together, the “Investors”). Pursuant to the Registration Rights Agreement, the Company agreed to prepare and file a registration statement with the Securities and Exchange Commission (the “SEC”) within 20 days after the closing of the 2017 PIPE for purposes of registering the resale of the shares of common stock issued and sold in the 2017 PIPE (the “Shares”), the Warrant Shares, and any shares of common stock issued as a dividend or other distribution with respect to the Shares or Warrant Shares. The registration statement was declared effective by the SEC on July 21, 2017. The Company agreed to indemnify the Investors, their officers, directors, members, employees and agents, successors and assigns under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the Registration Rights Agreement. Initial Public Offering On June 21, 2016, the Company completed its IPO and issued and sold 5,000,000 shares of common stock at a price to the public of $14.00 per share for net proceeds of $60.8 million after deducting underwriting discounts and commissions and offering expenses. On July 25, 2016, 289,633 additional shares of the Company’s common stock were sold to the underwriters pursuant to the exercise of their option to purchase additional shares of common stock at a price to the public of $14.00 per share resulting in additional net proceeds of approximately $3.7 million after deducting underwriting discounts, commissions and offering expenses, bringing the total IPO net proceeds to $64.5 million . Upon the closing of the Company's IPO on June 27, 2016, all outstanding shares of its convertible preferred stock automatically converted into 10,126,118 shares of the Company’s common stock. In addition, at this time, the warrants to purchase shares of the Company’s Series D and Series E convertible preferred stock were converted into warrants to purchase shares of the Company’s common stock. Common Stock As of December 31, 2018 , the Company has 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 22,471,776 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics: Voting The common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company. Dividends The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through December 31, 2018 , no dividends have been declared or paid on common stock. Liquidation Upon liquidation of the Company, the common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders. Reserved Shares The Company has authorized shares of common stock for future issuance as follows: Period ending December 31, 2018 December 31, 2017 Exercise of common warrants 95,619 176,432 Shares available for future stock incentive awards 1,586,925 923,440 Unvested restricted stock units 175,000 — Outstanding common stock options 4,093,979 2,657,187 Total 5,951,523 3,757,059

Stock Incentive Plans

Stock Incentive Plans	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Incentive Plans	Stock Incentive Plans Stock Options The Company maintains the 2008 Stock Incentive Plan (the “2008 Plan”) for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non‑qualified stock option and restricted stock awards as determined by the Board. At inception of the 2008 Plan, a total of 2,213,412 shares of common stock were authorized for grants under the 2008 Plan. The Company ceased granting awards under the 2008 Plan upon the effectiveness of the 2016 Plan (as defined below); however, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan and the applicable 2008 Plan agreement. Shares subject to awards that were granted under the 2008 Plan and that expire, lapse or terminate following the effectiveness of the 2016 Plan become available under the 2016 Plan as shares available for future grants. All unvested stock options granted under the 2008 Plan may be exercised into restricted stock subject to forfeiture upon termination prior to vesting. The 2008 Plan provided that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2008 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date. On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 21, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option, restricted stock and other stock and cash-based awards as determined by the Board. Shares subject to awards that are granted under the 2016 Plan and that expire, lapse or terminate are available for future grants under the 2016 Plan. At inception of the 2016 Plan, a total of 1,947,779 shares of common stock were authorized for future issuance under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. During the twelve months ended December 31, 2018 and 2017, the number of shares of common stock that may be issued under the 2016 Plan was increased by 893,730 shares and 737,550 shares, respectively. As of December 31, 2018 , 1,586,925 shares remain available for future issuance under the 2016 Plan. The 2016 Plan provides that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2016 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date. The Company’s 2018 Employment Inducement Incentive Award Plan (the “Inducement Incentive Award Plan”), which was adopted by the Board on September 25, 2018 without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules ("Rule 5635(c)(4)"), provides for the grant of equity-based awards in the form of non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock or cash based awards. In accordance with Rule 5635(c)(4), awards under the Inducement Incentive Award Plan may only be made to a newly hired employee who has not previously been a member of the Board, or an employee who is being rehired following a bona fide period of non-employment by the Company, as a material inducement to the employee’s entering into employment with the Company. The Company reserved 1,175,000 shares of its common stock for issuance under the Inducement Incentive Award Plan. Pursuant to an employment agreement entered into with Carsten Brunn, Ph.D., upon Dr. Brunn's commencing employment as the Company’s President and Chief Executive Officer on December 1, 2018, the Company issued Dr. Brunn equity awards from the Inducement Incentive Award Plan consisting of an option to purchase 1,000,000 shares of common stock and 175,000 restricted stock units. As of December 31, 2018, there are no shares available for future grant under the 2018 Inducement Incentive Award Plan. The fair value of each option award was estimated on the grant date using the Black‑Scholes option pricing model. Expected volatilities are based on historical volatilities from guideline companies because the Company's common stock has not traded for a period that is at least equal to the expected term of its stock option awards. The Company uses the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk‑free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on awards that are expected to vest. The estimated grant date fair values of employee stock option awards granted under the 2008 Plan, 2016 Plan and the Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.85 % 2.03 % 1.42 % Dividend yield — — — Expected term 6.06 5.90 6.04 Expected volatility 85.17 % 84.52 % 94.97 % Weighted-average fair value of common stock $ 8.45 $ 15.32 $ 13.25 The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2018 , 2017 and 2016 was $6.17 , $10.97 , and $10.23 , respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2018 , 2017 and 2016 was $0.6 million , $2.6 million and $0.8 million , respectively. As of December 31, 2018 and December 31, 2017 , total unrecognized compensation expense related to unvested employee stock options was $9.8 million and $10.2 million , respectively, which is expected to be recognized over a weighted average period of 3.1 years and 2.9 years, respectively. During the year ended December 31, 2017, the status of two executives changed from an employee to a consultant (non-employee). Stock-based compensation expense relating to the executives' stock option awards were recognized through the termination date. In accordance with ASC 505-50, 55,073 unvested stock option awards were subsequently remeasured using the then-current fair value of the Company's common stock and updated assumption inputs in the Black-Scholes option pricing model, while 62,546 shares in aggregate were forfeited and expired as of December 31, 2017. The right to exercise vested, exercisable stock option awards ( 126,457 shares in aggregate) as of December 31, 2017 were extended to March 31, 2019, resulting in a modification; and during the first quarter of 2018, a stock option award to purchase 20,000 shares of the Company's common stock was granted. In connection with the Company's adoption of ASU 2018-07 during the second quarter of 2018, final remeasurement of the executives' stock option awards were performed as of April 1, 2018. Stock-based compensation expense was recognized over the consulting period until services were completed. All granted stock option awards are presented in the employee awards stock option table below. The Company granted a stock option award covering 95,000 shares to a non-employee director during the second quarter of 2018, with a weighted average grant date fair value of $8.71 for consulting services in addition to his responsibilities as a director. Prior to the adoption of ASU 2018-07 during the second quarter of 2018, the unvested options held by non‑employees were revalued using the Company’s estimate of fair value on each vesting and reporting date through the remaining vesting period. Subsequent to the adoption of ASU 2018-07, non-employee share-based payment awards are measured on the date of grant, similar to share-based payment awards granted to employees. During the fourth quarter of 2018, an additional stock option award of 75,000 shares was granted to the non-employee director, with a weighted average grant date fair value of $4.35 . Effective November 30, 2018, an executive officer resigned and immediately entered into a consulting arrangement with the Company for the period December 1, 2018 to November 30, 2019. In connection with his consulting arrangement, (i) all of his unvested equity-based compensation awards continue to vest subject to his continued consulting services ( 84,604 shares in aggregate) and (ii) his right to exercise the vested and exercisable stock option awards ( 310,219 shares in aggregate) as of November 30, 2019 was extended 180 days to May 28, 2020, resulting in a modification. In addition, since the consulting agreement was not considered substantive, the Company recorded a charge for the fair value of the options in the fourth quarter for the acceleration of the vesting of his awards. The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.77 % — % 2.06 % Dividend yield — — — Expected term 5.81 0.00 8.80 Expected volatility 85.86 % — % 87.33 % As of December 31, 2018 and 2017 , total unrecognized compensation expense related to unvested non‑employee stock options was $1.1 million and $0.6 million , respectively, which is expected to be recognized over a weighted average period of 2.2 years years and 1.9 years , respectively. The following table summarizes the activity under the 2008 Plan, 2016 Plan, and Inducement Incentive Award Plan: Weighted‑average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employee Awards Outstanding at December 31, 2017 2,411,237 $ 10.58 7.50 $ 4,729 Granted 1,625,679 $ 8.45 Exercised (103,784 ) $ 4.85 Forfeited (251,557 ) $ 15.03 Outstanding at December 31, 2018 3,681,575 $ 9.49 7.77 $ 300 Vested at December 31, 2018 1,484,196 $ 9.01 5.83 $ 300 Vested and expected to vest at December 31, 2018 2,990,612 $ 9.27 7.49 $ 300 Non‑Employee Awards Outstanding at December 31, 2017 245,950 $ 4.32 5.10 $ 1,351 Granted 170,000 $ 9.38 Exercised — $ — Forfeited (3,546 ) $ 0.39 Outstanding at December 31, 2018 412,404 $ 6.44 6.42 $ 28 Vested at December 31, 2018 223,180 $ 4.15 3.91 $ 28 Vested and expected to vest at December 31, 2018 412,404 $ 6.44 6.42 $ 28 Restricted Stock Units During the second quarter of 2018, the Company awarded 40,000 restricted stock units under the 2016 Plan to certain employees subject to the achievement of performance conditions. These restricted stock units will vest in a single installment on the date the performance condition is achieved, on or prior to December 31, 2018. The restricted stock units granted during the second quarter of 2018 had a weighted average fair value of $12.75 per share based on the closing price of the Company’s common stock on the date of grant. These restricted stock units were valued at approximately $0.5 million on their grant date. During the third quarter of 2018, the Company determined that it was not probable the performance criteria would be achieved on or prior to December 31, 2018. For this reason, the Company reversed less than $0.1 million of previously-recognized stock-based compensation expense in the third quarter of 2018. The performance condition was not satisfied on or prior to December 31, 2018, therefore the restricted stock units are forfeited for no consideration. During the fourth quarter of 2018, the Company awarded 175,000 restricted stock units under the Inducement Incentive Award Plan. The restricted stock units granted had a weighted average fair value of $6.03 per share based on the closing price of the Company’s common stock on the date of grant. These restricted stock units were valued at approximately $ 1.1 million . Unrecognized compensation expense is $ 1.0 million as of December 31, 2018, which is expected to be recognized over a weighted average period of 3.9 years . The following table summarizes the status of the Company’s restricted stock units: Number of Shares (#) Weighted Average Fair Value ($) Unvested at December 31, 2017 — $ — Granted 215,000 7.28 Vested — — Forfeited (40,000 ) 12.75 Unvested at December 31, 2018 175,000 $ 6.03 Employee Stock Purchase Plan On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. The ESPP is intended to qualify as an "employee stock purchase plan" under Section 423 of the Internal Revenue Code of 1986 with the purpose of providing employees with an opportunity to purchase the Company's common stock through accumulated payroll deductions. Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1 st and the other beginning September 1 st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower. The first ESPP offering period began on March 1, 2017. At inception of the ESPP, a total of 173,076 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the year ended December 31, 2018 and 2017 , the number of shares of common stock that may be issued under the ESPP was increased by 223,432 shares and 184,387 shares, respectively. During the year ended December 31, 2018 , the Company issued 24,738 shares of common stock under the ESPP. As of December 31, 2018 , 539,894 shares remain available for future issuance under the ESPP. For the years ended December 31, 2018 and 2017 , the Company recognized less than $0.1 million and $0.1 million of stock-based compensation expense under the ESPP. The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2018 2017 2016 Research and development $ 2,453 $ 1,779 $ 1,183 General and administrative 3,261 2,302 847 Total stock-based compensation expense $ 5,714 $ 4,081 $ 2,030

Revenue Arrangements

Revenue Arrangements	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, Revenue Recognition (ASC 605), which is also referred to herein as "legacy GAAP" or the "previous guidance". The adoption of ASC 606 represents a change in accounting principle that will more closely align revenue recognition with the delivery of the Company's services and will provide financial statement readers with enhanced disclosures. Financial Statement Impact of Adopting ASC 606 The cumulative effect of applying the new guidance to all contracts with customers that were not completed as of January 1, 2018, was recorded as an adjustment to accumulated deficit as of the adoption date. As a result of applying the modified retrospective method to adopt the new revenue guidance, the following adjustments were made to the Company's accounts on the consolidated balance sheet as of January 1, 2018 (in thousands): Adjustments As Reported at Due to Balance at Consolidated Balance Sheet December 31, 2017 ASU 2014-09 January 1, 2018 Assets Cash, cash equivalents, and restricted cash $ 70,698 $ — $ 70,698 Short-term deposits and investments 25,940 — 25,940 Prepaid expenses and other current assets 2,042 — 2,042 Total current assets 98,680 — 98,680 Property and equipment, net 2,091 — 2,091 Restricted cash and other assets 329 — 329 Total assets $ 101,100 $ — $ 101,100 Liabilities and stockholders’ equity Current liabilities: Accounts payable $ 1,606 $ — $ 1,606 Accrued expenses 8,580 — 8,580 Deferred revenue, current portion 787 — 787 Total current liabilities 10,973 — 10,973 Non‑current liabilities: Deferred rent and lease incentive 151 — 151 Loan payable 21,042 — 21,042 Deferred revenue, net of current portion 15,919 (1,830 ) 14,089 Other long‑term liabilities 1,201 — 1,201 Total liabilities 49,286 (1,830 ) 47,456 Stockholders’ equity: Common stock 3 — 3 Additional paid-in capital 273,128 — 273,128 Accumulated deficit (216,897 ) 1,830 (215,067 ) Accumulated other comprehensive loss (4,420 ) — (4,420 ) Total stockholders’ equity 51,814 1,830 53,644 Total liabilities and stockholders’ equity $ 101,100 $ — $ 101,100 In connection with the adoption of ASC 606, the Company identified three collaboration/grant arrangements that required analysis to quantify the impact of adoption to its opening accumulated deficit balance as of January 1, 2018: Spark Therapeutics, Inc., Skolkovo Foundation and National Institutes of Health. Spark Therapeutics, Inc. Spark License Agreement In December 2016, the Company entered into a License and Option Agreement (“Spark License Agreement”) with Spark Therapeutics, Inc. (“Spark”) pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR technology. The Spark License Agreement provides Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies for an initial identified target. In addition to an upfront cash payment of $10.0 million under the Spark License Agreement, additional payments of an aggregate of $5.0 million in two payments of $2.5 million each were paid within twelve months of December 2, 2016 (“Contract Date”). The first of the two additional payments was scheduled to be made on or before May 31, 2017 (the “May 2017 License Payment”) (see “Spark Letter Agreement” below) and the second was made on October 31, 2017. Spark may also exercise options to research, develop and commercialize gene therapies utilizing the SVP technology for up to four additional targets. The Company is eligible to receive a variable fee up to $2.0 million for each additional target option elected, dependent on the incidence of the applicable indication. As per the agreement, the election period in which Spark can exercise additional targets is a term of three years from the Contract Date, or December 1, 2019. Assuming successful development and commercialization, the Company could receive up to an additional $65.0 million in development and regulatory milestone payments and $365.0 million in commercialization milestone payments for each indication. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-single to low-double digits, all of which apply on a target-by-target basis. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product and terminating upon the later of (i) ten years after the first commercial sale, (ii) expiration of the last to expire valid claim on patents covering the jointly invented field specific improvements, or (iii) the expiration of regulatory exclusivity in the applicable country for the licensed product. The Spark License Agreement may be terminated by Spark for convenience upon ninety days ’ notice. Either party may terminate the Spark License Agreement on a target-by-target basis for material breach with respect to such target. In December 2016, the Company also entered into a Share Purchase Agreement (the “Spark Purchase Agreement”) with Spark. Pursuant to the Spark Purchase Agreement, the Company sold 197,238 shares of the Company’s common stock to Spark for gross proceeds of $5.0 million , or $25.35 per share of common stock, at an initial closing (the “Initial Closing”). The purchase price per share represents an amount equal to 115% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Contract Date. Under the Spark Purchase Agreement, Spark has agreed not to dispose of any of the Initial Closing Shares or any Acquisition Right Shares that it may acquire until January 1, 2018 and, thereafter, transfers will be contractually subject to volume limitations applicable to an “affiliate” under Rule 144 of the Securities Act. Closings under the Spark Purchase Agreement are subject to customary conditions. Beyond the Initial Closing, the Spark Purchase Agreement contemplated potential future sales of shares by the Company to Spark as follows: • First Acquisition Right. During the period beginning on May 1, 2017 and ending on June 1, 2017, Spark had the right (the “First Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million . See "Spark Letter Agreement" below. • Second Acquisition Right. During the period beginning on October 1, 2017 and ending on November 1, 2017, Spark had the right (the “Second Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million . On October 31, 2017 Spark exercised this right and purchased 205,254 shares of common stock from the Company for $5.0 million , or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date. The First Acquisition Rights and Second Acquisition Rights are collectively referred to herein as the “Acquisition Rights”. The aggregate number of shares that the Company may issue pursuant to the Stock Purchase Agreement may not exceed the lesser of (i) 2,758,112 shares and (ii) such number of shares that would require the Company to obtain prior shareholder approval under the Nasdaq Marketplace Rules. In connection with the Spark License Agreement and Spark Purchase Agreement, the Company has made contractual payments defined in the MIT license agreement (Note 15) totaling $2.2 million for the MIT sub-license provided to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. Transition of Revenue Recognition from ASC Topic 605 to ASC Topic 606 The terms of the Spark Purchase Agreement and the Spark License Agreement were negotiated at the same time between the parties and the terms of the Spark Purchase Agreement are referenced in the Spark License Agreement in multiple sections. The pricing and terms of the agreements are unique and must be considered in contemplation with each other. There are provisions within the Spark License Agreement that link to the Spark Purchase Agreement related to provisions that constitute a material breach of the license agreement. Therefore, the Company concluded that the two agreements must be combined and evaluated as a single agreement. While the Spark Purchase Agreement and the Spark License agreement are considered to be a single agreement, the Company determined that the purchase of common stock and future acquisition rights are not within the scope of ASC 606. The Company determined that the initial purchase of common stock combined with the embedded future stock Acquisition Rights had a fair value of $2.7 million and this amount was recorded in equity as of the effective date. The remaining $2.3 million of cash received in exchange for the stock and acquisition rights is included in allocable consideration, as this represents the premium paid by Spark on the purchase of common stock, and should be allocated to the remaining performance obligations. The Company identified the following promises at the inception of the agreement: (1) certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property and a license to conduct certain research activities under the collaboration, or the License Obligation, (2) options to research, develop and commercialize gene therapies utilizing the ImmTOR technology for up to four additional target therapy options, or the Option Obligation, (3) manufactured supply of preclinical and clinical SVP, or the Supply Obligation at a discount, and (4) option to purchase manufactured supply of commercial SVP, or the Commercial Supply Obligation at fair value. In consideration for these promises, the Company received an upfront payment of $15.0 million . In addition, the Company is eligible to receive additional payments of up to $35.0 million based on the achievement by Spark of future specified development milestones, up to $30.0 million based on the achievement by Spark of future specified regulatory milestones, up to $110.0 million based on the achievement by Spark of future specified commercial milestones, and up to $255.0 million based on the achievement by Spark of future specified sales milestones. The Company will also be eligible to receive tiered royalty payments that reach low double-digits based on future net sales for the duration of the royalty term. The Company determined that the License Obligation was not capable of being distinct from the Supply Obligation. This is because Spark cannot derive benefit from the license without the simultaneous transfer of the preclinical and clinical supply. Therefore, the License Obligation and Supply Obligation are combined as a single performance obligation (the “Combined License and Supply Obligation”). The Company also determined that the Option Obligation, which includes the related Supply Obligation, provides the customer with a material right and is considered a performance obligation in the arrangement since it was priced at an incremental discount. The Company determined that the optional Commercial Supply Obligation does not provide the customer with a material right and is not considered to be a performance obligation because Spark can derive benefit from the Combined License and Supply Obligation without the delivery of the Commercial Supply Obligation and is not at an incremental discount. Therefore, the Company determined that the Spark agreement contains five distinct performance obligations: the Combined License and Supply Obligation, and the four separate Option Obligations. In determining the transaction price, the Company considered the future development milestones, regulatory milestones, commercial milestones, sales milestone, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Separately, any consideration related to sales-based milestones as well as royalties on net sales upon commercialization by Spark, will be recognized when the related sales occur as they were determined to relate predominantly to the intellectual property granted to Spark and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of the adoption date and as of December 31, 2018 , all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. As a result, the Company determined that the up-front payment of $12.3 million ( $15.0 million , less fair value of the equity totaling $2.7 million as discussed above) was included in the transaction price and was allocated to the performance obligations based on the Company’s best estimate of their relative stand-alone selling prices. The Company allocated $7.1 million to the Combined License and Supply Obligation and $5.2 million to the discount on the Option Obligation ( $1.3 million for each option) using the relative standalone selling price method to each obligation. The standalone selling price for the Combined License and Supply Obligation was determined using a discounted cash flow model. The standalone selling price for the Option Obligation was determined based on the fair value of the license minus the strike price of the option (the probability of exercise was included in the valuation) as well as the estimated discount of the Supply Obligation. The estimated proceeds to be received from the sale of the Supply Obligation was also included in the transaction price for the Combined License and Supply Obligation. The total consideration allocated to the Combined License and Supply Obligation will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term which is estimated to be approximately four years . The discount associated with the Option Obligation, along with the proceeds to be received upon exercise and estimated sale of the Supply Obligation, will be recognized when each of the options are exercised, over the related expected deliveries of its supply. If the options expire without exercise, the related deferred revenue associated with each option will be recognized upon expiration (December 1, 2019). The Company estimated that revenue recognized under ASC 605 is consistent with the revenue recognized in accordance with ASC 606 from contract inception. However, upon the initial adoption of ASC 606 on January 1, 2018, the Company recognized other assets of $2.6 million related to the incremental costs relating to the payments to MIT that would not have been incurred if the contract with Spark had not been obtained. Under the Company existing license agreement with MIT (See Note 15), in the event the Company sublicenses the MIT patents to a third party, it will be required to remit to MIT a percentage (ranging from 10% to 30% ) of sublicense income. The Company concluded that the payments made to MIT were analogous to sales commissions and represented the cost to obtain a contract, which were evaluated under ASC 340-40-25-1. Such amounts were capitalized as they were both incremental and recoverable. However, upon further review, the Company noted that the amounts paid to MIT represent the cost to fulfill a contract rather than a cost to obtain a contract as they represent the costs that were incurred in order to fulfill their supply obligations under the Spark License Agreement. Therefore, the incremental payments to MIT represent part of the cost to fulfill the contract. When determining the appropriate accounting guidance for the costs to fulfill a customer contract, ASC 340-40-25-6 indicates that any other applicable literature should be considered first. Since the intellectual property is being used exclusively for research and development, the accounting for the MIT costs were previously accounted for under ASC 730-10. Since all of the payments to MIT related to the underlying intellectual property, that does not have alternative future use, such amounts should not have been capitalized, and instead remained recorded as research and development expense upon adoption of ASC 606. The Company evaluated the impact of the error on previously issued financial statements included in the Form 10-Q for the quarters ended March 31, 2018, June 30, 2018 and September 30, 2018, noting that the impact was not material to the balance sheet, statement of operations or cash flows. However, correcting the error would be material to the quarterly trends in the statement of operations to expense the amount in the fourth quarter. Therefore, such amounts were corrected by reducing current and long-term assets by $0.2 million and $2.4 million , respectively and increasing accumulated deficit by $2.6 million as of January 1, 2018 by reversing the amounts initially recorded in transition. There was no impact to the consolidated statements of operations or cash flows for any of the quarters previously filed and no impact to any of the previously issued annual financial statements. During the twelve months ended December 31, 2018 , there were no deliveries and thus no revenue was recognized. The Company recognized revenues of less than $0.1 million related to the Spark License Agreement during the twelve months ended December 31, 2017. As of December 31, 2018 , there was a contract liability of $14.7 million representing deferred revenue associated with this agreement. A total of $1.0 million is presented as current and $13.7 million is presented as noncurrent in the accompanying consolidated balance sheet. As of December 31, 2017 , there was $14.7 million of deferred revenue related to this agreement. Spark Letter Agreement On June 6, 2017, the Company and Spark entered into a letter agreement (the “Letter Agreement”), pursuant to which the parties agreed that Spark would make the May 2017 License Payment by June 6, 2017. The May 2017 License Payment was received, and recorded as a liability as of June 30, 2017, of which some or all may potentially constitute the reimbursement described below. The parties also agreed that Spark would be deemed to have delivered notice on May 31, 2017 exercising its right to purchase the shares pursuant to the First Acquisition Right. The Letter Agreement further outlines a cost reimbursement arrangement, pursuant to which the Company agreed to reimburse Spark for all costs and expenses, including the cost of materials provided by the Company, associated with the preclinical research and toxicology studies being performed by Spark for any licensed products for a specified amount of time (the “Reimbursement Period”), up to an agreed upon cap of $2.5 million . Consistent with the First Acquisition Right, Spark purchased 324,362 shares of common stock pursuant to the Spark Purchase Agreement, as amended by the Letter Agreement, for an aggregate purchase price of $5.0 million , or $15.41 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the First Acquisition Right notification date. At the initial contract assessment, the Company allocated $2.7 million to equity (representing the fair value of the initial purchase of common stock combined with the embedded future stock Acquisition Rights). Upon exercise of the First Acquisition Right, the Company recorded the purchase amount to stockholders ’ equity. Under ASC 606, the Company determined that the Letter Agreement resulted in a modification to the original agreement. The amount received totaling $2.5 million and the reimbursements pursuant to the Letter Agreement totaling $2.5 million were both included in the transaction price, and a liability was recorded for the amount expected to be repaid. As repayments are made, the underlying liability will be reduced. To the extent that an amount is expected to be applied towards the clinical supply obligation, the analysis of variable consideration will be updated accordingly. The Company utilized the practical expedient relating to contract modifications that occurred prior to January 1, 2018. The practical expedient allows the Company to reflect the aggregate effect of all modifications that occurred before January 1, 2018 when identifying the satisfied and unsatisfied performance obligations, determining the transaction price and allocating the transaction price to the satisfied and unsatisfied performance obligations for the modified contract at transition. On October 31, 2017, Spark paid the Company a $2.5 million milestone payment pursuant to the Spark License Agreement, which was included in the transaction price and allocated to the performance obligations using the relative standalone selling price. In addition, Spark exercised the Second Acquisition Right set forth in Section 2.4 of the Spark Purchase Agreement and purchased 205,254 shares of common stock from the Company for $5.0 million , or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date. Skolkovo Foundation The Company receives grant funding from the Russia-based Development Fund of New Technologies Development and Commercialization Center ("Skolkovo"). On November 28, 2014, the Company executed a grant awarded by Skolkovo for the development of a therapeutic vaccine using nanoparticles to treat chronic infection caused by human papillomavirus (HPV) and diseases associated with this infection. The grant covered the period from August 1, 2014 through July 21, 2017. The grant provided for up to $2.7 million that covers 48.5% of the estimated total cost of the research plan with the remaining 51.5% of estimated costs to be contributed by the Company. From grant inception through December 31, 2018 , the Company received $2.0 million from Skolkovo. At any time during the term of the grant agreement, but not more than once per quarter, Skolkovo has the right to request information related to the project and to conduct an audit of the expenses incurred by the Company. In the event the project or the expenses do not meet predefined requirements, the Company may be required to reimburse the funds received up to three years after the completion of the project. As a result, in accordance with ASC 605, the Company determined that the grant funding was not fixed or determinable and the entire amount received through December 31, 2017 was recorded as deferred revenue in the consolidated balance sheet until the completion of the Skolkovo audit or the expiration of the audit term, which is expected April 2021. The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Skolkovo, is a customer. Furthermore, the Company determined that the agreement with Skolkovo has commercial substance, and that the grant should be accounted for as revenue. The Company identified the research and development services being provided to Skolkovo as the only performance obligation in the agreement. Based on the guidance in ASC 606, the Company concluded that the entire $2.0 million of grant funds received from Skolkovo is variable consideration. Although the Company believes it has an enforceable right to the amounts received, there is risk that an audit could result in the Company needing to refund certain amounts back to Skolkovo, resulting in variability in the transaction price. The Company utilized the “expected value” approach in determining the amount that can be recognized. The Company estimated that it will be entitled to revenue of $1.8 million from the Skolkovo grant, and recorded this amount as a cumulative-effect adjustment through accumulated deficit upon adoption of ASC 606. The remainder of $0.2 million was recorded as a contract liability upon adoption of ASC 606. During the year ended December 31, 2018 , the Company made a decision to cease work relating to the Skolkovo grant. As a result, Skolkovo performed a formal review of project expenses incurred by the Company. Skolkovo concluded that the Company should (i) return unused grant funds to Skolkovo in the amount of less than $0.1 million and (ii) reimburse $0.1 million of costs deemed to have been overspent relative to the cost share requirement stipulated in the grant. As of December 31, 2018 , a contract liability of $0.1 million remains on the balance sheet and will not be recognized as revenue until the expiration of the three-year audit period, or sooner, if resolution is reached with Skolkovo or there is a change in the estimate. National Institutes of Health On May 14, 2014, the National Institute on Drug Abuse (“NIDA”), part of the National Institutes of Health ("NIH"), provided the Company with a Notice of Award (“NoA”), for a grant relating to the preclinical development and clinical proof of concept for a synthetic nanoparticle (the “Project”). The grant will help advance the Company’s development of the nanoparticle from a preclinical through early clinical stage and will provide support for one or more clinical trials. Under the terms of the grant, NIDA agreed to provide up to $8.1 million in funding to be used to offset the qualifying expenses incurred by the Company. The grant was to be funded over the course of three years , according to budget forecasts prepared by the Company. In accordance with ASC 605, revenue associated with this grant was recognized as the related research and development work was performed, and expenses incurred. Revenue under this grant arrangement was recognized using a proportional performance method. Amounts received prior to satisfying the ASC 605 revenue recognition criteria (if any) was recorded as deferred revenue on the balance sheet. Through December 31, 2017, the Company received and recognized under ASC 605 approximately $7.2 million of grant revenue. The remaining $0.9 million available under the grant requires a change to the overhead rate, which is subject to final reconciliation and approval from NIDA and, therefore, was not recognized under ASC 605. The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, NIDA, is a customer. Furthermore, the Company determined that the agreement with NIDA has commercial substance, and that the grant should be accounted for as revenue. The Company identified the research and development services being provided to NIDA as the only performance obligation in the agreement. Upon adoption of ASC 606, the remaining $0.9 million was not recognized because the change to the overhead rate required approval from NIDA, and therefore, represents a contract modification. On December 19, 2018, the Company received supervisory review and approval of the overhead rate change from NIDA. During the fourth quarter of 2018, the Company recognized the full $0.9 million of grant revenue from NIDA. Impact of New Revenue Guidance on Financial Statement Line Items The following table compares the reported consolidated balance sheet and statement of operations, as of and for the year ended December 31, 2018 , to the pro-forma amounts had the previous guidance been in effect (in thousands): As of December 31, 2018 Consolidated Balance Sheet As reported Pro forma as if the previous accounting was in effect Assets Cash, cash equivalents, and restricted cash $ 37,403 $ 37,403 Prepaid expenses and other current assets 4,673 4,673 Total current assets 42,076 42,076 Property and equipment, net 2,127 2,127 Restricted cash and other assets 279 279 Total assets $ 44,482 $ 44,482 Liabilities and stockholders’ equity Current liabilities: Accounts payable $ 1,100 $ 1,100 Accrued expenses 11,700 11,700 Loan payable, current portion 21,385 21,385 Deferred revenue, current portion 959 959 Total current liabilities 35,144 35,144 Non‑current liabilities: Deferred rent and lease incentive 34 34 Deferred revenue, net of current portion 13,818 15,648 Other long‑term liabilities 904 904 Total liabilities 49,900 51,730 Stockholders’ equity: Common stock 3 3 Additional paid-in capital 279,539 279,539 Receivable from stock option exercises — — Accumulated deficit (280,403 ) (282,233 ) Accumulated other comprehensive loss (4,557 ) (4,557 ) Total stockholders’ equity (5,418 ) (7,248 ) Total liabilities and stockholders’ equity $ 44,482 $ 44,482 Total reported liabilities were $1.8 million less than the pro-forma balance sheet, which assumes the previous guidance remained in effect as of December 31, 2018 . The lower reported liabilities were due to reductions recognized to deferred revenue balances related to Skolkovo, which would remain deferred under ASC 605 as of December 31, 2018. There was no difference in the amounts reported in the statement of operations under ASC 606 during the twelve months ended December 31, 2018 , and the pro-forma statement of operations, which assumes the previous guidance remained in effect as of December 31, 2018 . Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed) and excludes unexercised contract options. As of December 31, 2018 , the aggregate amount of the transaction price allocated to remaining performance obligations was $8.6 million . The Company expects to recognize revenue on approximately 8.9% of the remaining performance obligations over the next 12 months. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract liabilities during the twelve months ended December 31, 2018 (in thousands): Balance at Balance at Beginning of Period Additions Deductions End of Period Year Ended December 31, 2018 Contract liabilities: Deferred revenue $ 14,876 $ 903 $ (1,002 ) $ 14,777 Other liabilities (i) 2,231 — (105 ) 2,126 Total contract liabilities $ 17,107 $ 903 $ (1,107 ) $ 16,903 (i) As of December 31, 2018 , the current portion of other liabilities in the amount of $1.2 million is presented in the accrued expenses line of the consolidated balance sheet. The non-current portion of other liabilities in the amount of $0.9 million is presented in the other long-term liabilities line of the consolidated balance sheet. Deferred revenue During the year ended December 31, 2018, the Company drew down $0.9 million of grant funds from NIDA and received the required written approval from NIDA on December 19, 2018, however, amounts were not deferred at the beginning of the period. During the year ended December 31, 2018 , the Company made a decision to cease work relating to the Skolkovo grant. As a result, Skolkovo performed a formal review of project expenses incurred by the Company. Skolkovo concluded that the Company should return unused grant funds to Skolkovo, which resulted in a reduction to deferred revenue of less than $0.1 million . Other liabilities In connection with its cost reimbursement arrangement with Spark, the Company received reimbursement invoices during the year ended December 31, 2018 amounting to approximately $0.1 million , which reduced the other liability balance to $2.1 million as of December 31, 2018 .

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related-Party Transactions	Related‑Party Transactions In connection with the 2017 PIPE, the Company sold to Timothy Springer, Ph.D., a member of its Board of Directors, an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017. In addition, the Company sold warrants to Dr. Springer to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years . The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. The Company incurred expenses for consulting services provided by its founders totaling $0.2 million , $0.2 million and $0.3 million during the years ended December 31, 2018 , 2017 and 2016

Technology License Agreements

Technology License Agreements	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Technology License Agreements	Technology License Agreements Massachusetts Institute of Technology On November 25, 2008, the Company entered into an Exclusive Patent License agreement with the Massachusetts Institute of Technology (“MIT”), which is referred to as the Exclusive Patent License. The Company received an exclusive royalty‑bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. In the event the Company sublicenses the MIT patents to a third party, it will be required to remit to MIT a percentage (ranging from 10% to 30% ) of sublicense income. In addition, the Company is obligated to pay MIT a certain amount upon the achievement of defined clinical milestones, up to a total of $1.5 million . On December 18, 2008, the Company entered into a patent‑cross‑license agreement with BIND Therapeutics, Inc. whereby each party receives a license for the use of the other patents in their respective fields of use. In exchange for this license, the Company paid a one‑time expense in 2008. As of December 31, 2018 , and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the Exclusive Patent License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The Company made no additional payments during the year ended December 31, 2018 . Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement with Shenyang Sunshine Pharmaceutical Co., Ltd. (“3SBio”), which is referred to as the 3SBio License. Pursuant to the 3SBio License, which was amended and restated in May 2017, the Company was granted an exclusive license to certain pegadricase (formerly pegsiticase)‑related patents and related “know‑how” owned or in‑licensed by 3SBio for the worldwide (except for Greater China and Japan) development and commercialization of products based thereupon for human therapeutic, diagnostic and prophylactic use. Pegadricase is the new United States Adopted Name (USAN) for pegsiticase. The Company was also granted a worldwide (except for Greater China) exclusive license to develop, commercialize and manufacture or have manufactured products combining the Company’s proprietary ImmTOR technology with pegadricase or related compounds supplied by 3SBio (or otherwise supplied if the Company’s rights to manufacture are in effect) for human therapeutic, diagnostic and prophylactic use. The Company was also granted a co‑exclusive license to manufacture and have manufactured pegadricase and related compounds for preclinical and clinical use or, if the 3SBio License is terminated for 3SBio’s material breach, for any use under the 3SBio License. Otherwise, the Company is obligated to obtain all of its supply of such compounds for Phase 3 clinical trials and commercial use from 3SBio under the terms of supply agreements to be negotiated. Pursuant to the 3SBio License, the Company is required to use commercially reasonable efforts to develop and commercialize a product containing pegadricase or a related compound. If the Company does not commercialize any such product in a particular country in Asia, Africa or South America within 48 months after approval of any such product in the United States or a major European country, then 3SBio will have the right to do so, but only until the Company commercializes a product combining the Company’s ImmTOR technology with any such compound in such country. The Company has paid to 3SBio an aggregate of $1.0 million in upfront and milestone‑based payments under the 3SBio License. An additional liability totaling $2.0 million for milestone payments was expensed in 2016 and was included within accounts payable on the balance sheet as of December 31, 2016. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $21.0 million for products containing the Company’s ImmTOR technology, and up to an aggregate of $41.5 million for products without the Company’s ImmTOR technology. The Company is also required to pay 3SBio tiered royalties on annual worldwide net sales (on a country‑by‑country and product‑by‑product basis) related to the pegadricase component of products at percentages ranging from the low‑to‑mid single digits for products containing the Company’s ImmTOR technology, and a range of no more than ten percent points from the mid‑single digits to low double‑digits for products without the Company’s ImmTOR technology. The Company will pay these royalties to 3SBio, subject to specified reductions, on a country‑by‑country and product‑by‑product basis until the later of (i) the date that all of the patent rights for that product have expired in that country, or (ii) a specified number of years from the first commercial sale of such product in such country. The 3SBio License expires on the date of expiration of all of the Company’s royalty payment obligations unless earlier terminated by either party for an uncured material default or for the other party’s bankruptcy. Any such termination by 3SBio for material default may be on a country‑by‑country or product‑by‑product basis in certain circumstances. The Company may also terminate the 3SBio License on a country‑by‑country or product‑by‑product basis for any reason effective upon 60 days’ prior written notice to 3SBio or, with respect to a given product, immediately upon written notice to 3SBio if the Company identifies a safety or efficacy concern related to such product. Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. In May 2016, the Company entered into a license agreement with the Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (collectively, “MEE”), which is referred to as the MEE License. Under the MEE License, the Company was granted an exclusive commercial worldwide license, with the right to grant sublicenses through multiple tiers, to make, have made, use, offer to sell, sell and import certain products and to practice certain processes, the sale, use or practice of which are covered by patents and proprietary know‑how owned or controlled by MEE, for use of Anc80 gene therapy vectors for gene augmentation therapies expressing certain target sequences. MEE also granted the Company exclusive options to exclusively license certain of their intellectual property rights relating to several additional target sequences and variations thereof each linked to a specified disease. During a defined option period, the Company may exercise this right for up to a designated number of target sequences. If the Company exercises its options, under certain circumstances, the Company may substitute alternative target sequences for previously selected target sequences. The Company agreed to use commercially reasonable efforts to develop and commercialize licensed products pursuant to a development plan, and to market and sell at least one product for each target sequence for which the Company exercised its option as soon as reasonably practicable. Subject to certain exceptions, following commercial launch, the Company must use commercially reasonable efforts to market, sell, and maintain public availability of licensed products in a certain number of specified major markets. Pursuant to the MEE License, the Company agreed to pay MEE a license fee in the low six figures, annual license maintenance fees ranging from the mid‑twenty thousands to mid‑seventy thousands and an option maintenance fee in the low five figures for each exercisable option. The Company also agreed to reimburse MEE for a specified percentage of the past patent expenses for the patents licensed to the Company. The Company also agreed to pay development milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $4.2 million to $37.0 million and sales milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $50.0 million to $70.0 million ; tiered royalties on a licensed product‑by‑licensed product and country‑by‑country basis equal to a percentage of net sales ranging from mid‑single digits to mid‑teens, subject to the prevalence of the targeted disease and certain reductions; and a percentage, in a range expected to be in the mid‑teens depending on timing, of any sublicense income the Company receives from sublicensing its rights granted thereunder, subject to certain reductions and exclusions. Upon exercise of each option, the Company agreed to pay MEE an option exercise fee ranging from low‑six figures to mid‑six figures, depending on the prevalence of the targeted disease. The MEE License will continue until the expiration of the last to expire of the patent rights licensed thereunder. The Company may terminate the MEE License in whole or in part upon prior written notice. MEE may terminate the MEE License on a target sequence‑by‑target sequence basis if the Company fails to make any scheduled payments in respect of such target sequence or if the Company materially breaches a diligence obligation in respect of such target sequence, in each case if the Company fails to cure within a specified time period. MEE may terminate the MEE License in its entirety if the Company materially breaches certain of its obligations related to diligence, representations and warranties, and maintenance of insurance; if the Company challenges the validity or enforceability of any patents licensed thereunder; if any of the Company’s executive officers are convicted of a felony relating to manufacture, use, sale or importation of licensed products; or upon the Company’s insolvency or bankruptcy. Through December 31, 2018 , the Company paid a total of $0.4 million in license fees due under the MEE License. No license fees are accrued under the MEE License as of December 31, 2018

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. For the years ended December 31, 2018 , 2017 and 2016 , the Company did not record a current or deferred income tax expense or benefit. The following table reconciles the federal statutory income rate to the Company's effective income tax rate: Year Ended December 31, 2018 2017 2016 Statutory U.S. federal rate 21.0 % 34.0 % 34.0 % State income taxes - net of federal benefit 7.0 % 5.8 % 5.8 % Permanent items (0.5 )% (0.1 )% (1.1 )% Research tax credits/other 1.6 % 0.5 % 1.3 % Change in enacted rates — % (35.8 )% — % Valuation allowance, net (29.1 )% (6.7 )% (39.8 )% Other — % 2.3 % (0.2 )% Effective income tax rate — % — % — % The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands): Year Ended December 31, 2018 2017 Deferred Tax Assets Net operating loss carryforwards $ 59,126 $ 44,146 Research and development credits 5,780 4,092 Stock-based compensations expense 1,946 959 Deferred rent and other expenses 458 263 Deferred revenue 4,037 4,021 Patent costs/amortization 4,881 4,264 Gross deferred tax assets 76,228 57,745 Deferred Tax Liabilities Depreciation $ (66 ) $ (108 ) Gross deferred tax liabilities (66 ) (108 ) Net deferred tax assets 76,162 57,637 Valuation allowance (76,162 ) (57,637 ) Net deferred tax assets $ — $ — The Company has provided a full valuation allowance against its net deferred tax assets, as the Company believes that it is more likely than not that the deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its deferred tax assets. The valuation allowance increased by $18.5 million and $4.7 million for the years ended December 31, 2018 and 2017, respectively, primarily as a result of an increase in net operating loss. In 2014, the Company's Russian subsidiary was granted a 10 year tax holiday in Russia. The Company's foreign operations continue to benefit from the tax holiday, which is set to expire on December 31, 2023, however the Company is in the process of closing down operations in Russia and does not expect any tax liability. At December 31, 2018, the Company had federal and state net operating loss carryforwards of $217.7 million and $212.1 million , respectively, which will expire at various times through 2038 . Of the federal net operating loss carryforwards, $55.2 million can be carried forward indefinitely. The Company also has federal and state research and development tax credit carryforwards of $3.6 million and $2.8 million , respectively, available to reduce future tax liabilities, which will expire at various times through 2038 . On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act ("Tax Reform Act"). The legislation significantly changes U.S. tax law by, among other things, lowering corporate income tax rates, implementing a territorial tax system, expanding the tax base and imposing a tax on deemed repatriated earnings of foreign subsidiaries. The Tax Reform Act permanently reduces the U.S. corporate federal income tax rate from a maximum of 35% to a flat 21% rate, effective January 1, 2018. The Company has recognized the impact of the Tax Reform Act in these consolidated financial statements and related disclosures. The impact of the remeasurement of the Company’s U.S. deferred tax assets and liabilities to 21% resulted in the reduction of deferred tax assets of approximately $23.4 million , which was offset by a full valuation allowance, thus there was no net effect. The Company recorded no tax expense related to the deemed repatriation tax because its foreign entity, Selecta (RUS) is a foreign disregarded entity, which is not subject to the repatriation tax. In December 2017, the SEC staff issued SAB 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of H.R.1. The Company did not record any adjustments in the year ended December 31, 2018 to provisional amounts. As of December 31, 2018, the Company’s accounting treatment is complete. Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 and 383 of the Internal Revenue Code due to ownership change limitations that have occurred previously, or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In January 2019, the Company completed an equity offering that it believes will result in an ownership change under Section 382 of the Internal Revenue Code. Based upon estimates, the Company believes that deferred tax assets of $49.5 million , related to federal and Massachusetts net operating losses and credits, will no longer be available going forward. Such amounts are offset by a full valuation allowance at December 31, 2018. The Company will finalize this calculation in 2019. The Company applies ASC 740, Income Taxes to uncertain tax positions. As of the adoption date on January 1, 2010 and through December 31, 2018, the Company had no unrecognized tax benefits or related interest and penalties accrued. The Company has not, as of yet, conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the consolidated balance sheets, statements of operations and comprehensive loss, or cash flows if an adjustment was required. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations. As of December 31, 2018, the Company had no accrued interest related to uncertain tax positions.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan The Company maintains a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaled $0.2 million , $0.2 million and $0.1 million during each of the years ended December 31, 2018 , 2017 and 2016

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Corporate Restructuring On December 21, 2018, the Board of Directors (the “Board”) of the Company approved a plan to reduce Selecta’s budgeted headcount by approximately 36% (the “Workforce Reduction”) following a strategic review of the Company’s business. The Workforce Reduction aimed to align Selecta’s workforce with the Company’s recently announced strategy to focus on the development of the Company’s lead product candidate, SEL-212, for the treatment of chronic refractory gout, and advancement of the Company’s ImmTOR technology in the area of gene therapy, specifically ImmTOR in combination with AAV gene therapy for the treatment of Crigler-Najjar Syndrome (“CN”), as well as the deprioritization of the Company’s oncology development program. While the Workforce Reduction generally affected employees in all areas of Selecta, primarily affected were those working in research and related general and accounting functions. The Workforce Reduction resulted in the termination of approximately 17 employment positions effective January 3, 2019, and the affected employees were notified on the same date. Since the Company does not have a pre-existing severance plan, the restructuring activity will be accounted for under ASC 420 and since the employees were notified after year end and exited within the minimum retention period, a charge totaling approximately $0.5 million will be recorded in the first quarter. Following the Workforce Reduction, Selecta expects to have approximately 45 full-time employment positions and to be appropriately resourced to continue executing on its current strategy. In addition, the Company decided to vacate certain office space in Watertown, MA on January 11, 2019 and consolidated all employees at its corporate headquarters also in Watertown, MA. The lease term expires in March 2020. Public Offering On January 23, 2019, the Company entered into an underwriting agreement (the “Underwriting Agreement”) with Stifel, Nicolaus & Company, Incorporated, as representative of the several underwriters named therein (collectively, the “Underwriters”), in connection with the public offering, issuance and sale by the Company of 20,000,000 shares of the Company’s common stock, $ 0.0001 par value per share, at a public offering price of $ 1.50 per share, less underwriting discounts and commissions, pursuant to an effective shelf registration statement on Form S-3 (Registration No. 333-219900) and a related prospectus supplement filed with the Securities and Exchange Commission. Under the terms of the Underwriting Agreement, the Company has also granted the Underwriters an option exercisable for 30 days to purchase up to an additional 3,000,000 shares of its common stock at the public offering price, less underwriting discounts and commissions. On January 25, 2019, the Company closed the offering of 20,000,000 shares at a public offering price of $ 1.50 per share. Effective January 29, 2019, an additional 2,188,706 shares were sold at a public offering price of $ 1.50 per share. The total net proceeds from the offering was $ 31.3 million , after deducting underwriting discounts and commissions.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited) The following table summarizes unaudited quarterly financial data for the years ended December 31, 2018 and 2017 (in thousands, except per share data). Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Grant and collaboration revenue $ — $ — $ — $ 903 Operating expenses $ 15,813 $ 18,769 $ 15,941 $ 15,402 Net loss $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss attributable to common stockholders $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.71 ) $ (0.84 ) $ (0.71 ) $ (0.65 ) Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Grant and collaboration revenue $ 137 $ 26 $ 27 $ 17 Operating expenses $ 14,919 $ 15,897 $ 13,881 $ 19,294 Net loss $ (15,134 ) $ (15,967 ) $ (14,676 ) $ (19,544 ) Net loss attributable to common stockholders $ (15,134 ) $ (15,967 ) $ (14,676 ) $ (19,544 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.82 ) $ (0.85 ) $ (0.66 ) $ (0.88 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.
Foreign Currency	Foreign CurrencyThe functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one
Cash Equivalents and Short Term Investments	Cash Equivalents, Short-term Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the reporting periods, there were no
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off‑Balance Sheet RiskFinancial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, short-term investments, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At December 31, 2018 , the carrying amount of the Company's loan payable approximates its estimated fair value due to the short-term nature of the instrument. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 3 inputs.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets The Company reviews long‑lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level ("asset group"). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Based on management's evaluation, the fair value of the asset group, measured as the market capitalization of the Company exceeds its carrying value, and for this reason the Company did no t recognize an impairment loss during the twelve months ended December 31, 2018 and 2017
Debt Issuance Costs	Debt Issuance CostsDebt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss)Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on available-for-sale securities.
Revenue Recognition	Revenue Recognition The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Prior to January 1, 2018, the Company recognized revenue in accordance with ASC Topic 605, Revenue Recognition ("ASC 605"). Under ASC 605, the Company recognized revenue when all of the following criteria were met: - Persuasive evidence of an arrangement exists; - Delivery has occurred or services have been rendered; - The seller’s price to the buyer is fixed or determinable; and - Collectability is reasonably assured. The Company enters into agreements that contain multiple deliverables. Prior to the adoption of ASC 606, when evaluating multiple element arrangements, the Company considered whether the deliverables under the arrangement represent separate units of accounting. In determining the units of accounting, management evaluated certain criteria, including whether the deliverables had standalone value. The consideration received was allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria was applied to each of the separate units. If deliverables had no standalone value, they were grouped together and revenue was recognized over the estimated performance period. Prior to the adoption of ASC 606, the Company determined the estimated selling price for deliverables within each agreement using vendor‑specific objective evidence (“VSOE”) of selling price, if available, third‑party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price if neither VSOE nor TPE was available. Since the Company is a clinical stage company without a marketable product the Company used its best estimate of selling price to estimate the selling price for licenses and deliverables related to the Company’s proprietary technology. Prior to the adoption of ASC 606, options for future deliverables were considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. When an option was considered substantive and there is no significant incremental discount, the option is not considered a deliverable in the arrangement and no consideration is allocated to it. Conversely, when an option was not considered substantive or it was considered substantive but was priced at an incremental discount, it would have been considered a deliverable at the inception of the arrangement and a corresponding amount would be included in the initial consideration. The Company may receive upfront payments when licensing its intellectual property in conjunction with a manufacturing or a research and development agreement. Prior to the adoption of ASC 606, when management believed the license to its intellectual property did not have stand‑alone value from the other deliverables to be provided in the arrangement, the Company generally recognized revenue attributed to the license over the Company’s contractual or estimated performance period. When management believed the license to its intellectual property had stand‑alone value, the Company generally recognized revenue attributed to the license upon delivery. Prior to the adoption of ASC 606, at the inception of each agreement that included milestone payments, the Company evaluated whether each milestone is substantive and at risk to both parties based on the contingent nature of the milestone. Revenues from milestones, if they were nonrefundable and deemed substantive, were recognized upon successful accomplishment of the milestones. Milestones that were not considered substantive were accounted for as license payments and recognized over the remaining period of performance. Under ASC 605, amounts received prior to satisfying the revenue recognition criteria were recognized as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date were classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date were classified as deferred revenue, net of current portion. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The Company adopted ASC 606, Revenue from Contracts with Customers , on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 guidance while the reported results for 2017 were prepared under the guidance of ASC 605, which is also referred to herein as "legacy GAAP" or the "previous guidance". Refer to Note 13 to its consolidated financial statements included elsewhere in this Annual Report on Form 10-K for the impact of adoption of ASU 2014-09 on the Company’s consolidated financial position, results of operations, equity and cash flows as of the adoption date and for the year ended December 31, 2018. Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark (see Note 13 to the Company's consolidated financial statements included elsewhere in this Annual Report on Form 10-K) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. The grants, collaboration and license agreements are within the scope of ASC 606. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. Licenses of intellectual property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties:
Research and Development Costs	Research and Development CostsCosts incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs	Clinical Trial CostsClinical trial expenses are a significant component of research and development expenses, and we outsource a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. We also record accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, we analyze progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs.
Income Taxes	Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized.
Warrants	Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity , and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock . Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares.
Stock-Based Compensation	Stock‑Based CompensationThe Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation awarded to employees is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Prior to the adoption of ASU 2018-07 during the second quarter of 2018 described below in "Recent Accounting Pronouncements", stock‑based compensation awarded to non‑employees was subject to revaluation over its vesting terms. Subsequent to the adoption of ASU 2018-07, non-employee share-based payment awards are measured on the date of grant, similar to share-based payment awards granted to employees. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.
Net Loss Per Share	Net Loss Per ShareThe Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.
Deferred Rent	Deferred RentRent expense and lease incentives from operating leases are recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments is recorded as deferred rent in the accompanying consolidated balance sheets.
Contingent Liabilities	Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies . A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of December 31, 2018 and December 31, 2017
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers ("ASU 2014-09"). ASU 2014-09 and its related amendments (collectively referred to as ASC 606) requires that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The new guidance requires the application of a five-step model to determine the amount and timing of revenue to be recognized. The underlying principle is that revenue is to be recognized for the transfer of goods or services to customers that reflects the amount of consideration that the Company expects to be entitled to in exchange for those goods or services. The Company adopted the standard as of the required effective date of January 1, 2018 using the modified retrospective method. See Note 13 for details. In January 2016, the Financial Accounting Standards Board ("FASB") issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”) . ASU 2016-01 supersedes the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and requires equity securities (including other ownership interests, such as partnerships, unincorporated joint ventures, and limited liability companies) to be measured at fair value with changes in fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU 2016-01 is effective for annual reporting beginning after December 15, 2017, including interim periods within the year of adoption, and calls for prospective application, with early application permitted. The Company adopted ASU 2016-01 during the first quarter of 2018. Because the Company held no equity investments upon adoption, there was no impact on its consolidated financial statements. In February 2016, the FASB issued Accounting Standards Update No. (“ASU”) 2016-02, “Leases (Topic 842).” ASU 2016-02 requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability. Leases are classified as either operating or finance based on criteria similar to current lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations. Subsequently, in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (“ASU 2018-11”), which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company has elected this transition approach, using a cumulative-effect adjustment on the effective date of the standard, with comparative periods presented in accordance with the existing guidance in ASC 840. We expect to adopt the new standard on January 1, 2019 and use the effective date as our date of initial application. Pursuant to the guidance under ASU 2016-02, the Company expects to take advantage of certain available expedients by electing the transition package of practical expedients permitted with ASU 2016-02, which allows the Company the option not to reassess previous accounting conclusions around, (i) whether expired or existing contracts contain leases, (ii) lease classification for any expired or existing leases, and (iii) the treatment of initial direct costs for any existing leases. The Company also made an accounting policy election to exclude leases with an initial term of 12 months or less from their balance sheet. The adoption of the new standard is expected to result in the recognition of a net additional lease liability up to $2.0 million , and right-of-use assets up to $2.0 million as of January 1, 2019 related to the Company’s operating leases. We further expect to provide enhanced new disclosures about our leasing arrangements. The Company does not expect that the new standard will have a material impact on the Company’s consolidated statement of operations or cash flows. In August 2016, the FASB issued ASU No. 2016-15, Statements of Cash Flows (Topic 230) - Classifications of Certain Cash Receipts and Cash Payments ("ASU 2016-15"), to clarify how companies present and classify certain cash receipts and cash payments in the statement of cash flows. This guidance is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The adoption of ASU 2016-15 did not have a retrospective impact because it classified cash payments for debt prepayment and debt extinguishment costs as cash outflows for financing activities for the year end 2017. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows, Restricted Cash ("ASU 2016-18"). This guidance requires that a statement of cash flows explain the total change during the period of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Amounts described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period to total amounts shown on the statement of cash flows. The Company adopted ASU 2016-18 during the first quarter ended March 31, 2018. The adoption of ASU 2016-18 resulted in the Company's cash, cash equivalents and restricted cash being included in the beginning and ending amounts for the periods shown on the statement of cash flows and was applied retroactively and reflected in the balances presented for any prior periods. The Company believes that the adoption of this guidance did not have a significant impact on its consolidated financial statements. In January 2017, the FASB issued ASU No. 2017-01, “Business Combinations (Topic 805): Clarifying the Definition of a Business” (ASU 2017-01), which clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This ASU is effective for public entities for fiscal years beginning after December 15, 2017, with early adoption permitted. The adoption of ASU 2017-01 did not have a material impact on the Company's consolidated financial statements. Effective January 1, 2018, the Company adopted ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting ("ASU 2017-09"). ASU 2017-09 specifies the types of changes to the terms or conditions of a share-based payment award that require an entity to apply modification accounting in accordance with Topic 718. The adoption of ASU 2017-09 did not have a material impact on the Company's consolidated financial statements. In June 2018, the FASB issued ASU No. 2018-07, “Improvements to Nonemployee Share-Based Payment Accounting" (ASU 2018-07), to simplify the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. This ASU is effective for public entities for fiscal years beginning after December 15, 2018, with early adoption permitted. The Company early adopted ASU 2018-07 during the second quarter of 2018. Prior to the adoption of ASU 2018-07, stock‑based compensation awarded to non‑employees was subject to revaluation over its vesting terms. Subsequent to the adoption of ASU 2018-07, non-employee share-based payment awards are measured on the date of grant, similar to share-based payment awards granted to employees. The adoption of ASU 2018-07 did not have a material impact on the Company's consolidated financial statements. In November 2018, the FASB issued ASU No. 2018-18, “Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606”

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows: Year Ended December 31, 2018 2017 2016 Cash and cash equivalents $ 37,403 $ 70,622 $ 58,656 Restricted cash — 76 78 Restricted cash included in other assets 279 329 316 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 37,682 $ 71,027 $ 59,050
Schedule of Restricted Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows: Year Ended December 31, 2018 2017 2016 Cash and cash equivalents $ 37,403 $ 70,622 $ 58,656 Restricted cash — 76 78 Restricted cash included in other assets 279 329 316 Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows $ 37,682 $ 71,027 $ 59,050
Components of Accumulated Other Comprehensive Income (Loss), Net of Tax	The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands): Foreign currency translation adjustment Unrealized gains (losses) on available-for-sale securities Accumulated other comprehensive income (loss) Balance at December 31, 2015 $ (4,986 ) $ — $ (4,986 ) Other comprehensive income (loss) during the year 504 (36 ) 468 Balance at December 31, 2016 $ (4,482 ) $ (36 ) $ (4,518 ) Other comprehensive income during the year 78 20 98 Balance at December 31, 2017 $ (4,404 ) $ (16 ) $ (4,420 ) Other comprehensive income (loss) during the period (153 ) 16 (137 ) Balance at December 31, 2018 $ (4,557 ) $ — $ (4,557 )

Available-for-Sale Marketable_2

Available-for-Sale Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Marketable Securities	The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of December 31, 2017 (in thousands): December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. government and agency securities $ 12,798 $ — $ (10 ) $ 12,788 Corporate bonds 13,158 — (6 ) 13,152 Total available-for-sale marketable securities $ 25,956 $ — $ (16 ) $ 25,940
Schedule of Fair Values and Amortized Cost of Marketable Debt Securities by Contractual Maturity	The fair values and amortized cost of marketable debt securities by contractual maturity were as follows (in thousands): December 31, 2017 Fair Value Amortized Cost Less than one year $ 25,940 $ 25,956

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data): Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (65,336 ) $ (65,321 ) $ (36,210 ) Less: accretion on preferred stock — — (4,566 ) Net loss $ (65,336 ) $ (65,321 ) $ (40,776 ) Denominator: Weighted‑average common shares outstanding—basic and diluted 22,389,286 20,425,050 10,493,939 Net loss per share —basic and diluted $ (2.92 ) $ (3.20 ) $ (3.89 )
Schedule of Potential Common Shares Issuable Upon Conversion of Warrants	Potential dilutive common share equivalents consist of the following: Year Ended December 31, 2018 2017 2016 Stock options to purchase common stock 4,093,979 2,657,187 2,128,346 Unvested restricted stock units 175,000 — — Stock warrants to purchase common stock 95,619 176,432 97,302 Total 4,364,598 2,833,619 2,225,648

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Assets Measured at Fair Value on a Recurring Basis	Below is a summary of assets measured at fair value on a recurring basis (in thousands): December 31, 2018 (Level 1) (Level 2) (Level 3) Total Cash equivalents: Money market funds $ 10,123 $ — $ — $ 10,123 Total cash equivalents $ 10,123 $ — $ — $ 10,123 December 31, 2017 (Level 1) (Level 2) (Level 3) Total Cash equivalents: Money market funds $ 39,478 $ — $ — $ 39,478 U.S. government and agency securities — 1,000 — 1,000 Total cash equivalents $ 39,478 $ 1,000 $ — $ 40,478 Short-term investments: Corporate bonds $ 13,152 $ — $ — $ 13,152 U.S. government and agency securities — 12,788 — 12,788 Total short-term investments $ 13,152 $ 12,788 $ — $ 25,940

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): December 31, December 31, 2018 2017 Laboratory equipment $ 5,379 $ 5,138 Computer equipment and software 561 480 Leasehold improvements 278 278 Furniture and fixtures 247 235 Office equipment 135 118 Construction in process 79 — Total property and equipment 6,679 6,249 Less accumulated depreciation (4,552 ) (4,158 ) Property and equipment, net $ 2,127 $ 2,091

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, December 31, 2018 2017 Payroll and employee related expenses $ 2,497 $ 1,902 Current portion of deferred rent and lease incentive 117 72 Collaboration and licensing 1,222 1,020 Accrued patent fees 736 268 Accrued external research and development costs 5,344 3,578 Accrued professional and consulting services 994 859 Accrued grant refund 175 175 Accrued interest 106 89 Other 509 617 Accrued expenses $ 11,700 $ 8,580

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	As of December 31, 2018 , the aggregate future minimum lease payments related to these leases are as follows (in thousands): Year ending December 31, 2019 1,482 2020 375 Total minimum lease payments $ 1,857

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments on the Term Loans	Future minimum principal and interest payments on the 2017 Term Loan as of December 31, 2018 are as follows (in thousands): 2019 4,742 2020 9,232 2021 8,718 2022 1,754 Total minimum debt payments $ 24,446 Less: Amount representing interest (2,396 ) Less: Debt discount and deferred charges (665 ) Less: Current portion of loan payable (21,385 ) Loan payable, net of current portion $ —

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as follows: Period ending December 31, 2018 December 31, 2017 Exercise of common warrants 95,619 176,432 Shares available for future stock incentive awards 1,586,925 923,440 Unvested restricted stock units 175,000 — Outstanding common stock options 4,093,979 2,657,187 Total 5,951,523 3,757,059

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of employee stock option awards granted under the 2008 Plan, 2016 Plan and the Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.85 % 2.03 % 1.42 % Dividend yield — — — Expected term 6.06 5.90 6.04 Expected volatility 85.17 % 84.52 % 94.97 % Weighted-average fair value of common stock $ 8.45 $ 15.32 $ 13.25 Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.77 % — % 2.06 % Dividend yield — — — Expected term 5.81 0.00 8.80 Expected volatility 85.86 % — % 87.33 %
Summary of Activity Under the 2008 Plan and the 2016 Plan	The following table summarizes the activity under the 2008 Plan, 2016 Plan, and Inducement Incentive Award Plan: Weighted‑average remaining Aggregate Number of Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Employee Awards Outstanding at December 31, 2017 2,411,237 $ 10.58 7.50 $ 4,729 Granted 1,625,679 $ 8.45 Exercised (103,784 ) $ 4.85 Forfeited (251,557 ) $ 15.03 Outstanding at December 31, 2018 3,681,575 $ 9.49 7.77 $ 300 Vested at December 31, 2018 1,484,196 $ 9.01 5.83 $ 300 Vested and expected to vest at December 31, 2018 2,990,612 $ 9.27 7.49 $ 300 Non‑Employee Awards Outstanding at December 31, 2017 245,950 $ 4.32 5.10 $ 1,351 Granted 170,000 $ 9.38 Exercised — $ — Forfeited (3,546 ) $ 0.39 Outstanding at December 31, 2018 412,404 $ 6.44 6.42 $ 28 Vested at December 31, 2018 223,180 $ 4.15 3.91 $ 28 Vested and expected to vest at December 31, 2018 412,404 $ 6.44 6.42 $ 28
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table summarizes the status of the Company’s restricted stock units: Number of Shares (#) Weighted Average Fair Value ($) Unvested at December 31, 2017 — $ — Granted 215,000 7.28 Vested — — Forfeited (40,000 ) 12.75 Unvested at December 31, 2018 175,000 $ 6.03
Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year Ended December 31, 2018 2017 2016 Research and development $ 2,453 $ 1,779 $ 1,183 General and administrative 3,261 2,302 847 Total stock-based compensation expense $ 5,714 $ 4,081 $ 2,030

Revenue Arrangements (Tables)

Revenue Arrangements (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles [Table Text Block]	As a result of applying the modified retrospective method to adopt the new revenue guidance, the following adjustments were made to the Company's accounts on the consolidated balance sheet as of January 1, 2018 (in thousands): Adjustments As Reported at Due to Balance at Consolidated Balance Sheet December 31, 2017 ASU 2014-09 January 1, 2018 Assets Cash, cash equivalents, and restricted cash $ 70,698 $ — $ 70,698 Short-term deposits and investments 25,940 — 25,940 Prepaid expenses and other current assets 2,042 — 2,042 Total current assets 98,680 — 98,680 Property and equipment, net 2,091 — 2,091 Restricted cash and other assets 329 — 329 Total assets $ 101,100 $ — $ 101,100 Liabilities and stockholders’ equity Current liabilities: Accounts payable $ 1,606 $ — $ 1,606 Accrued expenses 8,580 — 8,580 Deferred revenue, current portion 787 — 787 Total current liabilities 10,973 — 10,973 Non‑current liabilities: Deferred rent and lease incentive 151 — 151 Loan payable 21,042 — 21,042 Deferred revenue, net of current portion 15,919 (1,830 ) 14,089 Other long‑term liabilities 1,201 — 1,201 Total liabilities 49,286 (1,830 ) 47,456 Stockholders’ equity: Common stock 3 — 3 Additional paid-in capital 273,128 — 273,128 Accumulated deficit (216,897 ) 1,830 (215,067 ) Accumulated other comprehensive loss (4,420 ) — (4,420 ) Total stockholders’ equity 51,814 1,830 53,644 Total liabilities and stockholders’ equity $ 101,100 $ — $ 101,100 December 31, 2018 , to the pro-forma amounts had the previous guidance been in effect (in thousands): As of December 31, 2018 Consolidated Balance Sheet As reported Pro forma as if the previous accounting was in effect Assets Cash, cash equivalents, and restricted cash $ 37,403 $ 37,403 Prepaid expenses and other current assets 4,673 4,673 Total current assets 42,076 42,076 Property and equipment, net 2,127 2,127 Restricted cash and other assets 279 279 Total assets $ 44,482 $ 44,482 Liabilities and stockholders’ equity Current liabilities: Accounts payable $ 1,100 $ 1,100 Accrued expenses 11,700 11,700 Loan payable, current portion 21,385 21,385 Deferred revenue, current portion 959 959 Total current liabilities 35,144 35,144 Non‑current liabilities: Deferred rent and lease incentive 34 34 Deferred revenue, net of current portion 13,818 15,648 Other long‑term liabilities 904 904 Total liabilities 49,900 51,730 Stockholders’ equity: Common stock 3 3 Additional paid-in capital 279,539 279,539 Receivable from stock option exercises — — Accumulated deficit (280,403 ) (282,233 ) Accumulated other comprehensive loss (4,557 ) (4,557 ) Total stockholders’ equity (5,418 ) (7,248 ) Total liabilities and stockholders’ equity $ 44,482 $ 44,482
Schedule of Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the twelve months ended December 31, 2018 (in thousands): Balance at Balance at Beginning of Period Additions Deductions End of Period Year Ended December 31, 2018 Contract liabilities: Deferred revenue $ 14,876 $ 903 $ (1,002 ) $ 14,777 Other liabilities (i) 2,231 — (105 ) 2,126 Total contract liabilities $ 17,107 $ 903 $ (1,107 ) $ 16,903 (i) As of December 31, 2018 , the current portion of other liabilities in the amount of $1.2 million is presented in the accrued expenses line of the consolidated balance sheet. The non-current portion of other liabilities in the amount of $0.9 million

Income Taxes - (Tables)

Income Taxes - (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	The following table reconciles the federal statutory income rate to the Company's effective income tax rate: Year Ended December 31, 2018 2017 2016 Statutory U.S. federal rate 21.0 % 34.0 % 34.0 % State income taxes - net of federal benefit 7.0 % 5.8 % 5.8 % Permanent items (0.5 )% (0.1 )% (1.1 )% Research tax credits/other 1.6 % 0.5 % 1.3 % Change in enacted rates — % (35.8 )% — % Valuation allowance, net (29.1 )% (6.7 )% (39.8 )% Other — % 2.3 % (0.2 )% Effective income tax rate — % — % — %
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]	The tax effects of temporary differences that give rise to the Company's net deferred tax assets are as follows (in thousands): Year Ended December 31, 2018 2017 Deferred Tax Assets Net operating loss carryforwards $ 59,126 $ 44,146 Research and development credits 5,780 4,092 Stock-based compensations expense 1,946 959 Deferred rent and other expenses 458 263 Deferred revenue 4,037 4,021 Patent costs/amortization 4,881 4,264 Gross deferred tax assets 76,228 57,745 Deferred Tax Liabilities Depreciation $ (66 ) $ (108 ) Gross deferred tax liabilities (66 ) (108 ) Net deferred tax assets 76,162 57,637 Valuation allowance (76,162 ) (57,637 ) Net deferred tax assets $ — $ —

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table summarizes unaudited quarterly financial data for the years ended December 31, 2018 and 2017 (in thousands, except per share data). Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Grant and collaboration revenue $ — $ — $ — $ 903 Operating expenses $ 15,813 $ 18,769 $ 15,941 $ 15,402 Net loss $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss attributable to common stockholders $ (15,888 ) $ (18,796 ) $ (16,001 ) $ (14,651 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.71 ) $ (0.84 ) $ (0.71 ) $ (0.65 ) Three Months Ended (unaudited) March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Grant and collaboration revenue $ 137 $ 26 $ 27 $ 17 Operating expenses $ 14,919 $ 15,897 $ 13,881 $ 19,294 Net loss $ (15,134 ) $ (15,967 ) $ (14,676 ) $ (19,544 ) Net loss attributable to common stockholders $ (15,134 ) $ (15,967 ) $ (14,676 ) $ (19,544 ) Net loss per share attributable to common stockholders, basic and diluted $ (0.82 ) $ (0.85 ) $ (0.66 ) $ (0.88 )

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Jan. 29, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit		$ 280,403	$ 216,897		$ 215,067
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents		37,403	70,622	$ 58,656
Proceeds from issuance of common stock		0	$ 10,000	$ 2,743
Russian subsidiary | Unrestricted cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents		$ 400
Subsequent Event
Cash and Cash Equivalents [Line Items]
Proceeds from issuance of common stock	$ 31,300

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Segment Information (Details)	12 Months Ended
	Dec. 31, 2018segment
Accounting Policies [Abstract]
Number of operating segments	1

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Cash Equivalents, Short Term Investments and Restricted Cash (Details)	12 Months Ended
	Dec. 31, 2018USD ($)
Accounting Policies [Abstract]
Realized gains or losses on sale of investments	$ 0

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 37,403,000	$ 70,622,000	$ 58,656,000
Restricted cash	0	76,000	78,000
Restricted cash included in other assets	279,000	329,000	316,000
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 37,682,000	$ 71,027,000	$ 59,050,000	$ 32,785,000

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) - Russian subsidiary $ in Millions	Dec. 31, 2018USD ($)
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 0.4
U.S. dollars
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 0.4

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Laboratory and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Impairment of Long‑Lived Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Impairment of intangible assets, finite-lived	$ 0	$ 0

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss), Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ 51,814	$ 54,957	$ (116,493)
Other comprehensive loss during the year	(137)
Ending balance	(5,418)	51,814	54,957
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,404)	(4,482)	(4,986)
Other comprehensive loss during the year	(153)	78	504
Ending balance	(4,557)	(4,404)	(4,482)
Unrealized gains (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	(16)	(36)	0
Other comprehensive loss during the year	16	20	(36)
Ending balance	0	(16)	(36)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,420)	(4,518)	(4,986)
Other comprehensive loss during the year		98	468
Ending balance	$ (4,557)	$ (4,420)	$ (4,518)

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - Subsequent Event - Scenario, Forecast - ASU 2016-02 $ in Millions	Jan. 01, 2019USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liabilities	$ 2
Lease, right-of-use asset	$ 2

Available-for-Sale Marketable_3

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) $ in Thousands	Dec. 31, 2017USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 25,956
Gross Unrealized Gains	0
Gross Unrealized Losses	(16)
Fair Value	25,940
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,798
Gross Unrealized Gains	0
Gross Unrealized Losses	(10)
Fair Value	12,788
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,158
Gross Unrealized Gains	0
Gross Unrealized Losses	(6)
Fair Value	$ 13,152

Available-for-Sale Marketable_4

Available-for-Sale Marketable Securities - Schedule of Fair Values and Amortized Cost of Marketable Debt Securities by Contractual Maturity (Details) $ in Thousands	Dec. 31, 2017USD ($)
Fair Value
Less than one year	$ 25,940
Amortized Cost
Less than one year	$ 25,956

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (14,651)	$ (16,001)	$ (18,796)	$ (15,888)	$ (19,544)	$ (14,676)	$ (15,967)	$ (15,134)	$ (65,336)	$ (65,321)	$ (36,210)
Less: accretion on preferred stock									0	0	4,566
Net (loss) income attributable to common stockholders - basic & diluted	$ (14,651)	$ (16,001)	$ (18,796)	$ (15,888)	$ (19,544)	$ (14,676)	$ (15,967)	$ (15,134)	$ (65,336)	$ (65,321)	$ (40,776)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)									22,389,286	20,425,050	10,493,939
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.65)	$ (0.71)	$ (0.84)	$ (0.71)	$ (0.88)	$ (0.66)	$ (0.85)	$ (0.82)	$ (2.92)	$ (3.20)	$ (3.89)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Potential common shares
Total (in shares)	4,364,598	2,833,619	2,225,648
Stock options to purchase common stock
Potential common shares
Total (in shares)	4,093,979	2,657,187	2,128,346
Unvested restricted stock units
Potential common shares
Total (in shares)	175,000	0	0
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	95,619	176,432	97,302

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 10,123	$ 40,478
Total short-term investments		25,940
(Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	10,123	39,478
Total short-term investments		13,152
(Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	1,000
Total short-term investments		12,788
(Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Total short-term investments		0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		13,152
Corporate bonds | (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		13,152
Corporate bonds | (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		0
Corporate bonds | (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		0
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		12,788
U.S. government and agency securities | (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		0
U.S. government and agency securities | (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		12,788
U.S. government and agency securities | (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total short-term investments		0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	10,123	39,478
Money market funds | (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	10,123	39,478
Money market funds | (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Money market funds | (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 0	0
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		1,000
U.S. government and agency securities | (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		0
U.S. government and agency securities | (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		1,000
U.S. government and agency securities | (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents		$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	12 Months Ended
	Dec. 31, 2017
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity, term	179 days

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 6,679		$ 6,249
Less accumulated depreciation	(4,552)		(4,158)
Property and equipment, net	2,127	$ 2,091	2,091
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	5,379		5,138
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	561		480
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	278		278
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	247		235
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	135		118
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 79		$ 0

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 975	$ 750	$ 802

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 2,497		$ 1,902
Current portion of deferred rent and lease incentive	117		72
Collaboration and licensing	1,222		1,020
Accrued patent fees	736		268
Accrued external research and development costs	5,344		3,578
Accrued professional and consulting services	994		859
Accrued grant refund	175		175
Accrued interest	106		89
Other	509		617
Accrued expenses	$ 11,700	$ 8,580	$ 8,580

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2012USD ($)	Oct. 31, 2017ft²
Operating Leased Assets [Line Items]
Area of additional office space leased | ft²					5,100
Deferred rent and lease incentive liability	$ 200,000	$ 200,000
Current portion of deferred rent and lease incentive liability (prior year less than)	117,000	72,000
Rent expense	2,000,000	1,900,000	$ 1,600,000
Accrued expenses
Operating Leased Assets [Line Items]
Current portion of deferred rent and lease incentive liability (prior year less than)	100,000	$ 100,000
Letter of credit | Restricted cash and other assets
Operating Leased Assets [Line Items]
Letter of credit	$ 300,000
Maximum | Laboratory and office space
Operating Leased Assets [Line Items]
Tenant improvement allowance				$ 700,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Year ending December 31,
2019	$ 1,482
2020	375
Total minimum lease payments	$ 1,857

Debt - Term Loans (Narrative) (

Debt - Term Loans (Narrative) (Details) - USD ($)	Sep. 12, 2017	Dec. 31, 2015	Dec. 31, 2016	Jul. 31, 2014	Aug. 31, 2013	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 21, 2016	Aug. 09, 2013
Line of Credit Facility [Line Items]
Borrowings on loan and security agreement								$ 0	$ 20,957,000	$ 0
Loss on extinguishment of debt								0	673,000	0
Minimum assets trigger								300,000
Minimum amount in excess of indebtedness trigger								300,000
Interest expense related to the Term Loan								$ 1,494,000	1,206,000	1,253,000
Conversion of Series D Preferred
Line of Credit Facility [Line Items]
Expiration period of warrants								10 years
Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)		37,978
Expiration period of warrants		10 years
Conversion of Series E Preferred | Convertible Debt
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)		$ 11.32
Term Loans
Line of Credit Facility [Line Items]
Borrowing capacity		$ 12,000,000										$ 7,500,000
Borrowings on loan and security agreement				$ 4,500,000	$ 3,000,000
Interest rate per annum								8.10%
Number of monthly installments								30 months
Final payment fee (as a percent)								6.00%
Final payment fee						$ 700,000		$ 700,000
Initial grant date fair value of warrants		$ 100,000			$ 100,000
Number of warrants exercised to purchase common stock (in shares)			16,493
Net shares of common stock issued (in shares)			4,697
Repayments of debt	$ 10,000,000
Term Loans | Conversion of Series D Preferred | Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)											17,888
Term Loans | Conversion of Series D Preferred | Convertible Debt | Common stock
Line of Credit Facility [Line Items]
Exercise price (in dollars per share)											$ 16.77
Term Loans | Conversion of Series D Preferred | Maximum
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)				40,000	26,668						66,668
Term Loans | Conversion of Series E Preferred | Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares)		15,094
Term Loans | Prepaid after to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)						2.00%		2.00%
Term Loans | Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)						1.00%		1.00%
2017 Term Loans
Line of Credit Facility [Line Items]
Interest rate per annum								4.00%
Final payment fee (as a percent)								5.00%
Final payment fee						$ 1,100,000		$ 1,100,000
Term loan facility	21,000,000					21,000,000	$ 21,000,000	$ 21,000,000	$ 21,000,000
Loss on extinguishment of debt							700,000
Debt issuance costs, line of credit arrangements (less than)	$ 100,000
Interest expense related to the Term Loan						$ 1,500,000	$ 1,200,000			$ 1,300,000
2017 Term Loans | Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.05%
2017 Term Loans | Prepaid before the first anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)						3.00%		3.00%
2017 Term Loans | Prepaid after to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)						2.00%		2.00%
2017 Term Loans | Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)						1.00%		1.00%

Debt - Schedule of Future Minim

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Dec. 31, 2018USD ($)
Year ended December 31,
2019	$ 4,742
2020	9,232
2021	8,718
2022	1,754
Total minimum debt payments	24,446
Less: Amount representing interest	(2,396)
Less: Debt discount and deferred charges	(665)
Less: Current portion of loan payable	(21,385)
Loan payable, net of current portion	$ 0

Preferred Stock (Details)

Preferred Stock (Details) - $ / shares	Jun. 27, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Temporary Equity Disclosure [Abstract]
Authorized shares of Preferred Stock (in shares)				37,835,623
Preferred Stock, par value (in dollars per share)				$ 0.0001
Preferred stock
Preferred stock, shares outstanding		0	0
Preferred stock, shares issued		0	0
Preferred stock automatically converted into common stock upon the closing of the IPO
Preferred stock
Convertible preferred stock automatically converted into common stock (in shares)	10,126,118
Series A Redeemable Convertible Preferred Stock
Preferred stock
Preferred stock, shares outstanding				2,589,868
Preferred stock, shares issued				2,589,868
Series B Preferred
Preferred stock
Preferred stock, shares outstanding				7,437,325
Preferred stock, shares issued				7,437,325
Series C Redeemable Convertible Preferred Stock
Preferred stock
Preferred stock, shares outstanding				5,000,002
Preferred stock, shares issued				5,000,002
Series D Preferred Stock [Member]
Preferred stock
Preferred stock, shares outstanding				8,099,994
Preferred stock, shares issued				8,099,994
Series SRN Redeemable Convertible Preferred Stock
Preferred stock
Preferred stock, shares outstanding				2,111,109
Preferred stock, shares issued				2,111,109
Series E Preferred Stock [Member]
Preferred stock
Preferred stock, shares outstanding				8,888,888
Preferred stock, shares issued				8,888,888

Common Stock - Narrative (Detai

Common Stock - Narrative (Details)	Aug. 10, 2017USD ($)	Jun. 27, 2017USD ($)$ / sharesshares	Jul. 25, 2016USD ($)$ / sharesshares	Jun. 21, 2016USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)	Jun. 27, 2016shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares					22,471,776
Common stock, par value (in dollars per share) | $ / shares					$ 0.0001	$ 0.0001
Proceeds from issuance of common stock | $					$ 0	$ 10,000,000	$ 2,743,000
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions | $			$ 64,500,000		$ 0	$ 0	$ 68,870,000
Convertible preferred stock, shares issued upon conversion (in shares) | shares								10,126,118
Common stock, shares authorized (in shares) | shares					200,000,000	200,000,000
Number of votes per share that common stockholders are entitled to					1
Dividends declared or paid on common stock | $					$ 0
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Public securities offering, amount authorized | $	$ 200,000,000
Public stock offering, amount authorized | $					$ 50,000,000
Sale of stock, number of shares issued in transaction (in shares) | shares					0
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares		2,750,000
Common stock, par value (in dollars per share) | $ / shares		$ 0.0001
Share price (in dollars per share) | $ / shares		$ 16
Proceeds from issuance of common stock | $		$ 47,100,000
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares		$ 17.71
Springer Purchase Agreement | Common stock
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares) | shares		338,791
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares		79,130
Exercise price (in dollars per share) | $ / shares		$ 17.71
Expiration period of warrants		5 years
Share price (in dollars per share) | $ / shares		$ 0.125
IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares			289,633	5,000,000
Share price (in dollars per share) | $ / shares				$ 14
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions | $				$ 60,800,000
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares			$ 14
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions | $			$ 3,700,000

Common Stock - Schedule of Auth

Common Stock - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total (in shares)	5,951,523	3,757,059
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	1,586,925	923,440
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	175,000	0
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	4,093,979	2,657,187
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	95,619	176,432

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 20, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 01, 2018	Dec. 31, 2008
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		5,951,523				5,951,523	3,757,059
Stock‑based compensation expense						$ 5,714	$ 4,081	$ 2,051
Shares issued, value								$ 64,465
Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		1,586,925				1,586,925	923,440
Estimated forfeitures rate						10.00%
Weighted average grant date fair value of stock options (in dollars per share)						$ 6.17	$ 10.97	$ 10.23
Aggregate intrinsic value of stock options exercised		$ 600				$ 600	$ 2,600	$ 800
Unrecognized compensation expense related to unvested employee stock options		$ 9,800				$ 9,800	$ 10,200
Weighted average period for recognition						3 years 1 month 6 days	2 years 10 months 24 days
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested stock option awards (in shares)		55,073				55,073
Vested and expected to vest (in shares)									84,604
Vested and expected to vest, exercisable (in shares)									310,219
Unrecognized compensation expense related to unvested non-employee stock options		$ 1,100				$ 1,100	$ 600
Unrecognized compensation expense related to unvested non-employee stock options, period						2 years 2 months 12 days	1 year 10 months 24 days
Granted, net of forfeitures (in shares)		75,000		95,000
Granted (in dollars per share)		$ 4.35		$ 8.71
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		175,000				175,000	0
Stock‑based compensation expense			$ 100
Shares issued (in dollars per share)		$ 6.03				$ 6.03	$ 0
The 2008 Plan and the 2016 Plan | Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares exercises in period by employees (in shares)						103,784
Aggregate intrinsic value of stock options exercised		$ 300				$ 300
Forfeited (in shares)						251,557
Vested and expected to vest (in shares)		2,990,612				2,990,612
Granted (in shares)						1,625,679
Granted (in dollars per share)						$ 8.45
The 2008 Plan and the 2016 Plan | Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares exercises in period by employees (in shares)						0
Aggregate intrinsic value of stock options exercised		$ 28				$ 28
Forfeited (in shares)						3,546
Vested and expected to vest (in shares)		412,404				412,404
Granted (in shares)						170,000	0
Granted (in dollars per share)						$ 9.38
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)										2,213,412
2008 Plan | Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term						4 years
Award expiration term						10 years
2008 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)						10.00%
2008 Plan | Shares available for future stock incentive awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)						10.00%
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		1,947,779				1,947,779
Automatic increase in the number of shares that may be issued (as a percent)						0.04
Number of shares authorized, increase (in shares)						893,730	737,550
Common stock authorized and reserved for future issuance (in shares)		1,586,925				1,586,925
2016 Plan | Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term						4 years
Award expiration term						10 years
2016 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)						10.00%
2016 Plan | Shares available for future stock incentive awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)						10.00%
Employment Inducement Incentive Award Plan | Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognition						3 years 10 months 24 days
Shares issued (in shares)		175,000
Shares issued (in dollars per share)		$ 6.03				$ 6.03
Shares issued, value		$ 1,100
Unrecognized compensation expense		$ 1,000				$ 1,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)	85.00%
Automatic increase in the number of shares that may be issued (as a percent)	0.01
Number of shares authorized, increase (in shares)						223,432	184,387
Common stock authorized and reserved for future issuance (in shares)	173,076	539,894				539,894
Stock‑based compensation expense		$ 100					$ 100
Shares issued (in shares)						24,738
Tranche One | 2008 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)						100.00%
Tranche One | 2016 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)						100.00%
Tranche Two | 2008 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)						110.00%
Tranche Two | 2016 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)						110.00%
Chief Executive Officer | The 2008 Plan and the 2016 Plan | Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited (in shares)							62,546
Vested and expected to vest (in shares)							126,457
Granted (in shares)					20,000

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Non-employee stock options
Share-based Goods and Nonemployee Services Transaction [Abstract]
Risk-free interest rate	2.77%	0.00%	2.06%
Dividend yield	0.00%	0.00%	0.00%
Expected term	5 years 9 months 21 days	0 days	8 years 9 months 18 days
Expected volatility	85.86%	0.00%	87.33%
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.85%	2.03%	1.42%
Dividend yield	0.00%	0.00%	0.00%
Expected term	6 years 21 days	5 years 10 months 24 days	6 years 14 days
Expected volatility	85.17%	84.52%	94.97%
Weighted-average fair value of common stock (in dollars per share)	$ 8.45	$ 15.32	$ 13.25

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 01, 2018	Dec. 31, 2016
Employee Awards
Aggregate intrinsic value
Vested and expected to vest	$ 600		$ 600	$ 2,600		$ 800
Non‑Employee Awards
Number of Options
Vested and expected to vest (in shares)					84,604
Weighted-average exercise price
Granted (in dollars per share)	$ 4.35	$ 8.71
The 2008 Plan and the 2016 Plan | Employee Awards
Number of Options
Beginning balance (in shares)			2,411,237
Granted (in shares)			1,625,679
Exercised (in shares)			(103,784)
Forfeited (in shares)			(251,557)
Ending balance (in shares)	3,681,575		3,681,575	2,411,237
Vested (in shares)			1,484,196
Vested and expected to vest (in shares)	2,990,612		2,990,612
Weighted-average exercise price
Beginning balance (in dollars per share)			$ 10.58
Granted (in dollars per share)			8.45
Exercised (in dollars per share)			4.85
Forfeited (in dollars per share)			15.03
Ending balance (in dollars per share)	$ 9.49		9.49	$ 10.58
Vested (in dollars per share)			9.01
Vested and expected to vest (in dollars per share)	$ 9.27		$ 9.27
Weighted-average remaining contractual term
Outstanding, term			7 years 9 months 7 days	7 years 6 months
Vested, term			5 years 9 months 29 days
Vested and expected to vest, term			7 years 5 months 26 days
Aggregate intrinsic value
Outstanding	$ 300		$ 300	$ 4,729
Vested	300		300
Vested and expected to vest	$ 300		$ 300
The 2008 Plan and the 2016 Plan | Non‑Employee Awards
Number of Options
Beginning balance (in shares)			245,950
Granted (in shares)			170,000	0
Exercised (in shares)			0
Forfeited (in shares)			(3,546)
Ending balance (in shares)	412,404		412,404	245,950
Vested (in shares)			223,180
Vested and expected to vest (in shares)	412,404		412,404
Weighted-average exercise price
Beginning balance (in dollars per share)			$ 4.32
Granted (in dollars per share)			9.38
Exercised (in dollars per share)			0
Forfeited (in dollars per share)			0.39
Ending balance (in dollars per share)	$ 6.44		6.44	$ 4.32
Vested (in dollars per share)			4.15
Vested and expected to vest (in dollars per share)	$ 6.44		$ 6.44
Weighted-average remaining contractual term
Outstanding, term			6 years 5 months 1 day	5 years 1 month 6 days
Vested, term			3 years 10 months 28 days
Vested and expected to vest, term			6 years 5 months 1 day
Aggregate intrinsic value
Outstanding	$ 28		$ 28	$ 1,351
Vested	28		28
Vested and expected to vest	$ 28		$ 28

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Stock‑based compensation expense			$ 5,714	$ 4,081	$ 2,051
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Shares outstanding at December 31, 2017			0
Grants in period (in shares)		40,000	215,000
Vested in period (in shares)			0
Forfeited in period (in shares)			(40,000)
Shares outstanding at December 31, 2018			175,000	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Shares outstanding at December 31, 2017 (in dollars per share)			$ 0
Grants in period (in dollars per share)		$ 12.75	7.28
Vested in period (in dollars per share)			0
Forfeited in period (in dollars per share)			12.75
Shares outstanding at December 31, 2018 (in dollars per share)			$ 6.03	$ 0
Stock‑based compensation expense	$ 100
Grants in period, amount		$ 500

Stock Incentive Plans - Employe

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) $ in Thousands	Jun. 20, 2016offering_periodshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		5,951,523	5,951,523	3,757,059
Stock-based compensation expense | $			$ 5,714	$ 4,081	$ 2,051
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of offering periods under the ESPP | offering_period	2
Term of each offering period	6 months
Eligible compensation withheld for use (up to) (as a percent)	25.00%
Purchase price of shares (not less than) (as a percent)	85.00%
Common stock authorized and reserved for future issuance (in shares)	173,076	539,894	539,894
Number of shares of common stock outstanding (as a percent)	0.01
Number of shares authorized, increase (in shares)			223,432	184,387
Shares of common stock issued to employees under the ESPP (in shares)			24,738
Stock-based compensation expense | $		$ 100		$ 100

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 5,714	$ 4,081	$ 2,051
Stock Options And Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	5,714	4,081	2,030
Stock Options And Restricted Common Stock [Member] | Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,453	1,779	1,183
Stock Options And Restricted Common Stock [Member] | General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 3,261	$ 2,302	$ 847

Stock Incentive Plans - Employm

Stock Incentive Plans - Employment Inducement Incentive Award Plan (Details) - shares	Dec. 31, 2018	Dec. 01, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	5,951,523		3,757,059
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	175,000		0
President | Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	1,175,000	1,000,000
Number of shares authorized for grants (in shares)	0
President | Employment Inducement Incentive Award Plan | Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		175,000

Revenue Arrangements - Narrativ

Revenue Arrangements - Narrative (Details)	Oct. 31, 2017USD ($)$ / sharesshares	Jun. 06, 2017USD ($)$ / sharesshares	Dec. 02, 2016USD ($)contracttargetobligation	Nov. 28, 2014USD ($)	Dec. 31, 2016USD ($)$ / sharesshares	May 31, 2014USD ($)	Dec. 31, 2018USD ($)agreement	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)shares	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)agreementshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)$ / shares	Mar. 31, 2018USD ($)	Jan. 01, 2018USD ($)agreement	Nov. 25, 2008
Disaggregation of Revenue [Line Items]
Number of collaboration/grant agreements that required analysis to quantify the impact of adoption | agreement																			3
Number of additional targets | target			4
Sale of stock, number of shares issued in transaction (in shares) | shares															22,471,776
Deferred revenue related to agreement							$ 16,903,000				$ 17,107,000				$ 16,903,000	$ 17,107,000
Deferred revenue, current portion							959,000				787,000				959,000	787,000			$ 787,000
Deferred revenue, net of current portion							13,818,000				15,919,000				13,818,000	15,919,000			14,089,000
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights																10,000,000	$ 2,743,000
Remaining performance obligation							8,600,000								8,600,000
Current assets							(42,076,000)				(98,680,000)				(42,076,000)	(98,680,000)			(98,680,000)
Accumulated deficit							280,403,000				216,897,000				280,403,000	216,897,000			215,067,000
Revenue recognized															1,002,000
Payments received															0	0	(461,000)
Adoption of new accounting principle																			1,830,000
Grant and collaboration revenue							903,000	$ 0	$ 0	$ 0	17,000	$ 27,000	$ 26,000	$ 137,000	903,000	207,000	$ 8,083,000
Reimbursement invoices															100,000
Other liabilities							2,100,000								2,100,000
Spark Letter Agreement
Disaggregation of Revenue [Line Items]
Sale common stock	$ 2,500,000
Premium of average daily VWAP (as a percent)	115.00%
Deferred revenue related to agreement	$ 2,500,000
Aggregate purchase price		$ 5,000,000
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) | shares	205,254	324,362
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights			$ 2,700,000
Aggregate purchase price per share (in dollar per share) | $ / shares	$ 24.36	$ 15.41
First Acquisition Right
Disaggregation of Revenue [Line Items]
Aggregate purchase price	$ 5,000,000
Skolkovo
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement							100,000								100,000				200,000
Revenue recognized															1,800,000
Aggregate amount of grant				$ 2,700,000
Percentage of estimated total cost of research plan				48.50%
Remaining percentage of estimated costs to be contributed by the Company				51.50%
Payments received																		$ 2,000,000
Grants from entities, unused grant funds									100,000						100,000
Overspent costs									$ 100,000
Period after completion of project when Company may be required to reimburse the funds received				3 years
Second Acquisition Right
Disaggregation of Revenue [Line Items]
Share price (in dollars per share) | $ / shares	$ 24.36
Premium of average daily VWAP (as a percent)	115.00%
Consecutive calendar days leading up to and ending on the day prior to the Contract Date	30 days
Aggregate price of shares Spark will have the right to purchase	$ 5,000,000
Spark License Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment			10,000,000
Aggregate additional payments due within twelve months			$ 5,000,000
Term loan number of payments | contract			2
Two payments due within 12 months of contract date			$ 2,500,000
Number of additional targets | target			4
Variable fee the Company is eligible to receive for each additional target option elected			$ 2,000,000
Election period to exercise additional targets			3 years
Additional development and regulatory milestone payments			$ 65,000,000
Commercialization milestone payments			365,000,000
Sales milestone payments			$ 255,000,000
Period after first commercial sale when the Company is eligible to receive royalties			10 years
Period for prior written notice to terminate license			90 days
Sale of stock, number of shares issued in transaction (in shares) | shares					197,238
Sale common stock					$ 5,000,000
Share price (in dollars per share) | $ / shares					$ 25.35												$ 25.35
Premium of average daily VWAP (as a percent)					115.00%												115.00%
Deferred revenue related to agreement							14,700,000				14,700,000				14,700,000	14,700,000
Deferred revenue, current portion							1,000,000				800,000				1,000,000	800,000
Deferred revenue, net of current portion							$ 13,700,000				$ 13,900,000				$ 13,700,000	$ 13,900,000
Consecutive calendar days leading up to and ending on the day prior to the Contract Date					30 days
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) | shares											2,758,112					2,758,112
Number of agreements combined and evaluated into a single agreement | agreement							2								2
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights			$ 2,700,000
Remaining performance obligation			12,300,000				$ 2,300,000								$ 2,300,000
Performance obligation per each option			1,300,000
Incremental costs of obtaining a contract			2,600,000
Spark License Agreement | MIT
Disaggregation of Revenue [Line Items]
License and option agreement, additional sub-license payment received liability																$ 2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement															400,000	400,000
Spark License Agreement | Consideration for Promises
Disaggregation of Revenue [Line Items]
Upfront cash payment			15,000,000
Spark License Agreement | Development Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments			35,000,000
Spark License Agreement | Regulatory Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments			30,000,000
Spark License Agreement | Commercial Milestones
Disaggregation of Revenue [Line Items]
Commercialization milestone payments			$ 110,000,000
Spark License Agreement | License and Supply Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation			5
Remaining performance obligation			$ 7,100,000
Spark License Agreement | Discount on Option Obligation
Disaggregation of Revenue [Line Items]
Number of obligations | obligation			4
Remaining performance obligation			$ 5,200,000
First Grant | NIH
Disaggregation of Revenue [Line Items]
Aggregate amount of grant						$ 8,100,000
Period after completion of project when Company may be required to reimburse the funds received						3 years
Remaining amount not recognized						$ 900,000
Accumulated deficit
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle																			1,830,000
Accumulated deficit | ASU 2014-09 | Skolkovo
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle																			$ 1,800,000
Grant Revenue | NIDA
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue							900,000
Grant Revenue | First Grant | NIH
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue																$ 7,200,000
Restatement Adjustment | Error Recognized Upon the Initial Adoption of ASC 606
Disaggregation of Revenue [Line Items]
Current assets							200,000								200,000
Long-term assets							2,400,000								2,400,000
Accumulated deficit							$ 2,600,000								$ 2,600,000
Minimum | MIT
Disaggregation of Revenue [Line Items]
Percentage of sublicense income																				10.00%
Maximum | MIT
Disaggregation of Revenue [Line Items]
Percentage of sublicense income																				30.00%

Revenue Arrangements - Schedule

Revenue Arrangements - Schedule of Impact of New Accounting Pronouncements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Cash, cash equivalents, and restricted cash	$ 37,403	$ 70,698	$ 70,698
Short-term deposits and investments	0	25,940	25,940
Prepaid expenses and other current assets	4,673	2,042	2,042
Total current assets	42,076	98,680	98,680
Property and equipment, net	2,127	2,091	2,091
Restricted cash and other assets	279	329	329
Total assets	44,482	101,100	101,100
Current liabilities:
Accounts payable	1,100	1,606	1,606
Accrued expenses	11,700	8,580	8,580
Loan payable, current portion	21,385		0
Deferred revenue, current portion	959	787	787
Total current liabilities	35,144	10,973	10,973
Deferred rent and lease incentive	34	151	151
Loan payable, net of current portion	0	21,042	21,042
Deferred revenue, net of current portion	13,818	14,089	15,919
Other long‑term liabilities	904	1,201	1,201
Total liabilities	49,900	47,456	49,286
Stockholders’ (deficit) equity:
Common stock	3	3	3
Additional paid-in capital	279,539	273,128	273,128
Receivable from stock option exercises	0
Accumulated deficit	(280,403)	(215,067)	(216,897)
Accumulated other comprehensive loss	(4,557)	(4,420)	(4,420)
Total stockholders’ (deficit) equity	(5,418)	53,644	51,814	$ 54,957	$ (116,493)
Total liabilities and stockholders’ equity	44,482	101,100	$ 101,100
Pro forma as if the previous accounting was in effect
Assets
Cash, cash equivalents, and restricted cash	37,403
Prepaid expenses and other current assets	4,673
Total current assets	42,076
Property and equipment, net	2,127
Restricted cash and other assets	279
Total assets	44,482
Current liabilities:
Accounts payable	1,100
Accrued expenses	11,700
Loan payable, current portion	21,385
Deferred revenue, current portion	959
Total current liabilities	35,144
Deferred rent and lease incentive	34
Deferred revenue, net of current portion	15,648
Other long‑term liabilities	904
Total liabilities	51,730
Stockholders’ (deficit) equity:
Common stock	3
Additional paid-in capital	279,539
Receivable from stock option exercises	0
Accumulated deficit	(282,233)
Accumulated other comprehensive loss	(4,557)
Total stockholders’ (deficit) equity	(7,248)
Total liabilities and stockholders’ equity	$ 44,482
Adjustments due to ASU 2014-09 | ASU 2014-09
Assets
Cash, cash equivalents, and restricted cash		0
Short-term deposits and investments		0
Prepaid expenses and other current assets		0
Total current assets		0
Property and equipment, net		0
Restricted cash and other assets		0
Total assets		0
Current liabilities:
Accounts payable		0
Accrued expenses		0
Deferred revenue, current portion		0
Total current liabilities		0
Deferred rent and lease incentive		0
Loan payable, net of current portion		0
Deferred revenue, net of current portion		(1,830)
Other long‑term liabilities		0
Total liabilities		(1,830)
Stockholders’ (deficit) equity:
Common stock		0
Additional paid-in capital		0
Accumulated deficit		1,830
Accumulated other comprehensive loss		0
Total stockholders’ (deficit) equity		1,830
Total liabilities and stockholders’ equity		$ 0

Revenue Arrangements - Transact

Revenue Arrangements - Transaction Price Allocated to Future Performance Obligation (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 02, 2016
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 8.6
Remaining performance obligation, percentage	8.90%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	12 months
Spark License Agreement
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 2.3	$ 12.3
Spark License Agreement | License and Supply Obligation
Revenue from Contract with Customer [Abstract]
Remaining performance obligation		$ 7.1
Spark License Agreement | License and Supply Obligation | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2016-12-03
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period		4 years

Revenue Arrangements - Schedu_2

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018
Contract liabilities:
Deferred revenue, beginning of period	$ 14,876
Other liabilities, beginning of period	2,231
Contract liabilities, beginning of period	17,107
Deferred revenue, additions	903
Other liabilities, additions	0
Contract liabilities, additions	903
Deferred revenue, deductions	(1,002)
Other liabilities, deductions	(105)
Contract liabilities, deductions	(1,107)
Deferred revenue, end of period	14,777
Other liabilities, end of period	2,231	$ 2,126
Contract liabilities, end of period	$ 16,903
Other liabilities, current portion		1,200
Other liabilities, noncurrent portion		$ 900

Related-Party Transactions (Det

Related-Party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions	Jun. 27, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		22,471,776
Founders
Related Party Transaction [Line Items]
Consulting services expenses		$ 0.2	$ 0.2	$ 0.3
Springer Purchase Agreement
Related Party Transaction [Line Items]
Share price (in dollars per share)	$ 17.71
Springer Purchase Agreement | Common stock
Related Party Transaction [Line Items]
Stock issued (in shares)	338,791
Stock warrants to purchase common stock
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	79,130
Exercise price (in dollars per share)	$ 17.71
Expiration period of warrants	5 years
Share price (in dollars per share)	$ 0.125

Technology License Agreements -

Technology License Agreements - MIT (Narrative) (Details) - USD ($)	Nov. 25, 2008	Dec. 31, 2018	Dec. 31, 2017
Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments defined in the Exclusive Patent License agreement		$ 0
MIT
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment amount upon achievement of defined clinical milestones	$ 1,500,000
MIT | Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments defined in the Exclusive Patent License agreement		2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement		$ 400,000	$ 400,000
MIT | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	10.00%
MIT | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	30.00%

Technology License Agreements_2

Technology License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License - USD ($) $ in Millions	1 Months Ended
	May 31, 2014	Dec. 31, 2018	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period after approval to commercialize product	48 months
Aggregate amount of upfront and milestone-based payments		$ 1
Additional liability for milestone payments expensed in 2016			$ 2
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing SVP technology	$ 21
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products without SVP technology	$ 41.5
Maximum paid on tiered royalties (percent)	10.00%
Period for prior written notice to terminate license	60 days

Technology License Agreements_3

Technology License Agreements - Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (Narrative) (Details) - MEE	1 Months Ended	12 Months Ended
	May 31, 2016USD ($)product	Dec. 31, 2018USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fees due under agreement		$ 400,000
Accrued license fees		$ 0
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Minimum number of products for each target sequence | product	1
Payment under development milestone method	$ 4,200,000
Payment under sales milestone method	50,000,000
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment under development milestone method	37,000,000
Payment under sales milestone method	$ 70,000,000

Income Taxes (Details)

Income Taxes (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Jan. 22, 2019
Income Tax Holiday [Line Items]
Valuation allowance, deferred tax asset, increase (decrease), amount			$ 18,500,000	$ 4,700,000
Current income tax expense or benefit	$ 0	$ 0	0	0
Deferred income tax expense or benefit	0	0	0	0
Term of tax holiday granted to Russian subsidiary					10 years
Increase in net operating losses, deferred tax assets	59,126,000	44,146,000	59,126,000	44,146,000
Research and development credits	5,780,000	4,092,000	5,780,000	4,092,000
Tax Cuts and Jobs Act, reduction of deferred tax assets			23,400,000
Full valuation allowance			23,400,000
Unrecognized tax benefits	0		0
Related interest and penalties accrued	0		0
Accrued interest related to uncertain tax positions		$ 0		$ 0
Domestic Tax Authority
Income Tax Holiday [Line Items]
Operating loss carryforwards	217,700,000		217,700,000
Operating loss carryforwards, carried forward indefinitely	55,200,000		55,200,000
Research and development credits	3,600,000		3,600,000
State and Local Jurisdiction
Income Tax Holiday [Line Items]
Operating loss carryforwards	212,100,000		212,100,000
Research and development credits	$ 2,800,000		$ 2,800,000
Scenario, Forecast | Domestic And State Tax Authority
Income Tax Holiday [Line Items]
Operating loss carryforwards, subject to expiration						$ 49,500,000

Income Taxes Reconciliation of

Income Taxes Reconciliation of Income Taxes (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory U.S. federal rate	21.00%	34.00%	34.00%
State income taxes - net of federal benefit	7.00%	5.80%	5.80%
Permanent items	(0.50%)	(0.10%)	(1.10%)
Research tax credits/other	1.60%	0.50%	1.30%
Change in enacted rates	0.00%	(35.80%)	0.00%
Valuation allowance, net	(29.10%)	(6.70%)	(39.80%)
Other	0.00%	2.30%	(0.20%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes Deferred Tax Asset

Income Taxes Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets
Net operating loss carryforwards	$ 59,126	$ 44,146
Research and development credits	5,780	4,092
Stock-based compensations expense	1,946	959
Deferred rent and other expenses	458	263
Deferred revenue	4,037	4,021
Patent costs/amortization	4,881	4,264
Gross deferred tax assets	76,228	57,745
Deferred Tax Liabilities
Depreciation	(66)	(108)
Gross deferred tax liabilities	(66)	(108)
Net deferred tax assets	76,162	57,637
Valuation allowance	(76,162)	(57,637)
Net deferred tax assets	$ 0	$ 0

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Vesting period		4 years
Employer contribution made	$ 0.1	$ 0.2	$ 0.2	$ 0.1

Subsequent Events (Details)

Subsequent Events (Details) $ / shares in Units, $ in Thousands	Jan. 29, 2019USD ($)shares	Jan. 25, 2019$ / sharesshares	Mar. 31, 2019USD ($)position	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)	Jan. 03, 2019position	Dec. 21, 2018	Jul. 25, 2016$ / shares
Subsequent Event [Line Items]
Workforce reduction, percentage								36.00%
Common stock, shares issued | shares				22,471,776
Common stock, par value (in dollars per share)				$ 0.0001	$ 0.0001
Proceeds from issuance of common stock | $				$ 0	$ 10,000	$ 2,743
Over-Allotment Option
Subsequent Event [Line Items]
Share price (in dollars per share)									$ 14
Subsequent Event
Subsequent Event [Line Items]
Number of employment positions terminated | position							17
Number of employment positions | position			45
Proceeds from issuance of common stock | $	$ 31,300
Subsequent Event | Public Stock Offering
Subsequent Event [Line Items]
Common stock, shares issued | shares	2,188,706	20,000,000
Common stock, par value (in dollars per share)		$ 0.0001
Share price (in dollars per share)		$ 1.50
Subsequent Event | Over-Allotment Option
Subsequent Event [Line Items]
Common stock, shares issued | shares		3,000,000
Sale Of Stock, Granted Exercisable Share Option, Time Period To Purchase		30 days
Subsequent Event | One-time Termination Benefits
Subsequent Event [Line Items]
Payments for restructuring | $			$ 500

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Grant and collaboration revenue	$ 903	$ 0	$ 0	$ 0	$ 17	$ 27	$ 26	$ 137	$ 903	$ 207	$ 8,083
Operating expenses	15,402	15,941	18,769	15,813	19,294	13,881	15,897	14,919	65,925	63,991	42,753
Net loss	(14,651)	(16,001)	(18,796)	(15,888)	(19,544)	(14,676)	(15,967)	(15,134)	(65,336)	(65,321)	(36,210)
Net (loss) income attributable to common stockholders - basic & diluted	$ (14,651)	$ (16,001)	$ (18,796)	$ (15,888)	$ (19,544)	$ (14,676)	$ (15,967)	$ (15,134)	$ (65,336)	$ (65,321)	$ (40,776)
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.65)	$ (0.71)	$ (0.84)	$ (0.71)	$ (0.88)	$ (0.66)	$ (0.85)	$ (0.82)	$ (2.92)	$ (3.20)	$ (3.89)