Cartesian Therapeutics (RNAC) 10-Q 10 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 10, 2016
Document and Entity Information
Document Type	10-Q
Amendment Flag	false
Document and Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q3
Entity Registrant Name	SELECTA BIOSCIENCES INC
Entity Central Index Key	1,453,687
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		18,190,180

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 59,488	$ 32,337
Short term deposits and investments	19,788	4,125
Restricted cash	335	133
Accounts receivable	483	824
Prepaid expenses and other current assets	3,303	1,494
Total current assets	83,397	38,913
Property and equipment, net	2,066	2,029
Restricted cash and other deposits	316	316
Other assets	0	1,566
Total assets	85,779	42,824
Current liabilities:
Accounts payable	1,040	2,179
Accrued expenses	2,434	3,378
Loans payable, current portion	2,913	0
Deferred revenue, current portion	1,041	1,313
Contingently repayable grant funding	262	420
Total current liabilities	7,690	7,290
Non‑current liabilities:
Deferred rent and lease incentive	234	105
Loans payable, net of current portion	9,064	11,855
Deferred revenue, net of current portion	3,348	2,295
Other long‑term liabilities	0	290
Total liabilities	20,336	21,835
Commitments and contingencies (Notes 8 and 13)
Total redeemable convertible preferred stock	0	137,482
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value; 10,000,000 and 0 shares authorized; 0 shares issued and outstanding at September 30, 2016 and December 31, 2015, respectively.	0	0
Common stock, $0.0001 par value; 200,000,000 and 62,164,377 shares authorized at September 30, 2016 and December 31, 2015 respectively; 18,190,180 and 2,180,976 shares issued, 18,188,313 and 2,173,399 shares outstanding as of September 30, 2016 and December 31, 2015, respectively.	1	0
Additional paid-in capital	207,489	1
Accumulated deficit	(137,493)	(111,508)
Accumulated other comprehensive loss	(4,554)	(4,986)
Total stockholders’ equity (deficit)	65,443	(116,493)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity	85,779	42,824
Series A redeemable convertible preferred stock, $0.0001 par value; 0 and 2,589,868 shares authorized; 0 and 2,589,868 shares issued and outstanding; as of September 30, 2016 and December 31, 2015 respectively.
Total redeemable convertible preferred stock	0	3,644
Series B redeemable convertible preferred stock, $0.0001 par value; 0 and 7,437,325 shares authorized; 0 and 7,437,325 shares issued and outstanding; as of September 30, 2016 and December 31, 2015 respectively.
Total redeemable convertible preferred stock	0	21,448
Series C redeemable convertible preferred stock, $0.0001 par value; 0 and 5,000,002 shares authorized; 0 and 5,000,002 shares issued and outstanding; as of September 30, 2016 and December 31, 2015 respectively.
Total redeemable convertible preferred stock	0	20,178
Series D redeemable convertible preferred stock, $0.0001 par value; 0 and 8,166,662 shares authorized; 0 and 8,099,994 shares issued and outstanding; as of September 30, 2016 and December 31, 2015 respectively.
Total redeemable convertible preferred stock	0	42,902
Series SRN redeemable convertible preferred stock, $0.0001 par value; 0 and 5,611,112 shares authorized; 0 and 2,111,109 shares issued and outstanding; as of September 30, 2016 and December 31, 2015 respectively.
Total redeemable convertible preferred stock	0	12,082
Series E redeemable convertible preferred stock, $0.0001 par value; 0 and 9,030,654 shares authorized; 0 and 8,888,888 shares issued and outstanding; as of September 30, 2016 and December 31, 2015 respectively.
Total redeemable convertible preferred stock	$ 0	$ 37,228

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
Convertible preferred stock, par value (in dollars per share)	$ 0.0001
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
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	62,164,377
Common stock, shares issued	18,190,180	2,180,976
Common stock, shares outstanding	18,188,313	2,173,399
Series A redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	2,589,868
Convertible preferred stock, shares issued (in shares)	0	2,589,868
Convertible preferred stock, shares outstanding	0	2,589,868
Series B redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	7,437,325
Convertible preferred stock, shares issued (in shares)	0	7,437,325
Convertible preferred stock, shares outstanding	0	7,437,325
Series C redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	5,000,002
Convertible preferred stock, shares issued (in shares)	0	5,000,002
Convertible preferred stock, shares outstanding	0	5,000,002
Series D redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	8,166,662
Convertible preferred stock, shares issued (in shares)	0	8,099,994
Convertible preferred stock, shares outstanding	0	8,099,994
Series SRN redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	5,611,112
Convertible preferred stock, shares issued (in shares)	0	2,111,109
Convertible preferred stock, shares outstanding	0	2,111,109
Series E redeemable convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	9,030,654
Convertible preferred stock, shares issued (in shares)	0	8,888,888
Convertible preferred stock, shares outstanding	0	8,888,888

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 1,048	$ 1,607	$ 5,153	$ 3,877
Operating expenses:
Research and development	6,021	5,483	18,669	15,769
General and administrative	2,495	2,195	7,294	6,305
Total operating expenses	8,516	7,678	25,963	22,074
Loss from operations	(7,468)	(6,071)	(20,810)	(18,197)
Investment income	98	25	121	149
Foreign currency transaction gain (loss), net	(51)	668	(429)	616
Interest expense	(311)	(334)	(931)	(843)
Other expense, net	4	(13)	(78)	(50)
Net loss	(7,728)	(5,725)	(22,127)	(18,325)
Other comprehensive loss:
Foreign currency translation adjustment	15	(800)	416	(763)
Unrealized gain (loss) on securities	16	0	16	0
Comprehensive loss	(7,697)	(6,525)	(21,695)	(19,088)
Net loss	(7,728)	(5,725)	(22,127)	(18,325)
Accretion of redeemable convertible preferred stock	0	(1,836)	(4,566)	(4,959)
Net loss attributable to common stockholders	$ (7,728)	$ (7,561)	$ (26,693)	$ (23,284)
Net loss per share attributable to common stockholders
Basic and diluted (in dollars per share)	$ (0.43)	$ (3.50)	$ (3.39)	$ (10.86)
Weighted average common shares outstanding
Basic and diluted (in shares)	18,108,014	2,159,658	7,881,625	2,144,731

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - 9 months ended Sep. 30, 2016 - USD ($)	Total	Common stock	Additional paid-In Capital	Accumulated deficit	Accumulated other comprehensive loss	Series A redeemable convertible preferred stock	Series B redeemable convertible preferred stock	Series C redeemable convertible preferred stock	Series D redeemable convertible preferred stock	Series D redeemable convertible preferred stockAdditional paid-In Capital	Series SRN redeemable convertible preferred stock	Series E redeemable convertible preferred stock	Series E redeemable convertible preferred stockAdditional paid-In Capital
Beginning balance (in shares) at Dec. 31, 2015						2,589,868	7,437,325	5,000,002	8,099,994		2,111,109	8,888,888
Beginning balance at Dec. 31, 2015	$ 137,482,000					$ 3,644,000	$ 21,448,000	$ 20,178,000	$ 42,902,000		$ 12,082,000	$ 37,228,000
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of preferred stock to redemption value	4,566,000					$ 75,000	$ 449,000	$ 446,000	$ 1,131,000		$ 913,000	$ 1,552,000
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)
Conversion of convertible stock upon listing						$ (3,719,000)	$ (21,897,000)	$ (20,624,000)	$ (44,033,000)		$ (12,995,000)	$ (38,780,000)
Ending balance (in shares) at Sep. 30, 2016						0	0	0	0		0	0
Ending balance at Sep. 30, 2016	$ 0					$ 0	$ 0	$ 0	$ 0		$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2015	2,173,399	2,173,399
Beginning balance at Dec. 31, 2015	$ (116,493,000)	$ 0	$ 1,000	$ (111,508,000)	$ (4,986,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)		6,647
Vesting of restricted common stock	16,000		16,000
Issuance of common stock upon exercise of options (in shares)		25,210
Issuance of common stock upon exercise of options	48,000		48,000
Stock‑based compensation expense	1,281,000		1,281,000
Accretion of preferred stock to redemption value	(4,566,000)		(708,000)	(3,858,000)
Exercise of common warrants (in shares)		567,306
Exercise of common warrants	0
Conversion of convertible stock upon listing (in shares)		10,126,118
Conversion of convertible stock upon listing	142,048,000	$ 1,000	142,047,000
Issuance of common stock, Initial public offering net of issuance costs	64,501,000	$ 5,289,633	64,501,000
Conversion of preferred stock warrants into warrants for the purchase of common stock									$ 189,000	$ 189,000		$ 114,000	$ 114,000
Currency translation adjustment	416,000				416,000
Unrealized gains (losses) on securities	16,000				16,000
Net loss	$ (22,127,000)			(22,127,000)
Ending balance (in shares) at Sep. 30, 2016	18,188,313	18,188,313
Ending balance at Sep. 30, 2016	$ 65,443,000	$ 1,000	$ 207,489,000	$ (137,493,000)	$ (4,554,000)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Operating activities
Net loss	$ (22,127)	$ (18,325)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	545	866
Amortization of premiums and accretion of discounts on investments	113	0
Stock‑based compensation expense	1,281	869
Non‑cash interest expense	176	139
Change in fair value of redeemable convertible preferred stock warrant	12	(17)
Changes in operating assets and liabilities:
Accounts receivable	341	112
Prepaid expenses and other assets	(1,668)	(608)
Restricted cash and other deposits	507	1,388
Accounts payable	(35)	363
Deferred revenue	586	(34)
Contingently repayable grant funding	(207)	(446)
Accrued expenses and other liabilities	(763)	870
Net cash used in operating activities	(21,239)	(14,823)
Investing activities
Maturities of short term government obligations	6,900	0
Purchase of short term investments	(23,318)	(739)
Purchases of property and equipment	(478)	(1,031)
Net cash used in investing activities	(16,896)	(1,770)
Financing activities
Net proceeds from issuance of preferred stock and warrants	0	32,669
Vesting of restricted stock	16	0
Principle payments on loan payable	0	(1,619)
Deferred IPO costs paid	(4,068)	0
Issuance of convertible note	0	7,092
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions	68,871	0
Exercise of stock options	48	78
Net cash provided by financing activities	64,867	38,220
Effect of exchange rate changes on cash	419	(705)
Net increase in cash and cash equivalents	27,151	20,922
Cash and cash equivalents at beginning of period	32,337	16,592
Cash and cash equivalents at end of period	59,488	37,514
Cash paid during the year for:
Interest	729	418
Supplemental noncash investing and financing activities:
Purchase of property and equipment not yet paid	31	11
Reclassification of deferred IPO costs from non-current assets to additional paid-in capital	4,369	0
Accrued dividends and accretion of preferred stock to redemption value	4,566	4,959
Conversion of bridge loans into Series E preferred	0	7,288
Unrealized gain on marketable securities	$ 16	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2016
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 pre-clinical 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. In connection with the initial public offering (“IPO”), the Company’s Board of Directors and stockholders approved a one-for-3.9 reverse stock split of the Company’s common stock. The reverse stock split became effective June 7, 2016. All share and per share amounts in these condensed interim financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. On June 21, 2016 the Company completed an IPO of its common stock 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 IPO on June 27, 2016, all outstanding shares of the Company’s 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. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three and nine months ended September 30, 2016 and 2015 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2015 included in the Company’s final prospectus filed under the Securities Act of 1933, as amended, with the SEC pursuant to Rule 424(b)(4) on June 23, 2016. The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Company’s financial statements for the three and nine months ended September 30, 2016 and 2015 and its cash flows for the nine months ended September 30, 2016 and 2015 . Such adjustments are of a normal and recurring nature. The results of operations for the three and nine months ended September 30, 2016 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2016 . Liquidity The Company has incurred losses since inception and negative cash flows from operating activities. As of September 30, 2016 and December 31, 2015 , the Company had an accumulated deficit of $137.5 million and $111.5 million , respectively. The Company’s cash and cash equivalents as of September 30, 2016 and December 31, 2015 , includes $2.8 million and $3.0 million of unrestricted cash held by its Russian subsidiary. The future success of the Company is dependent upon its ability to obtain additional capital through issuances of equity and debt securities and from collaboration and grant agreements in order to further the development of its technology and product candidates, and ultimately upon its ability to attain profitable operations. There can be no assurance that the Company will be able to obtain the necessary financing to successfully develop and market its product candidates or attain profitability. 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 September 30, 2016 , the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2016
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 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, the fair value of common stock and other equity instruments, accounting for stock-based compensation, income taxes, collectability of accounts receivable, useful lives of long-lived assets, accrued expenses, and accounting for project development. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. The Company’s management made significant estimates and assumptions in determining the fair value of its common stock for those periods reported prior to the completion of the IPO. The Company utilized various valuation methodologies in accordance with the framework of the 2004 American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology included estimates and assumptions that require the Company’s judgment. These estimates and assumptions included a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector, the prices at which the Company sold shares of preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an initial public offering or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date. 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 and Short Term Investments 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. The Company, as part of its cash management strategy, may invest in reverse repurchase agreements. All reverse repurchase agreements are tri-party and have maturities of three months or less at the time of investment. These agreements are collateralized by U.S. treasury securities for an amount no less than 102% of their value. 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. 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 rubles and U.S. dollars. As of September 30, 2016 , the Company maintained approximately $3.1 million in Russian bank accounts, of which $2.8 million was held in U.S. dollars. The Company has minimal credit risk as the majority of accounts receivable relates to amounts due under a government sponsored grant, collaboration with large pharmaceutical companies or grants from well‑known and supported non‑profit organizations. The Company did not have any off balance sheet arrangements as of September 30, 2016 and December 31, 2015 . Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, short‑term investments, restricted cash, accounts receivable, accounts payable, loans payable, common stock warrants, and redeemable convertible preferred 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. The carrying amount of loans payable approximates their estimated fair value. 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. 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 nine months ended September 30, 2016 or the year ended December 31, 2015 . 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, five years for equipment and three years for computer and office 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 periodically evaluates its long‑lived assets for potential impairment. Impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends, and product development cycles. Impairment in the carrying value of each asset is assessed when the undiscounted expected future cash flows derived from the asset are less than their carrying value. The Company did not recognize any impairment charges through September 30, 2016 . Debt Issuance Costs Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are accounted for as additional debt discount and 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. Revenue Recognition The Company’s revenue is primarily generated from research grants in both the United States and Russia, and, prior to its termination, a license and research collaboration agreement with Sanofi. The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition . Accordingly, revenue is recognized when all of the following criteria are 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. Amounts received prior to satisfying the revenue recognition criteria are 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 are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Collaboration Revenue When evaluating multiple element arrangements such as the agreement with Sanofi discussed in Note 13, the Company considers whether the deliverables under the arrangement represent separate units of accounting. This evaluation requires subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have standalone value, based on the consideration of the relevant facts and circumstances for each arrangement. The consideration received is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria are applied to each of the separate units. The Company determines 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 is available. Determining the best estimate of selling price for a deliverable requires significant judgment. The Company has used its best estimate of selling price to estimate the selling price for licenses to the Company’s proprietary technology, since the Company does not have VSOE or TPE of selling price for these deliverables. In those circumstances, the Company considers market conditions as well as entity‑specific factors, including those factors contemplated in negotiating the agreements, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating the Company’s best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine the best estimate of selling price will have a significant effect on the allocation of arrangement consideration between multiple deliverables. The Company may receive upfront payments when licensing its intellectual property in conjunction with a research and development agreement. When management believes the license to its intellectual property does not have stand‑alone value from the other deliverables to be provided in the arrangement, the Company generally recognizes revenue attributed to the license over the Company’s contractual or estimated performance period. When management believes the license to its intellectual property has stand‑alone value, the Company generally recognizes revenue attributed to the license upon delivery. The periods over which revenue should be recognized are subject to estimates by management and may change over the course of the research and development agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Payments or reimbursements resulting from the Company’s research and development efforts are recognized as the services are performed. At the inception of each agreement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific and other risks that must be overcome to achieve the milestone, as well as the level of effort and investment required. Revenues from milestones, if they are nonrefundable and deemed substantive, are recognized upon successful accomplishment of the milestones. Milestones that are not considered substantive are accounted for as license payments and recognized over the remaining period of performance. Grant Agreements Grant revenue is generally recognized as the related research and development work is performed. Grant arrangements frequently include payment milestones which the Company has judged to be non‑substantive milestones as they are typically entitled to receive payment regardless of the outcome of the research work. Revenue under such arrangements is recognized using a proportional performance method, but not in excess of cash actually received. Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets. 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 costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activation, and other information provided to the Company by its vendors. 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 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. 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. Should such costs be incurred, they would be classified as a component of income tax expense. Warrants The Company has issued common stock warrants and redeemable convertible preferred stock warrants to investors and lenders. Common stock warrants are classified as a component of permanent equity because they are freestanding financial instruments that are legally detachable and separately exercisable from other debt and equity instruments, are contingently exercisable, do not embody an obligation for the Company to repurchase its own shares, and permit the holders to receive a fixed number of common shares upon exercise. In addition, such warrants require physical settlement and do not provide any guarantee of value or return. Common stock warrants are initially recorded at their issuance date fair value and are not subsequently re‑measured. These warrants are valued using the Black‑Scholes option pricing model (“Black‑Scholes”). In connection with the automatic conversion of the Company’s convertible preferred stock, which occurred upon the closing of the IPO on June 27, 2016, the preferred stock warrants became warrants to purchase common stock. The Company performed the final mark to market adjustment on the preferred stock warrant using the fair value of the underlying common shares of $14.00 per share on June 27, 2016 and recorded the change in fair value in other income (expense), net in the consolidated statement of operations and comprehensive loss. The preferred stock warrant liability was then reclassified to additional paid-in-capital as the preferred stock warrants became warrants to purchase common stock. 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 of stock option grants and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Stock‑based compensation awarded to non‑employees are subject to revaluation over their vesting terms. 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 financial statements is based on awards that are ultimately expected to vest. Comprehensive Loss Comprehensive 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 loss consists of both: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. For all periods presented, other comprehensive loss is comprised solely of foreign currency translation adjustments. Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after giving consideration to 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 September 30, 2016 and December 31, 2015 , the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014‑9, Revenue from Contracts with Customers (“ASU 2014‑9”), which amends the guidance for revenue recognition to replace numerous industry‑ specific requirements. ASU 2014‑9 implements a five‑step process for customer contract revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards. ASU 2014‑9 also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenues and cash flows from contracts with customers. Other major provisions include ensuring the time value of money is considered in the transaction price, and allowing estimates of variable consideration to be recognized before contingencies are resolved in certain circumstances. The amendments in ASU 2014‑9 are effective for reporting periods beginning after December 15, 2017. Early adoption is permitted, but not before December 15, 2016. Entities can transition to the standard either retrospectively or as a cumulative‑effect adjustment as of the date of adoption. The Company is currently in the process of evaluating the effect the adoption of ASU 2014‑9 may have on its financial statements. In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern. ASU 2014-15 requires management of all entities to evaluate whether there are conditions and events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the financial statements are issued, and to make certain disclosures if it concludes that substantial doubt exists or when its plans alleviate substantial doubt about the entity’s ability to continue as a going concern. ASU 2014-15 is effective for the Company for annual reporting periods beginning in 2016 and for interim reporting periods starting in the first quarter of 2017. The Company is currently evaluating the impact that the standard will have on the financial statements, and has not yet determined what effect, if any, the impact of adoption will be. In February 2016, FASB issued ASU No.2016‑2, Leases (“ASU 2016‑2”). ASU 2016‑2 requires a lessee to separate the lease components from the non‑lease components in a contract and recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right‑of‑use asset representing its right to use the underlying asset for the lease term. It also aligns lease accounting for lessors with the revenue recognition guidance in ASU 2014‑9. ASU 2016‑2 is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years, and is to be applied at the beginning of the earliest period presented using a modified retrospective approach. In March 2016, the FASB issued ASU 2016-9, Improvements to Employee Share-Based Payment Accounting, which simplifies several aspects of accounting for share-based payment transactions including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The new standard is effective for fiscal years beginning after December 15, 2016 and interim periods within those fiscal years with early adoption permitted. The Company is evaluating the impact that the adoption of this standard will have on its financial statements.

Available-for-Sale Marketable S

Available-for-Sale Marketable Securities	9 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Marketable Securities	Available-for-Sale Marketable Securities As of September 30, 2016 and December 31, 2015 , the Company’s available-for-sale marketable securities consisted of debt securities issued by the U.S. government and corporate debt securities. The following tables summarize the Company’s available-for-sale marketable securities by major type of security as of September 30, 2016 and December 31, 2015 (in thousands): September 30, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value U.S. Treasury securities $ — $ — $ — $ — Corporate bonds 19,804 6 (23 ) 19,788 Total available-for-sale marketable securities $ 19,804 $ 6 $ (23 ) $ 19,788 December 31, 2015 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value U.S. Treasury securities $ 3,516 $ — $ — $ 3,516 Corporate bonds — — — — Total available-for-sale marketable securities $ 3,516 $ — $ — $ 3,516 All available-for-sale marketable securities are classified in the Company’s Condensed Balance Sheets as Short term deposits and investments. The Company classifies its marketable debt securities based on their contractual maturity dates. As of September 30, 2016 , the Company’s marketable debt securities mature at various dates through August 2017. The fair values and amortized cost of marketable debt securities by contractual maturity were as follows (in thousands): September 30, 2016 December 31, 2015 Fair Value Amortized Cost Fair Value Amortized Cost Less than one year $ 19,788 $ 19,804 $ 3,516 $ 3,516 As of September 30, 2016 the Company held a total of 11 out of 13 positions that were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. Based on the Company’s review of these securities, the Company believes that the cost basis of its available-for-sale marketable securities is recoverable and that, therefore, it had no other-than-temporary impairments on these securities as of September 30, 2016 . The Company does not intend to sell these debt securities and the Company believes it is not more likely than not that it will be required to sell these securities before the recovery of their amortized cost basis, which may be maturity.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share Because the Company has reported a net loss attributable to common stockholders for all periods presented, basic and diluted net loss per share attributable to common stockholders are the same for those periods. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data): Three Months Ended Nine Months Ended 2016 2015 2016 2015 (unaudited) (unaudited) Numerator: Net (loss) $ (7,728 ) $ (5,725 ) $ (22,127 ) $ (18,325 ) Less: accretion on preferred stock — (1,836 ) (4,566 ) (4,959 ) Net loss attributable to common stockholders $ (7,728 ) $ (7,561 ) $ (26,693 ) $ (23,284 ) Denominator: Weighted‑average common shares outstanding—basic and diluted 18,108,014 2,159,658 7,881,625 2,144,731 Net loss per share attributable to common stockholders—basic and diluted $ (0.43 ) $ (3.50 ) $ (3.39 ) $ (10.86 ) Potential common shares issuable upon conversion of warrants to purchase common stock and stock options that are excluded from the computation of diluted weighted average shares outstanding are as follows: September 30, 2016 2015 (unaudited) Redeemable convertible preferred stock — 6,872,090 Stock options to purchase common stock 2,140,295 2,165,187 Stock warrants to purchase common stock 113,795 17,888 Total 2,254,090 9,055,165

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The tables below present information about the Company’s financial assets and liabilities that are measured and carried at fair value as of September 30, 2016 and December 31, 2015 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): September 30, 2016 (level 1) (level 2) (level 3) Total Money market funds, included in cash equivalents $ 67 $ — $ — $ 67 Tri-party repurchase agreements, included in cash equivalents $ — $ 32,000 $ — $ 32,000 US Treasury obligations, included in investments $ — $ — $ — $ — Corporate bonds, included in investments $ 19,788 $ — $ — $ 19,788 Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities $ — $ — $ — $ — December 31, 2015 (level 1) (level 2) (level 3) Total US Treasury obligations, included in cash equivalents $ 14,486 $ — $ — $ 14,486 US Treasury obligations, included in investments $ 3,516 $ — $ — $ 3,516 Corporate bonds, included in investments $ — $ — $ — $ — Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities $ — $ — $ 290 $ 290 At September 30, 2016 , all cash and cash equivalent investments where held in money market funds and tri-party repurchase agreements which are collateralized by government securities for an amount not less than 102% of their value. All reverse repurchase agreements are tri-party and have maturities of three months or less at the time of investment. At December 31, 2015 the average maturity for the US Treasury obligations was 106 days. Fair value of US Treasury obligations approximates amortized value. In July 2015, the Company issued warrants for the purchase of 80,813 shares of common stock at an exercise price of $17.55 in connection with the issuance of convertible notes. These warrants expire three years from date of issuance. Common stock warrants are classified as permanent equity which are initially recorded at issuance date fair value and are not subsequently re‑measured. In August 2013 and July 2014, in conjunction with the execution of a loan and security agreement (Note 9), the Company issued warrants to the lenders for the purchase of up to 66,668 shares of the Company’s Series D redeemable convertible preferred stock (“Series D Preferred”) at an exercise price of $4.50 per share. At the IPO, these warrants were converted to warrants to purchase 17,888 of common stock at an exercise price of $16.77 . These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire four years from the date of issuance. In December 2015, in conjunction with the execution of a loan and security agreement (Note 9), the Company issued warrants to the lenders for the purchase of up to 37,978 shares of the Company’s Series E Preferred at an exercise price of $4.50 per share. At the IPO, these warrants were converted to warrants to purchase 15,094 of common stock at an exercise price of $11.32 . These warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire four years from the date of issuance.

Property and Equipment

Property and Equipment	9 Months Ended
	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): September 30, December 31, 2016 2015 (unaudited) Laboratory equipment $ 4,617 $ 4,028 Computer equipment and software 501 409 Leasehold improvements 165 91 Furniture and fixtures 225 222 Office equipment 62 62 P,P&E—Construction in process — 144 Total property and equipment 5,570 4,956 Less accumulated depreciation (3,504 ) (2,927 ) Property and equipment, net $ 2,066 $ 2,029 Depreciation expense for the three months ended September 30, 2016 and 2015 was $0.1 million and $0.2 million respectively and for the nine months ended September 30, 2016 and 2015 , was $0.5 million and $0.9 million , respectively.

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): September 30, December 31, 2016 2015 (unaudited) Payroll $ 297 $ — Legal 59 213 Bonus 519 669 Current portion of deferred rent and lease incentive 12 405 Accrued patent fees 184 219 Accrued external research and development costs 662 1,649 Other 701 223 Accrued expenses $ 2,434 $ 3,378

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2016
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 in Watertown, Massachusetts. As part of the 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 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 April 2015, the Company amended the lease agreement to exchange 13,711 square feet of space for another 15,174 square feet of space within the same building. Rental payments on the prior space ceased as of March 31, 2015 and rental payments on the new space began on October 1, 2015. The Company subleased a portion of its facility to a tenant with a term that would have expired in March 2017. In March 2015, the tenant terminated the sublease and vacated the space. The sublease amount from the tenant was recorded as a reduction of lease expense and totaled $0.2 million for the year ended December 31, 2015 . In August 2016, the Company signed an amendment to the operating lease, which extends the lease term of the Company's laboratory and office space through March 31, 2020. The lease agreement includes a rent escalation clause, and accordingly, rent expense is being recognized on a straight-line basis over the lease term. Deferred rent and lease incentive liability totaled $0.2 million and $0.5 million as of September 30, 2016 and December 31, 2015 , respectively. Included in that amount, the current portion of deferred rent and lease incentive liability is classified as accrued expenses and was less than $0.1 million at September 30, 2016 and $0.4 million at December 31, 2015 , respectively. The Company has a month‑to‑month facility agreement for its Moscow, Russia facility. Rent expense is recognized as incurred. Rent expense, net of sublease payments, for the three months ended September 30, 2016 and September 30, 2015 was $0.4 million , and $0.5 million respectively, and for the nine months ended September 30, 2016 and September 30, 2015 was $1.1 million and $1.0 million , respectively. As of December 31, 2015 , the future minimum lease payments under the lease, as amended by the lease amendment are as follows (in thousands): Year ended December 31, 2016 $ 1,176 2017 1,244 2018 1,291 2019 1,330 2020 335 Total minimum lease payments $ 5,376 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. The Company does not expect significant claims related to these indemnification obligations and, accordingly, has concluded that the fair value of these obligations is negligible, and no related reserves have been established. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect on its financial position or results of operations.

Debt

Debt	9 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Debt	Debt Term Loans On August 9, 2013, the Company entered into a loan and security agreement with two lenders 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. The amounts borrowed are collectively referred to as “Term Loans.” In December 2015, the Company refinanced its existing debt facility that was originally entered into on August 9, 2013, as amended with Oxford Finance LLC (“Oxford”) and Square 1Bank (“Square 1”), to increase the amount of the borrowing to $12.0 million and to extend the repayment term. 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. As of September 30, 2016 and December 31, 2015 , the outstanding principal balance under the Term Loans was $12.0 million . According to ASC 470‑50‑40, the refinancing and modification of the prior debt in a non‑troubled debt situation must be treated as either an extinguishment or a modification based on whether the present value of the cash flows under the terms of the new debt instrument is different by greater than, or less than, 10% from the present value of the remaining cash flows under the terms of the original instrument. For cash flow changes greater than 10%, the debt modification is accounted for as a debt extinguishment, whereby the original debt is derecognized and the new debt is initially recorded at fair value, with the difference recognized as an extinguishment gain or loss. For cash flow changes of less than 10%, the new loan is considered a modification and no gain or loss is recognized. In considering all cash flow changes, the Company concluded that the refinancing of the debt as of December 31, 2015 is a modification of the debt and not a debt extinguishment, and as a result the debt is initially recorded at its amortizable value net of discounts and deferred costs. The Term Loans are collateralized by the assets of the Company and bear interest at 8.1% per annum. The monthly payments for the Term Loans are initially interest only through January 2017. Principal repayments for the Term Loans are due over 30 monthly installments beginning on February 1, 2017. The Term Loans may be prepaid at the Company’s option at any time prior to maturity subject to a prepayment fee of 3% if prepaid prior to the first anniversary of the borrowing date, 2% if prepaid after the first anniversary but before the second anniversaries, and 1% if prepaid after the second anniversary. The Term Loans do not include any financial covenants. The Term Loans require 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 is recorded as a loan discount. In addition, the Term Loans contain 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. To date, there have been no such events and the lender has not exercised its right under this clause. As a result, the Company concluded that a material adverse change has not occurred and is unlikely to occur, therefore, no liability has been recorded in connection with the clause. In connection with the Term Loans, 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 17,888 shares of the Company’s common stock. Additionally, with the refinancing of the Term Loans 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 15,094 shares of Company’s common stock. The initial grant date fair value of the warrants of $0.1 million , $0.1 million and $0.1 million for each issuance respectively, was recorded as a loan discount. Term Loan discounts are amortized as additional interest expense over the term of the loans. Interest expense for the three months ended September 30, 2016 and 2015 totaled $0.3 million and $0.1 million , respectively, and for the nine months ended September 30, 2016 and 2015 was $0.9 million and $0.5 million , respectively. Future minimum payments on the Term Loans as of December 31, 2015 are as follows (in thousands): Year ended December 31, 2016 $ 972 2017 5,319 2018 5,318 2019 3,379 Total debt payments 14,988 Less: Amount representing interest (2,268 ) Less: Debt discount and deferred charges (919 ) Less: Current portion of issuance costs 54 Loans payable, net of current portion $ 11,855 Convertible Notes In April 2015, the Company issued convertible notes as a bridge loan to be automatically converted into the Company’s capital stock upon the consummation of a private placement of the Company’s Preferred Stock. The convertible notes bore interest at 8% per annum, compounding monthly. In the event the Company was unable to consummate the private placement by July 15, 2015, the Company would be required to issue warrants to purchase shares of the Company’s common stock equal to 20% of the convertible note principal divided by $17.55 . On July 24, 2015, the Company issued warrants to the convertible note holders to purchase up to 80,813 shares of the Company’s common stock at an exercise price of $17.55 per share for a term of three years. The carrying value and accrued interest of the outstanding convertible notes were automatically converted into 1,619,550 shares of Series E Preferred. As part of the Series E Preferred issuance, the convertible note holders also received warrants to purchase up to 103,817 shares of the Company’s common stock (Note 10). The difference between the carrying value and accrued interest of the convertible notes that were converted and the combined fair value of the Series E Preferred shares and common stock warrants issued were negligible. There was no interest expense related to the convertible notes for the three and nine months ended September 30, 2016 , and $0.1 million and $0.3 million for the three and nine months ended September 30, 2015 , respectively.

Preferred Stock

Preferred Stock	9 Months Ended
	Sep. 30, 2016
Temporary Equity Disclosure [Abstract]
Preferred Stock	Preferred Stock The Company issued Preferred Stock with a $0.0001 par value to investors for cash or as settlement for outstanding debt under convertible notes. The Company had issued 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. In April 2014 and August 2014, the Company issued an additional 3,211,105 shares of Series D Preferred at $4.50 per share for total net proceeds of $14,349,239 . In July 2014, the Company issued an additional 1,333,332 shares of Series SRN Preferred at $4.50 per share for total net proceeds of $5.8 million . In connection with the issuance of the additional shares of Series SRN Preferred, the Series SRN Preferred terms were amended. Significant terms that were amended included a change of the Series SRN Preferred optional and mandatory conversion price (other than a special conversion event, as defined in the certificate of incorporation) to $16.77 per share, the elimination of a time‑based tranche investment requirement, and the removal of a call option for the Company to repurchase the Series SRN Preferred shares. Based upon the qualitative characteristics of the amendments, the Company determined that the changes significantly modified the terms of Series SRN Preferred resulting in an extinguishment of the then outstanding SRN Preferred shares. As a result, the carrying value of Series SRN Preferred of $5.0 million at the date of the amendment was derecognized, and the amended Series SRN Preferred shares were recorded at their fair value of $4.50 per share. The difference of $1.5 million was recorded as additional paid in capital. In August 2015 and September 2015, the Company issued an aggregate of 7,269,338 shares of Series E Preferred at $4.50 per share for total gross proceeds of $32.7 million with issuance costs totaling $0.2 million . In addition, the Company issued 1,619,550 shares of Series E Preferred in connection with the conversion of convertible notes (Note 9). In connection with the Series E Preferred issuances, each Series E Preferred stockholder also received warrants to purchase a number of shares of the Company’s common stock that equal to 25% of the number of Series E Preferred shares issued. The fair value of the issued common stock warrants is accounted for as an issuance discount on the Series E Preferred. The common stock warrants are classified as permanent equity and were recorded as additional paid‑in capital. All outstanding shares of the Company’s convertible preferred stock automatically converted into 10,126,118 shares of the Company’s common stock upon the closing of the IPO on June 27, 2016.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2016
Stockholders' Equity Note [Abstract]
Common Stock	Common Stock 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. Common stock voting rights on certain matters are subject to the powers, preferences, and rights of the Senior Preferred. Dividends The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. The Company may not declare or pay any cash dividends to the common stockholders unless dividends are first declared and paid to the holders of Preferred Stock in accordance with their respective terms. Through September 30, 2016 , no dividends have been declared or paid on common stock. Liquidation After holders of Preferred Stock are satisfied of their liquidation preferences upon any voluntary or involuntary liquidation, dissolution or winding up of the affairs of the Company, the common stockholders are then entitled to receive that portion of the remaining funds to be distributed to all holders of the Company’s stock on an as‑converted basis. Reserved Shares The Company has authorized shares of common stock for future issuance as follows: Periods ending September 30, 2016 December 31, 2015 (unaudited) Conversion of Series A Preferred — 664,068 Conversion of Series B Preferred — 1,907,006 Conversion of Series C Preferred — 1,282,051 Conversion of Series D Preferred — 2,191,412 Conversion of Series SRN Preferred — 1,798,433 Conversion of Series E Preferred — 2,662,885 Exercise of common warrants 113,795 651,618 Shares available for future stock incentive awards 783,494 100,034 Exercise of outstanding common stock options 2,140,295 1,569,379 Total 3,037,584 12,826,886

Stock Incentive Plans

Stock Incentive Plans	9 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Incentive Plans	Stock Incentive Plans The Company maintained 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. As of September 30, 2016 , a total of 2,213,412 shares of common stock are authorized for grants under the 2008 Plan with 10,163 shares available for future grant. All stock options granted under the 2008 Plan may be exercised into restricted stock subject to forfeiture provisions upon termination. The 2008 Plan provided that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the common stock on the grant date for participants who own less than 10% 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 granted under the 2008 Plan vest over periods as determined by the Board, which are generally four years and with terms that generally expire ten years from the grant date. The fair value of each option award was estimated on the grant date using Black‑Scholes. Expected volatilities were based on historical volatilities from guideline companies, since there was no active market for the Company’s common stock. The Company used 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, any cash dividends in the foreseeable future. Forfeitures were estimated at the time of grant and were adjusted, if necessary, in subsequent periods if actual forfeitures differed from those estimates. The Company had estimated a forfeitures rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on the awards that are expected to vest. As of the S-1 registration date of May 24, 2016, the Company ceased granting awards under the 2008 Plan; however, awards issued under the 2008 Plan remain subject to the terms of the applicable 2008 Plan agreement. 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 and restricted stock awards as determined by the Board. As of September 30, 2016 , a total of 1,210,256 shares of common stock are authorized for grants under the 2016 Plan with 783,494 shares available for future grant. The 2016 Plan provides that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the common stock on the grant date for participants who own less than 10% 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 granted under the 2016 Plan vest over periods as determined by the Board, which are generally four years and with terms that generally expire ten years from the grant date. On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. A total of 173,076 shares of common stock were reserved for issuance under the ESPP. In addition, 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 (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. The weighted average assumptions used for employee stock option grants issued for the nine month period ended September 30, 2016 and in 2015 : Nine Months Ended September 30, 2016 2015 Risk-free interest rate 1.39 % 1.71 % Expected dividend yield — — Expected life 6.05 6.00 Expected volatility 95.62 % 84.60 % Weighted-average fair value of common stock $ 9.79 $ 1.41 The resulting weighted average grant date fair value of stock options granted to employees during the nine months ended September 30, 2016 and year ended December 31, 2015 was $9.79 and $5.03 , respectively. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2016 and year ended December 31, 2015 was $0.1 million and $0.3 million , respectively. As of September 30, 2016 and December 31, 2015 , total unrecognized compensation expense related to unvested employee stock options was $5.5 million and $2.3 million , respectively, which is expected to be recognized over a weighted average period of 3.2 years and 3.1 years, respectively. The weighted average assumptions used for unvested non‑employee stock options are as follows: Nine Months Ended September 30, 2016 2015 Risk‑free interest rate 1.58 % 1.82 % Expected dividend yield — — Expected life (in years) 9.41 6.36 Expected volatility 87.75 % 96.75 % The unvested options held by non‑employees are revalued using the Company’s estimate of fair value on each vesting and reporting date through the remaining vesting period. Non‑employee stock‑based compensation expense of $0.2 million and $0.4 million was recorded during the nine months ended September 30, 2016 and for the year ended December 31, 2015 , respectively. As of September 30, 2016 and December 31, 2015 , total unrecognized compensation expense related to unvested non‑employee stock options was $1.0 million and $0.1 million , respectively. The following table summarizes the activity under the Plan and the 2016 Plan since December 31, 2015 : Weighted‑average Weighted‑average remaining Aggregate Number of exercise contractual term intrinsic value options price (in years) (in thousands) Employee Outstanding at December 31, 2015 1,247,160 $ 5.05 7.41 $ 3,130 Granted 511,039 $ 12.60 Exercised (21,088 ) $ 2.16 Forfeited (9,124 ) $ 7.74 Outstanding at September 30, 2016 1,727,987 $ 7.30 7.57 $ 12,004 Vested at September 30, 2016 805,229 $ 3.70 5.62 $ 8,493 Vested and expected to vest at September 30, 2016 1,315,411 $ 5.23 7.41 $ 11,582 Non‑Employee Outstanding at December 31, 2015 322,220 $ 2.93 5.50 $ 1,288 Granted 98,718 $ 7.70 Exercised (5,085 ) $ 0.47 Forfeited (3,546 ) $ 0.47 Outstanding at September 30, 2016 412,307 $ 4.12 5.94 $ 4,176 Vested at September 30, 2016 325,519 $ 3.12 5.03 $ 3,622 Vested and expected to vest at September 30, 2016 412,097 $ 4.12 5.93 $ 4,176 Restricted Stock During the year ended December 31, 2013, the Company issued 30,317 shares of restricted common stock to employees upon the early exercise of stock options. During the year ended December 31, 2014, the Company issued 2,564 shares of restricted common stock to employees. Under the terms of each agreement, the Company has a repurchase provision whereby the Company has the right to repurchase any unvested shares when/if the shareholders terminate their business relationship with the Company, at a price equal to the original exercise price. Accordingly, the Company recorded the cumulative payments received of $0.1 million for the purchase of the restricted shares as a liability. The Company records payment received from the granting of restricted stock as a liability which is amortized over the vesting period. As of September 30, 2016 and December 31, 2015 , the remaining liability was less than $0.1 million . Total fair value of restricted shares that vested during the nine month period ended September 30, 2016 was less than $0.1 million . The following table summarizes the restricted stock award activity of the 2008 Plan and the 2016 Plan since December 31, 2015 : Weighted‑ average exercise Shares price Unvested as of December 31, 2015 7,574 $ 2.77 Issued 940 9.36 Vested (6,647 ) 3.72 Unvested as of September 30, 2016 1,867 $ 2.77 As of September 30, 2016 and December 31, 2015 , total unrecognized compensation expense related to restricted stock awards was less than $0.1 million , which the Company expects to recognize over a weighted average period of approximately 0.25 years and 1.0 years, respectively. The Company recorded stock-based compensation expense related to stock options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Research and development $ 353 $ 40 $ 747 $ 321 General and administrative 280 212 534 548 Total $ 633 $ 252 $ 1,281 $ 869

Revenue Arrangements

Revenue Arrangements	9 Months Ended
	Sep. 30, 2016
Revenue Recognition [Abstract]
Revenue Arrangements	Revenue Arrangements Sanofi Collaboration Agreement On November 27, 2012, the Company and Sanofi entered into a license and research collaboration agreement focused on the identification and development of vaccines against food allergies (the “Sanofi Agreement”). Under the arrangement, the Company agreed to perform research to identify an initial vaccine candidate for development and commercialization by Sanofi under an exclusive license. Pursuant to the Sanofi Agreement, the Company received an upfront payment of $2.0 million for the initial indication in November 2012 and an additional payment of $3.0 million in August 2013. In November 2014, Sanofi exercised the option to include celiac disease as an additional indication, and in May 2015, the Sanofi Agreement was amended to add terms specific to the celiac disease indication and to terminate Sanofi's right to exercise its option for any additional indications. Sanofi paid the Company an additional $2.0 million upon the exercise of the option in May 2015 and an additional $1.0 million in July 2016 upon attaining the first milestone for the celiac disease indication. To date, Sanofi has paid the Company $8.0 million in the aggregate under the Sanofi Agreement. Except as authorized by Sanofi or permitted under the Sanofi Agreement, during the term of the Sanofi Agreement, exclusivity obligations prevent the Company from researching, developing, or commercializing products in these indications or granting third party licenses under the intellectual property rights and technologies licensed to Sanofi for use in these indications. As per the agreement, the research term expired for the first indication on the third anniversary (November 27, 2015) of the agreement. The Company completed its research obligations within the initial three year period and is not obligated to perform any further research on the specific indication under the agreement. A vaccine candidate for development and commercialization was not selected by Sanofi by the end of the research plan, and therefore no further milestone payments have been received. The Company identified the deliverables under the arrangement as the license, the research necessary to identify the development candidate, and participation of the Joint Research Committee ("JRC"). The Company determined that the exclusive license granted to Sanofi did not have standalone value from the research to be performed to identify the vaccine development candidate. As a result, each upfront and milestone consideration was allocated to the combined unit of account comprising the license and research services, and is being recognized over the estimated development period using a proportional performance method. The consideration allocated to participation on the JRC was not material. The Company recognized revenue in the amount of $0.2 million and $0.5 million for the three months ended September 30, 2016 and 2015 , respectively, and $0.6 million and $1.5 million for the nine months ended September 30, 2016 and 2015 , respectively. Termination of the Sanofi Collaboration Agreement On November 9, 2016, the Company received written notice from Sanofi that Sanofi has elected to terminate in its entirety the Sanofi Agreement. The termination of the Sanofi Agreement will be effective on May 8, 2017, or the Termination Date, which is six months from the date of the notice. As discussed above, Sanofi has paid the Company $8.0 million in the aggregate under the Sanofi Agreement to date. The Company would have been eligible to receive additional development-based, regulatory-based and sales-based milestone payments and tiered royalties on net sales of any approved product generated by the collaboration had the Sanofi Agreement not been terminated. As of September 30, 2016, the Company had $2.2 million of deferred revenue associated with the Sanofi Agreement, which is expected to be recognized within the termination period. Upon timely exercise by the Company, all rights granted to Sanofi will terminate and revert to the Company effective on the Termination Date, and Sanofi is required to grant to the Company a royalty bearing, exclusive license, with the right to grant sublicenses, under certain Sanofi intellectual property solely to the extent necessary to research, develop, make, have made, use, offer for sale, import, export and otherwise commercialize the vaccine candidates developed under the Sanofi Agreement. The exclusivity obligations discussed above will also expire on the Termination Date. The Company intends to exercise its right to acquire the development programs under the Sanofi Agreement. The Company will be solely responsible for performing and funding any development and clinical trial activities relating to further development of vaccine candidates that it chooses to undertake after the Termination Date. Other Research and Collaboration Agreements The Company has entered into other research and collaboration agreements in 2016 and 2015 for which the Company recognized revenue of zero and $0.3 million for the three months ended September 30, 2016 and 2015 , respectively, and $0.1 million and $0.4 million for the nine months ended September 30, 2016 and 2015 , respectively. Grant Agreements The Company receives funding in the form of grants from the National Institutes of Health (“NIH”), the Juvenile Diabetes Research Foundation (“JDRF”), the Bill and Melinda Gates Foundation, the Russian Ministry of Industry and Trade (“Minpromtorg”), and the Russia based Development Fund of New Technologies Development and Commercialization Center (“Skolkovo”). NIH The Company has two grants through the NIH. The first grant, for an aggregate amount of $8.1 million , was awarded in May 2014 to support research in the development of a next generation vaccine for smoking cessation and relapse prevention. The Company recognized revenue in the amount of $0.7 million and $0.5 million for the three months ended September 30, 2016 and 2015 , respectively, and $3.7 million and $1.5 million for the nine months ended September 30, 2016 and 2015 , respectively under the arrangement. The second grant is for an aggregate amount of $0.2 million , which was awarded in September 2015 for the development of nanoparticles for immune tolerance to factor VIII. The Company recognized revenue in an amount less than $0.1 million for the three and nine months ended September 30, 2016 related to this grant. JDRF The JDRF grant is a joint grant with Sanofi entered into in September 2014 for $0.4 million to conduct Type 1 Diabetes research. The Company recognized revenue in the amount of less than $0.1 million and zero for the three months ended September 30, 2016 and 2015 , respectively, and $0.1 million and $0.2 million for the nine months ended September 30, 2016 and 2015 , respectively, related to this grant. Bill and Melinda Gates Foundation The Company received a grant in 2013 from the Bill and Melinda Gates Foundation for $1.2 million to fund the Company’s immunology research on malaria antigens. During 2014, the grant amount was increased to a total of $1.6 million and the term was extended to a three -year research term. Revenue is recognized on a proportional performance basis as it relates to employee time expended on the research, along with reimbursement for external costs directly related to, and approved, by the grant terms. The Company recognized revenue in the amount of $0.1 million and $0.3 million for the three months ended September 30, 2016 and 2015 , respectively, and $0.4 million and $0.3 million for the nine months ended September 30, 2016 and 2015 , respectively, related to this grant. Minpromtorg The Company had a contract awarded from Minpromtorg for approximately $4.6 million to fund the Company’s nicotine cessation vaccine clinical trial to be conducted in Russia. The grant covered a term from July 9, 2013 through December 31, 2015, and provided for reimbursement of expenses incurred by the Company from the clinical trial. Under the agreement term, the Company was subject to a penalty in the event that the clinical trial was delayed or terminated prior to completion. As a result of the penalty provision, the Company concluded the amounts received under the agreement were not fixed or determinable. In 2014, the Company terminated its plan to conduct the clinical trial in Russia subjecting the Company to the penalty obligation. In February 2015, the Company received an executed final settlement agreement from Minpromtorg that included the repayment of funds previously received by the Company totaling $0.2 million , and a penalty fee that equaled to 10% of the contract value, or $0.2 million . The Company paid the settlement payment in March 2015 and all mutual claims under the contract were terminated. According to the terms of the agreement, Minpromtorg has the right to audit the expenditure incurred under the agreement for a period up to three years from each research milestone date. All grant funding received in excess of the penalty settlement will remain as a liability on the balance sheet until such time the audit period has expired and at which time, the amount will be recognized as revenue. Through September 30, 2016 , the Company received payments totaling approximately $1.4 million . The first audit period expired on December 31, 2015, and as a result $0.4 million of revenue was recognized for the year ended December 31, 2015. The second audit expired during the nine months ended September 30, 2016, and as a result $0.2 million of revenue was recognized for the nine months ended September 30, 2016 . 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 HPV and diseases associated with this infection. The grant covers a period from August 1, 2014 through July 21, 2017. The grant provides 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. The Company has received from Skolkovo $1.8 million through the nine months ended September 30, 2016 . 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, the Company has determined that the grant funding is not fixed or determinable and all amounts received to date are recorded as deferred revenue in the consolidated balance sheet until the completion of Skolkovo’s audit or the expiration of the audit term.

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2016
Related Party Transactions [Abstract]
Related-Party Transactions	Related‑Party Transactions As part of the Series B Preferred and Series D Preferred financings (as described in Note 10), the Company’s landlord (the “Landlord”) purchased 49,254 shares of Series B Preferred at $2.0303 per share for total proceeds of $0.1 million and 488,888 shares of Series D Preferred at $4.50 per share for total proceeds of $2.2 million . Additionally, in April 2015, the Landlord participated in the Company’s bridge loan in the amount of $0.2 million , which converted into Series E Preferred (see Note 10). The Landlord paid the same price as the price paid by other investors in each of these Preferred Stock purchases. At the IPO, all preferred stock was converted to common stock. The Company incurred expenses for consulting services provided by its founders totaling $0.1 million during each of the three months ended September 30, 2016 and 2015 , and $0.2 million for each of the nine months ended September 30, 2016 and 2015 .

Technology License Agreements

Technology License Agreements	9 Months Ended
	Sep. 30, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Technology License Agreements	Technology License Agreements MIT On November 25, 2008, the Company entered into an Exclusive Patent License agreement with the Massachusetts Institute of Technology (“MIT”). 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. 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, the Company was granted an exclusive license to certain 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. 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 SVP technology with pegsiticase 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 pegsiticase 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 pegsiticase 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 U.S. 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 SVP 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. 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 SVP technology, and up to an aggregate of $41.5 million for products without the Company’s SVP 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 pegsiticase component of products at percentages ranging from the low‑to‑mid single digits for products containing the Company’s SVP 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 SVP 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”) 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 Agreement, 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 $4,175,000 to $37,025,000 and sales milestones on a licensed product‑by‑licensed product basis, totaling up to an aggregate of between $50,000,000 to $70,000,000 ; 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.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company did not provide for any income taxes in any of the three or nine month periods ended September 30, 2016 or 2015 . The Company has evaluated the positive and negative evidence bearing upon the realizability of its U.S. net deferred tax assets. As required by the provisions of ASC 740, Income Taxes, management has determined that it is more-likely-than-not that the Company will not utilize the benefits of federal and state U.S. net deferred tax assets for financial reporting purposes. Accordingly, the net deferred tax assets are subject to a valuation allowance at September 30, 2016 and December 31, 2015 .

401(k) Savings Plan

401(k) Savings Plan	9 Months Ended
	Sep. 30, 2016
Compensation and Retirement Disclosure [Abstract]
401(k) Savings Plan	401(k) Savings Plan The Company maintains a defined‑contribution savings 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 totaling less than $0.1 million for the three months ended September 30, 2016 and 2015 , and $0.1 million for each of the nine months ended September 30, 2016 and 2015 , have been recorded in the consolidated statements of operations and comprehensive loss.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2016
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events As previously disclosed in our Current Report on Form 8-K, filed with the SEC on November 10, 2016, and as discussed in Note 13, on November 9, 2016, we received written notice from Sanofi that Sanofi has elected to terminate in its entirety the Sanofi Agreement. The termination of the Sanofi Agreement will be effective on May 8, 2017, which is six months from the date of the notice.

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2016
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 Currency The functional currency of Selecta (RUS) is the 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, the fair value of common stock and other equity instruments, accounting for stock-based compensation, income taxes, collectability of accounts receivable, useful lives of long-lived assets, accrued expenses, and accounting for project development. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. The Company’s management made significant estimates and assumptions in determining the fair value of its common stock for those periods reported prior to the completion of the IPO. The Company utilized various valuation methodologies in accordance with the framework of the 2004 American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology included estimates and assumptions that require the Company’s judgment. These estimates and assumptions included a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector, the prices at which the Company sold shares of preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an initial public offering or sale. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.
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 operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.
Cash Equivalents and Short Term Investments	Cash Equivalents and Short Term Investments 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. The Company, as part of its cash management strategy, may invest in reverse repurchase agreements. All reverse repurchase agreements are tri-party and have maturities of three months or less at the time of investment. These agreements are collateralized by U.S. treasury securities for an amount no less than 102% of their value. 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.
Concentrations of Credit Risk and Off-Balance Sheet Risk	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 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 receivable, accounts payable, loans payable, common stock warrants, and redeemable convertible preferred 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. The carrying amount of loans payable approximates their estimated fair value. 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. 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.
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, five years for equipment and three years for computer and office 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	Impairment of Long‑Lived Assets The Company periodically evaluates its long‑lived assets for potential impairment. Impairment is assessed when there is evidence that events or changes in circumstances indicate that the carrying amount of an asset may not be recovered. Recoverability of these assets is assessed based on undiscounted expected future cash flows from the assets, considering a number of factors, including past operating results, budgets and economic projections, market trends, and product development cycles. Impairment in the carrying value of each asset is assessed when the undiscounted expected future cash flows derived from the asset are less than their carrying value.
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are accounted for as additional debt discount and 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.
Revenue Recognition	Revenue Recognition The Company’s revenue is primarily generated from research grants in both the United States and Russia, and, prior to its termination, a license and research collaboration agreement with Sanofi. The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition . Accordingly, revenue is recognized when all of the following criteria are 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. Amounts received prior to satisfying the revenue recognition criteria are 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 are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.
Collaborative Revenue	Collaboration Revenue When evaluating multiple element arrangements such as the agreement with Sanofi discussed in Note 13, the Company considers whether the deliverables under the arrangement represent separate units of accounting. This evaluation requires subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. In determining the units of accounting, management evaluates certain criteria, including whether the deliverables have standalone value, based on the consideration of the relevant facts and circumstances for each arrangement. The consideration received is allocated among the separate units of accounting using the relative selling price method, and the applicable revenue recognition criteria are applied to each of the separate units. The Company determines 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 is available. Determining the best estimate of selling price for a deliverable requires significant judgment. The Company has used its best estimate of selling price to estimate the selling price for licenses to the Company’s proprietary technology, since the Company does not have VSOE or TPE of selling price for these deliverables. In those circumstances, the Company considers market conditions as well as entity‑specific factors, including those factors contemplated in negotiating the agreements, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating the Company’s best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine the best estimate of selling price will have a significant effect on the allocation of arrangement consideration between multiple deliverables. The Company may receive upfront payments when licensing its intellectual property in conjunction with a research and development agreement. When management believes the license to its intellectual property does not have stand‑alone value from the other deliverables to be provided in the arrangement, the Company generally recognizes revenue attributed to the license over the Company’s contractual or estimated performance period. When management believes the license to its intellectual property has stand‑alone value, the Company generally recognizes revenue attributed to the license upon delivery. The periods over which revenue should be recognized are subject to estimates by management and may change over the course of the research and development agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Payments or reimbursements resulting from the Company’s research and development efforts are recognized as the services are performed. At the inception of each agreement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific and other risks that must be overcome to achieve the milestone, as well as the level of effort and investment required. Revenues from milestones, if they are nonrefundable and deemed substantive, are recognized upon successful accomplishment of the milestones. Milestones that are not considered substantive are accounted for as license payments and recognized over the remaining period of performance.
Grant Agreements	Grant Agreements Grant revenue is generally recognized as the related research and development work is performed. Grant arrangements frequently include payment milestones which the Company has judged to be non‑substantive milestones as they are typically entitled to receive payment regardless of the outcome of the research work. Revenue under such arrangements is recognized using a proportional performance method, but not in excess of cash actually received. Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets.
Research and Development Costs	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 Costs Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activation, and other information provided to the Company by its vendors.
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 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. 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. Should such costs be incurred, they would be classified as a component of income tax expense.
Warrants	Warrants The Company has issued common stock warrants and redeemable convertible preferred stock warrants to investors and lenders. Common stock warrants are classified as a component of permanent equity because they are freestanding financial instruments that are legally detachable and separately exercisable from other debt and equity instruments, are contingently exercisable, do not embody an obligation for the Company to repurchase its own shares, and permit the holders to receive a fixed number of common shares upon exercise. In addition, such warrants require physical settlement and do not provide any guarantee of value or return. Common stock warrants are initially recorded at their issuance date fair value and are not subsequently re‑measured. These warrants are valued using the Black‑Scholes option pricing model (“Black‑Scholes”).
Stock-Based Compensation	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 of stock option grants and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. Stock‑based compensation awarded to non‑employees are subject to revaluation over their vesting terms. 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 financial statements is based on awards that are ultimately expected to vest.
Comprehensive Loss	Comprehensive Loss Comprehensive 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 loss consists of both: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss.
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after giving consideration to 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 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	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.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014‑9, Revenue from Contracts with Customers (“ASU 2014‑9”), which amends the guidance for revenue recognition to replace numerous industry‑ specific requirements. ASU 2014‑9 implements a five‑step process for customer contract revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards. ASU 2014‑9 also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenues and cash flows from contracts with customers. Other major provisions include ensuring the time value of money is considered in the transaction price, and allowing estimates of variable consideration to be recognized before contingencies are resolved in certain circumstances. The amendments in ASU 2014‑9 are effective for reporting periods beginning after December 15, 2017. Early adoption is permitted, but not before December 15, 2016. Entities can transition to the standard either retrospectively or as a cumulative‑effect adjustment as of the date of adoption. The Company is currently in the process of evaluating the effect the adoption of ASU 2014‑9 may have on its financial statements. In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern. ASU 2014-15 requires management of all entities to evaluate whether there are conditions and events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the financial statements are issued, and to make certain disclosures if it concludes that substantial doubt exists or when its plans alleviate substantial doubt about the entity’s ability to continue as a going concern. ASU 2014-15 is effective for the Company for annual reporting periods beginning in 2016 and for interim reporting periods starting in the first quarter of 2017. The Company is currently evaluating the impact that the standard will have on the financial statements, and has not yet determined what effect, if any, the impact of adoption will be. In February 2016, FASB issued ASU No.2016‑2, Leases (“ASU 2016‑2”). ASU 2016‑2 requires a lessee to separate the lease components from the non‑lease components in a contract and recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right‑of‑use asset representing its right to use the underlying asset for the lease term. It also aligns lease accounting for lessors with the revenue recognition guidance in ASU 2014‑9. ASU 2016‑2 is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years, and is to be applied at the beginning of the earliest period presented using a modified retrospective approach. In March 2016, the FASB issued ASU 2016-9, Improvements to Employee Share-Based Payment Accounting, which simplifies several aspects of accounting for share-based payment transactions including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The new standard is effective for fiscal years beginning after December 15, 2016 and interim periods within those fiscal years with early adoption permitted. The Company is evaluating the impact that the adoption of this standard will have on its financial statements.

Available-for-Sale Marketable26

Available-for-Sale Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2016
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 September 30, 2016 and December 31, 2015 (in thousands): September 30, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value U.S. Treasury securities $ — $ — $ — $ — Corporate bonds 19,804 6 (23 ) 19,788 Total available-for-sale marketable securities $ 19,804 $ 6 $ (23 ) $ 19,788 December 31, 2015 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value U.S. Treasury securities $ 3,516 $ — $ — $ 3,516 Corporate bonds — — — — Total available-for-sale marketable securities $ 3,516 $ — $ — $ 3,516
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): September 30, 2016 December 31, 2015 Fair Value Amortized Cost Fair Value Amortized Cost Less than one year $ 19,788 $ 19,804 $ 3,516 $ 3,516

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2016
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): Three Months Ended Nine Months Ended 2016 2015 2016 2015 (unaudited) (unaudited) Numerator: Net (loss) $ (7,728 ) $ (5,725 ) $ (22,127 ) $ (18,325 ) Less: accretion on preferred stock — (1,836 ) (4,566 ) (4,959 ) Net loss attributable to common stockholders $ (7,728 ) $ (7,561 ) $ (26,693 ) $ (23,284 ) Denominator: Weighted‑average common shares outstanding—basic and diluted 18,108,014 2,159,658 7,881,625 2,144,731 Net loss per share attributable to common stockholders—basic and diluted $ (0.43 ) $ (3.50 ) $ (3.39 ) $ (10.86 )
Potential Common Shares Issuable Upon Conversion of Warrants to Purchase Common Stock and Stock Options Excluded from Computation of Diluted Weighted Average Shares Outstanding	Potential common shares issuable upon conversion of warrants to purchase common stock and stock options that are excluded from the computation of diluted weighted average shares outstanding are as follows: September 30, 2016 2015 (unaudited) Redeemable convertible preferred stock — 6,872,090 Stock options to purchase common stock 2,140,295 2,165,187 Stock warrants to purchase common stock 113,795 17,888 Total 2,254,090 9,055,165

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured and Carried at Fair Value	The tables below present information about the Company’s financial assets and liabilities that are measured and carried at fair value as of September 30, 2016 and December 31, 2015 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): September 30, 2016 (level 1) (level 2) (level 3) Total Money market funds, included in cash equivalents $ 67 $ — $ — $ 67 Tri-party repurchase agreements, included in cash equivalents $ — $ 32,000 $ — $ 32,000 US Treasury obligations, included in investments $ — $ — $ — $ — Corporate bonds, included in investments $ 19,788 $ — $ — $ 19,788 Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities $ — $ — $ — $ — December 31, 2015 (level 1) (level 2) (level 3) Total US Treasury obligations, included in cash equivalents $ 14,486 $ — $ — $ 14,486 US Treasury obligations, included in investments $ 3,516 $ — $ — $ 3,516 Corporate bonds, included in investments $ — $ — $ — $ — Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities $ — $ — $ 290 $ 290

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): September 30, December 31, 2016 2015 (unaudited) Laboratory equipment $ 4,617 $ 4,028 Computer equipment and software 501 409 Leasehold improvements 165 91 Furniture and fixtures 225 222 Office equipment 62 62 P,P&E—Construction in process — 144 Total property and equipment 5,570 4,956 Less accumulated depreciation (3,504 ) (2,927 ) Property and equipment, net $ 2,066 $ 2,029

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Summary of Accrued Expenses	Accrued expenses consist of the following (in thousands): September 30, December 31, 2016 2015 (unaudited) Payroll $ 297 $ — Legal 59 213 Bonus 519 669 Current portion of deferred rent and lease incentive 12 405 Accrued patent fees 184 219 Accrued external research and development costs 662 1,649 Other 701 223 Accrued expenses $ 2,434 $ 3,378

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2016
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments as Amended by Lease Amendment	As of December 31, 2015 , the future minimum lease payments under the lease, as amended by the lease amendment are as follows (in thousands): Year ended December 31, 2016 $ 1,176 2017 1,244 2018 1,291 2019 1,330 2020 335 Total minimum lease payments $ 5,376

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Future Minimum Payments on the Term Loans	Future minimum payments on the Term Loans as of December 31, 2015 are as follows (in thousands): Year ended December 31, 2016 $ 972 2017 5,319 2018 5,318 2019 3,379 Total debt payments 14,988 Less: Amount representing interest (2,268 ) Less: Debt discount and deferred charges (919 ) Less: Current portion of issuance costs 54 Loans payable, net of current portion $ 11,855

Common Stock (Tables)

Common Stock (Tables)	9 Months Ended
	Sep. 30, 2016
Stockholders' Equity Note [Abstract]
Schedule of authorized shares of common stock reserved for future issuance	The Company has authorized shares of common stock for future issuance as follows: Periods ending September 30, 2016 December 31, 2015 (unaudited) Conversion of Series A Preferred — 664,068 Conversion of Series B Preferred — 1,907,006 Conversion of Series C Preferred — 1,282,051 Conversion of Series D Preferred — 2,191,412 Conversion of Series SRN Preferred — 1,798,433 Conversion of Series E Preferred — 2,662,885 Exercise of common warrants 113,795 651,618 Shares available for future stock incentive awards 783,494 100,034 Exercise of outstanding common stock options 2,140,295 1,569,379 Total 3,037,584 12,826,886

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	9 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Weighted Average Assumptions Used for Stock Option Grants	The weighted average assumptions used for employee stock option grants issued for the nine month period ended September 30, 2016 and in 2015 : Nine Months Ended September 30, 2016 2015 Risk-free interest rate 1.39 % 1.71 % Expected dividend yield — — Expected life 6.05 6.00 Expected volatility 95.62 % 84.60 % Weighted-average fair value of common stock $ 9.79 $ 1.41 The weighted average assumptions used for unvested non‑employee stock options are as follows: Nine Months Ended September 30, 2016 2015 Risk‑free interest rate 1.58 % 1.82 % Expected dividend yield — — Expected life (in years) 9.41 6.36 Expected volatility 87.75 % 96.75 %
Summary of Activity	The following table summarizes the activity under the Plan and the 2016 Plan since December 31, 2015 : Weighted‑average Weighted‑average remaining Aggregate Number of exercise contractual term intrinsic value options price (in years) (in thousands) Employee Outstanding at December 31, 2015 1,247,160 $ 5.05 7.41 $ 3,130 Granted 511,039 $ 12.60 Exercised (21,088 ) $ 2.16 Forfeited (9,124 ) $ 7.74 Outstanding at September 30, 2016 1,727,987 $ 7.30 7.57 $ 12,004 Vested at September 30, 2016 805,229 $ 3.70 5.62 $ 8,493 Vested and expected to vest at September 30, 2016 1,315,411 $ 5.23 7.41 $ 11,582 Non‑Employee Outstanding at December 31, 2015 322,220 $ 2.93 5.50 $ 1,288 Granted 98,718 $ 7.70 Exercised (5,085 ) $ 0.47 Forfeited (3,546 ) $ 0.47 Outstanding at September 30, 2016 412,307 $ 4.12 5.94 $ 4,176 Vested at September 30, 2016 325,519 $ 3.12 5.03 $ 3,622 Vested and expected to vest at September 30, 2016 412,097 $ 4.12 5.93 $ 4,176
Summary of Restricted Stock Award Activity	The following table summarizes the restricted stock award activity of the 2008 Plan and the 2016 Plan since December 31, 2015 : Weighted‑ average exercise Shares price Unvested as of December 31, 2015 7,574 $ 2.77 Issued 940 9.36 Vested (6,647 ) 3.72 Unvested as of September 30, 2016 1,867 $ 2.77
Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	The Company recorded stock-based compensation expense related to stock options and restricted common stock in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Research and development $ 353 $ 40 $ 747 $ 321 General and administrative 280 212 534 548 Total $ 633 $ 252 $ 1,281 $ 869

Nature of the Business and Ba35

Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands	Jul. 25, 2016USD ($)$ / sharesshares	Jun. 21, 2016USD ($)$ / sharesshares	Jun. 07, 2016	Jul. 25, 2016USD ($)$ / shares	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Jun. 27, 2016shares
Subsidiary, Sale of Stock [Line Items]
Net proceeds from initial public offering | $					$ 68,871	$ 0
Common stock issued for convertible preferred stock (in shares) | shares							10,126,118
IPO
Subsidiary, Sale of Stock [Line Items]
Reverse stock split ratio			0.2564
Common stock issued and sold (in shares) | shares		5,000,000
Share price (in dollars per share) | $ / shares		$ 14
Net proceeds from initial public offering | $		$ 60,800		$ 64,500
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Common stock issued and sold (in shares) | shares	289,633
Share price (in dollars per share) | $ / shares	$ 14			$ 14
Net proceeds from initial public offering | $	$ 3,700

Nature of the Business and Ba36

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 137,493	$ 111,508
Russian subsidiary
Cash and Cash Equivalents [Line Items]
Unrestricted cash held by subsidiary	$ 2,800	$ 3,000

Summary of Significant Accoun37

Summary of Significant Accounting Policies - Segment Information (Details)	9 Months Ended
	Sep. 30, 2016segment
Accounting Policies [Abstract]
Number of operating segments	1

Summary of Significant Accoun38

Summary of Significant Accounting Policies - Cash Equivalents and Short Term Investments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Cash and Cash Equivalents [Line Items]
Realized gains or losses on sale of investments	$ 0	$ 0	$ 0	$ 0
Minimum
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents collateralized by government securities (as a percent)	102.00%		102.00%

Summary of Significant Accoun39

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) $ in Millions	Sep. 30, 2016USD ($)
Rubles
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 3.1
U S dollars
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 2.8

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Property and Equipment (Details)	9 Months Ended
	Sep. 30, 2016
Furniture
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Warrants (Details)	Jun. 27, 2016$ / shares
Accounting Policies [Abstract]
Convertible preferred stock underlying share fair value price (in dollars per share)	$ 14

Available-for-Sale Marketable42

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 19,804	$ 3,516
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(23)	0
Estimated Fair Value	19,788	3,516
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	3,516
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	0	3,516
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,804	0
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(23)	0
Estimated Fair Value	$ 19,788	$ 0

Available-for-Sale Marketable43

Available-for-Sale Marketable Securities - Schedule of Fair Values and Amortized Cost of Marketable Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value
Less than one year	$ 19,788	$ 3,516
Amortized Cost
Less than one year	$ 19,804	$ 3,516

Available-for-Sale Marketable44

Available-for-Sale Marketable Securities - Narrative (Details)	9 Months Ended
	Sep. 30, 2016USD ($)position
Investments, Debt and Equity Securities [Abstract]
Number of positions in an unrealized loss position, less than 12 months	11
Number of investment positions	13
Other-than-temporary impairments | $	$ 0

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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net (loss)	$ (7,728)	$ (5,725)	$ (22,127)	$ (18,325)
Less: accretion on preferred stock	0	(1,836)	(4,566)	(4,959)
Net loss attributable to common stockholders	$ (7,728)	$ (7,561)	$ (26,693)	$ (23,284)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)	18,108,014	2,159,658	7,881,625	2,144,731
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.43)	$ (3.50)	$ (3.39)	$ (10.86)

Net Loss Per Share - Potential

Net Loss Per Share - Potential Common Shares Issuable Upon Conversion of Warrants to Purchase Common Stock and Stock Options Excluded from Computation of Diluted Weighted Average Shares Outstanding (Details) - shares	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Potential common shares
Total (in shares)	2,254,090	9,055,165
Redeemable convertible preferred stock
Potential common shares
Total (in shares)	0	6,872,090
Stock options to purchase common stock
Potential common shares
Total (in shares)	2,140,295	2,165,187
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	113,795	17,888

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets and Liabilities Measured and Carried at Fair Value (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	$ 0	$ 290
(level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	0	0
(level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	0	0
(level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase redeemable convertible preferred stock, included in other long term liabilities	0	290
US Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	3,516
US Treasury obligations | (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	3,516
US Treasury obligations | (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
US Treasury obligations | (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Corporate bonds, included in investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	19,788	0
Corporate bonds, included in investments | (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	19,788	0
Corporate bonds, included in investments | (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Corporate bonds, included in investments | (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Money market funds, included in cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	67
Money market funds, included in cash equivalents | (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	67
Money market funds, included in cash equivalents | (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Money market funds, included in cash equivalents | (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Tri-party repurchase agreements, included in cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	32,000
Tri-party repurchase agreements, included in cash equivalents | (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Tri-party repurchase agreements, included in cash equivalents | (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	32,000
Tri-party repurchase agreements, included in cash equivalents | (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 0
US Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		14,486
US Treasury obligations | (level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		14,486
US Treasury obligations | (level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		0
US Treasury obligations | (level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - $ / shares	Jun. 21, 2016	Jul. 24, 2015	Jul. 31, 2015	Dec. 31, 2015	Sep. 30, 2016	Jul. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Average maturity for US Treasury obligations, term				106 days
Warrants issued for purchase of stock (in shares)		80,813	80,813
Term of warrants		3 years	3 years
Exercise price (in dollars per share)			$ 17.55
Conversion of Series D Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)						66,668
Term of warrants	4 years
Exercise price (in dollars per share)						$ 4.50
Conversion of Series E Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)				37,978
Term of warrants	4 years
Exercise price (in dollars per share)				$ 4.50
Convertible Debt | Conversion of Series D Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)	17,888
Exercise price (in dollars per share)	$ 16.77
Convertible Debt | Conversion of Series E Preferred
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued for purchase of stock (in shares)	15,094
Exercise price (in dollars per share)	$ 11.32
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents collateralized by government securities (as a percent)					102.00%

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 5,570	$ 4,956
Less accumulated depreciation	(3,504)	(2,927)
Property and equipment, net	2,066	2,029
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	4,617	4,028
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	501	409
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	165	91
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	225	222
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	62	62
P,P&E—Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 0	$ 144

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 100	$ 200	$ 545	$ 866

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Payroll	$ 297	$ 0
Legal	59	213
Bonus	519	669
Current portion of deferred rent and lease incentive	12	405
Accrued patent fees	184	219
Accrued external research and development costs	662	1,649
Other	701	223
Accrued expenses	$ 2,434	$ 3,378

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2012USD ($)	Apr. 30, 2015ft²
Sublease income recorded as a reduction of lease expense					$ 200,000
Deferred rent and lease incentive liability	$ 200,000		$ 200,000		500,000
Current portion of deferred rent and lease incentive	12,000		12,000		405,000
Rent expense, net of sublease payments	400,000	$ 500,000	1,100,000	$ 1,000,000
Restricted cash and other deposits | Letter of Credit
Letter of credit	300,000		300,000
Accrued expenses
Current portion of deferred rent and lease incentive	$ 100,000		$ 100,000		$ 400,000
Laboratory and office space
Area exchanged (in square feet) | ft²							13,711
Laboratory And Office Space Agreement Amendment April 2015
Area exchanged (in square feet) | ft²							15,174
Maximum | Laboratory and office space
Tenant improvement allowance						$ 700,000

Commitments and Contingencies53

Commitments and Contingencies - Schedule of Future Minimum Lease Payments as Amended by Lease Amendment (Details) $ in Thousands	Dec. 31, 2015USD ($)
Year ended December 31,
2,016	$ 1,176
2,017	1,244
2,018	1,291
2,019	1,330
2,020	335
Total minimum lease payments	$ 5,376

Debt - Term Loans (Narrative) (

Debt - Term Loans (Narrative) (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2014USD ($)shares	Aug. 31, 2013USD ($)shares	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)installment	Sep. 30, 2015USD ($)	Jun. 21, 2016USD ($)shares	Dec. 31, 2015USD ($)shares	Jul. 31, 2015shares	Jul. 24, 2015shares	Aug. 09, 2013USD ($)lender
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares) | shares									80,813	80,813
Interest expense			$ 311,000	$ 334,000	$ 931,000	$ 843,000
Conversion of Series D Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares) | shares	66,668
Conversion of Series E Preferred
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares) | shares								37,978
Term Loan
Line of Credit Facility [Line Items]
Number of lenders | lender											2
Loan and security agreement, maximum borrowing capacity								$ 12,000,000			$ 7,500,000
Borrowings on loan and security agreement	$ 4,500,000	$ 3,000,000
Outstanding principal balance under Term Loans			12,000,000		$ 12,000,000			$ 12,000,000
Interest rate of Term Loans					8.10%
Number of monthly installments | installment					30
Final payment fee (as a percent)					6.00%
Total final payment fee					$ 700,000
Initial grant date fair value of warrants	$ 100,000	$ 100,000					$ 100,000
Interest expense			$ 300,000	$ 100,000	$ 900,000	$ 500,000
Term Loan | Prepaid prior to first anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)			3.00%		3.00%
Term Loan | Prepaid prior to first anniversary but before second
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)			2.00%		2.00%
Term Loan | Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)			1.00%		1.00%
Term Loan | Conversion of Series D Preferred | Maximum
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares) | shares	40,000	26,668					66,668
Term Loan | Conversion of Series D Preferred | Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares) | shares							17,888
Term Loan | Conversion of Series E Preferred | Common stock
Line of Credit Facility [Line Items]
Warrants issued for purchase of stock (in shares) | shares							15,094

Debt - Future Minimum Payments

Debt - Future Minimum Payments on the Term Loans (Details) - Term Loan $ in Thousands	Dec. 31, 2015USD ($)
Year ended December 31,
2,016	$ 972
2,017	5,319
2,018	5,318
2,019	3,379
Total debt payments	14,988
Less: Amount representing interest	(2,268)
Less: Debt discount and deferred charges	(919)
Less: Current portion of issuance costs	54
Loans payable, net of current portion	$ 11,855

Debt - Convertible Notes (Narra

Debt - Convertible Notes (Narrative) (Details) - USD ($)	Jun. 21, 2016	Jul. 24, 2015	Jul. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Apr. 30, 2015
Debt Instrument [Line Items]
Exercise price (in dollars per share)			$ 17.55
Warrants issued for purchase of common stock (in shares)		80,813	80,813
Term of warrants		3 years	3 years
Conversion of Series E Preferred
Debt Instrument [Line Items]
Exercise price (in dollars per share)								$ 4.50
Warrants issued for purchase of common stock (in shares)								37,978
Term of warrants	4 years
Convertible Notes Payable
Debt Instrument [Line Items]
Interest rate of convertible notes									8.00%
Percentage of convertible notes for which warrants were issued to purchase common stock									20.00%
Exercise price (in dollars per share)		$ 17.55
Interest expense related to convertible notes				$ 0	$ 100,000	$ 0	$ 300,000
Convertible Notes Payable | Conversion of Series E Preferred
Debt Instrument [Line Items]
Shares issued on conversion of outstanding convertible notes (in shares)		1,619,550
Convertible Notes Payable | Common stock
Debt Instrument [Line Items]
Exercise price (in dollars per share)									$ 17.55
Convertible Notes Payable | Common stock | Conversion of Series E Preferred | Maximum
Debt Instrument [Line Items]
Shares issued on conversion of outstanding convertible notes (in shares)		103,817

Preferred Stock (Details)

Preferred Stock (Details) - USD ($)	Jun. 27, 2016	Jul. 24, 2015	Jul. 31, 2014	Sep. 30, 2015	Aug. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001
Preferred stock
Carrying value of Series SRN Preferred						$ 0		$ 137,482,000
Issuance costs						$ 4,068,000	$ 0
Conversion of convertible stock upon listing (in shares)	10,126,118
Conversion of Series A Preferred
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001		$ 0.0001
Preferred stock
Convertible preferred stock, shares issued (in shares)						0		2,589,868
Carrying value of Series SRN Preferred						$ 0		$ 3,644,000
Conversion of convertible stock upon listing (in shares)						(2,589,868)
Conversion of Series B Preferred
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001		$ 0.0001
Preferred stock
Convertible preferred stock, shares issued (in shares)						0		7,437,325
Carrying value of Series SRN Preferred						$ 0		$ 21,448,000
Conversion of convertible stock upon listing (in shares)						(7,437,325)
Conversion of Series C Preferred
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001		$ 0.0001
Preferred stock
Convertible preferred stock, shares issued (in shares)						0		5,000,002
Carrying value of Series SRN Preferred						$ 0		$ 20,178,000
Conversion of convertible stock upon listing (in shares)						(5,000,002)
Conversion of Series D Preferred
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001		$ 0.0001
Preferred stock
Convertible preferred stock, shares issued (in shares)						0		8,099,994
Additional shares issued (in shares)					3,211,105
Price per share (in dollars per share)					$ 4.50
Net proceeds					$ 14,349,239
Carrying value of Series SRN Preferred						$ 0		$ 42,902,000
Conversion of convertible stock upon listing (in shares)						(8,099,994)
Conversion of Series SRN Preferred
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001		$ 0.0001
Preferred stock
Convertible preferred stock, shares issued (in shares)						0		2,111,109
Additional shares issued (in shares)			1,333,332
Price per share (in dollars per share)			$ 4.50
Net proceeds			$ 5,800,000
Conversion price (in dollars per share)			$ 16.77
Carrying value of Series SRN Preferred			$ 5,000,000			$ 0		$ 12,082,000
Difference recorded in additional paid in capital			$ 1,500,000
Conversion of convertible stock upon listing (in shares)						(2,111,109)
Conversion of Series E Preferred
Temporary Equity Disclosure [Abstract]
Convertible preferred stock, par value (in dollars per share)						$ 0.0001		$ 0.0001
Preferred stock
Convertible preferred stock, shares issued (in shares)						0		8,888,888
Additional shares issued (in shares)				7,269,338
Price per share (in dollars per share)				$ 4.50			$ 4.50
Carrying value of Series SRN Preferred						$ 0		$ 37,228,000
Gross proceeds				$ 32,700,000
Issuance costs				$ 200,000
Warrants to purchase common stock, percentage on preferred shares		25.00%
Conversion of convertible stock upon listing (in shares)						(8,888,888)
Conversion of Series E Preferred | Convertible Notes Payable
Preferred stock
Additional shares issued (in shares)		1,619,550

Common Stock (Details)

Common Stock (Details)	9 Months Ended
	Sep. 30, 2016USD ($)shares	Dec. 31, 2015shares
Stockholders' Equity Note [Abstract]
Votes per share of common stock	1
Common stock dividend | $	$ 0
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	3,037,584	12,826,886
Exercise of common warrants
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	113,795	651,618
Shares available for future stock incentive awards
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	783,494	100,034
Stock options to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	2,140,295	1,569,379
Conversion of Series A Preferred
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	664,068
Conversion of Series B Preferred
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	1,907,006
Conversion of Series C Preferred
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	1,282,051
Conversion of Series D Preferred
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	2,191,412
Conversion of Series SRN Preferred
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	1,798,433
Conversion of Series E Preferred
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	2,662,885

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Narrative) (Details) $ / shares in Units, $ in Millions	Jun. 21, 2016shares	Sep. 30, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (in shares)		3,037,584	12,826,886
Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (in shares)		783,494	100,034
Weighted-average fair value of stock options (in dollars per share) | $ / shares		$ 9.79	$ 5.03
Aggregate intrinsic value | $		$ 0.1	$ 0.3
Unrecognized compensation expense related to unvested employee stock options | $		$ 5.5	$ 2.3
Weighted average period for recognizing, unrecognized compensation expense		3 years 2 months 12 days	3 years 1 month 6 days
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non‑employee stock‑based compensation expense | $		$ 0.2	$ 0.4
Unrecognized compensation expense related to unvested non-employee stock options | $		$ 1	$ 0.1
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognizing, unrecognized compensation expense		3 months	1 year
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants		2,213,412
Common stock reserved for future issuance (in shares)		10,163
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
Forfeitures rate		10.00%
2008 Plan | Shares available for future stock incentive awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employees holding voting rights of all classes of stock (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]
Employees holding voting rights of all classes of stock (as a percent)		10.00%
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants		1,210,256
Common stock reserved for future issuance (in shares)		783,494
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]
Employees holding voting rights of all classes of stock (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]
Employees holding voting rights of all classes of stock (as a percent)	10.00%
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (in shares)	173,076
Maximum percentage of shares available may increase each year	0.01
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 options on common stock (as a percent)		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 options on common stock (as a percent)	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 options on common stock (as a percent)		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 options on common stock (as a percent)	110.00%

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Weighted Average Assumptions for Employee Stock Option Grants (Details) - 2008 Plan - Shares available for future stock incentive awards - $ / shares	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.39%	1.71%
Expected dividend yield	0.00%	0.00%
Expected life	6 years 18 days	6 years
Expected volatility	95.62%	84.60%
Weighted-average fair value of common stock (in dollars per share)	$ 9.79	$ 1.41

Stock Incentive Plans - Sched61

Stock Incentive Plans - Schedule of Weighted Average Assumptions for Unvested Non Employee Stock Options (Details) - Non-employee stock options	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Share-based Goods and Nonemployee Services Transaction [Line Items]
Risk‑free interest rate	1.58%	1.82%
Expected dividend yield	0.00%	0.00%
Expected life (in years)	9 years 4 months 28 days	6 years 4 months 10 days
Expected volatility	87.75%	96.75%

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Employee
Aggregate intrinsic value
Vested and expected to vest	$ 100	$ 300
The Plan and the 2016 Plan | Employee
Number of Options
Beginning balance (in shares)	1,247,160
Granted (in shares)	511,039
Exercised (in shares)	(21,088)
Forfeited (in shares)	(9,124)
Ending balance (in shares)	1,727,987	1,247,160
Vested (in shares)	805,229
Vested and expected to vest (in shares)	1,315,411
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 5.05
Granted (in dollars per share)	12.60
Exercised (in dollars per share)	2.16
Forfeited (in dollars per share)	7.74
Ending balance (in dollars per share)	7.30	$ 5.05
Vested (in dollars per share)	3.70
Vested and expected to vest (in dollars per share)	$ 5.23
Weighted-average remaining contractual term
Outstanding (period)	7 years 6 months 26 days	7 years 4 months 28 days
Vested (period)	5 years 7 months 13 days
Vested and expected to vest (period)	7 years 4 months 28 days
Aggregate intrinsic value
Outstanding	$ 12,004	$ 3,130
Vested	8,493
Vested and expected to vest	$ 11,582
The Plan and the 2016 Plan | Non‑Employee
Number of Options
Beginning balance (in shares)	322,220
Granted (in shares)	98,718
Exercised (in shares)	(5,085)
Forfeited (in shares)	(3,546)
Ending balance (in shares)	412,307	322,220
Vested (in shares)	325,519
Vested and expected to vest (in shares)	412,097
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 2.93
Granted (in dollars per share)	7.70
Exercised (in dollars per share)	0.47
Forfeited (in dollars per share)	0.47
Ending balance (in dollars per share)	4.12	$ 2.93
Vested (in dollars per share)	3.12
Vested and expected to vest (in dollars per share)	$ 4.12
Weighted-average remaining contractual term
Outstanding (period)	5 years 11 months 9 days	5 years 6 months
Vested (period)	5 years 11 days
Vested and expected to vest (period)	5 years 11 months 5 days
Aggregate intrinsic value
Outstanding	$ 4,176	$ 1,288
Vested	3,622
Vested and expected to vest	$ 4,176

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock (Narrative) (Details) - Restricted Stock Units (RSUs) - USD ($) $ in Millions	9 Months Ended	12 Months Ended			24 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued upon the early exercise of stock options				30,317
Number of shares issued			2,564
Payments received for the purchase of restricted shares					$ 0.1
Outstanding liability (less than)	$ 0.1	$ 0.1
Total fair value of restricted shares (less than)	0.1
Unrecognized compensation expense related to unvested employee stock options (less than)	$ 0.1	$ 0.1
Weighted average period for recognizing, unrecognized compensation expense	3 months	1 year

Stock Incentive Plans - Summa64

Stock Incentive Plans - Summary of Restricted Stock Award Activity (Details) - The 2008 Plan and the 2016 Plan - Restricted Stock Units (RSUs)	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Shares
Beginning balance (in shares) | shares	7,574
Issued (in shares) | shares	940
Vested (in shares) | shares	(6,647)
Ending balance (in shares) | shares	1,867
Weighted-average exercise price
Beginning balance (in dollars per share) | $ / shares	$ 2.77
Issued (in dollars per share) | $ / shares	9.36
Vested (in dollars per share) | $ / shares	3.72
Ending balance (in dollars per share) | $ / shares	$ 2.77

Stock Incentive Plans - Stock-B

Stock Incentive Plans - Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	$ 633	$ 252	$ 1,281	$ 869
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	353	40	747	321
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total	$ 280	$ 212	$ 534	$ 548

Revenue Arrangements - Sanofi C

Revenue Arrangements - Sanofi Collaboration Agreement (Narrative) (Details) - USD ($) $ in Thousands	Nov. 27, 2012	Jul. 31, 2016	May 31, 2016	Aug. 31, 2013	Nov. 30, 2012	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Grant and collaboration revenue						$ 1,048	$ 1,607	$ 5,153	$ 3,877
Sanofi
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial payment for first indication			$ 2,000		$ 2,000
Additional payment for preclinical research for each indication				$ 3,000
Additional payment upon attaining first milestone for celiac disease indication		$ 1,000
Aggregate payments under Sanofi Agreement										$ 8,000
Initial period to complete research obligations	3 years
Grant and collaboration revenue						$ 200	$ 500	$ 600	$ 1,500

Revenue Arrangements - Terminat

Revenue Arrangements - Termination of the Sanofi Collaboration Agreement (Narrative) (Details) - Sanofi $ in Millions	47 Months Ended
	Sep. 30, 2016USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate payments under Sanofi Agreement	$ 8
Deferred revenue expected to be recognized within the termination period	$ 2.2

Revenue Arrangements - Other Re

Revenue Arrangements - Other Research and Collaboration Agreements (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Grant and collaboration revenue	$ 1,048,000	$ 1,607,000	$ 5,153,000	$ 3,877,000
Other Research And Collaboration Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Grant and collaboration revenue	$ 0	$ 300,000	$ 100,000	$ 400,000

Revenue Arrangements - NIH (Nar

Revenue Arrangements - NIH (Narrative) (Details) - NIH $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2015USD ($)	May 31, 2014USD ($)	Sep. 30, 2016USD ($)grant	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)grant	Sep. 30, 2015USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of grants | grant			2		2
First Grant
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant		$ 8.1
Revenue recognized			$ 0.7	$ 0.5	$ 3.7	$ 1.5
Second Grant
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 0.2
Revenue recognized			$ 0.1		$ 0.1

Revenue Arrangements - JDRF (Na

Revenue Arrangements - JDRF (Narrative) (Details) - JDRF - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 400,000
Revenue recognized		$ 100,000	$ 0	$ 100,000	$ 200,000

Revenue Arrangements - Bill and

Revenue Arrangements - Bill and Melinda Gates Foundation (Narrative) (Details) - Bill and Melinda Gates Foundation - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant					$ 1.6	$ 1.2
Extended research term					3 years
Revenue recognized	$ 0.1	$ 0.3	$ 0.4	$ 0.3

Revenue Arrangements - Minpromt

Revenue Arrangements - Minpromtorg (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended		12 Months Ended	20 Months Ended	30 Months Ended
	Feb. 28, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Sep. 30, 2016	Dec. 31, 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingently repayable grant funding		$ (207)	$ (446)
Minpromtorg
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant						$ 4,600
Repayment of funds	$ 200
Penalty fee, as percent of contract value	10.00%
Penalty fee	$ 200
Period to audit expenditure incurred	3 years
Contingently repayable grant funding					$ 1,400
Revenue recognized		$ 200		$ 400

Revenue Arrangements - Skolkovo

Revenue Arrangements - Skolkovo (Narrative) (Details) - USD ($)	Nov. 28, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Contingently repayable grant funding		$ (207,000)	$ (446,000)
Skolkovao
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate amount of grant	$ 2,700,000
Percentage of estimated cost of research plan	48.50%
Remaining percentage contributed by the entity	51.50%
Contingently repayable grant funding				$ 1,800,000
Period to audit expenditure incurred				3 years

Related-Party Transactions (Det

Related-Party Transactions (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Apr. 30, 2015	Aug. 31, 2014
Related Party Transaction [Line Items]
Convertible preferred stock, par value (in dollars per share)	$ 0.0001		$ 0.0001
Founder
Related Party Transaction [Line Items]
Expenses for consulting services	$ 0.1	$ 0.1	$ 0.2	$ 0.2
Conversion of Series B Preferred
Related Party Transaction [Line Items]
Issued (in shares)	0		0		7,437,325
Convertible preferred stock, par value (in dollars per share)	$ 0.0001		$ 0.0001		$ 0.0001
Conversion of Series B Preferred | Landlord
Related Party Transaction [Line Items]
Issued (in shares)	49,254		49,254
Share price (in dollars per share)	$ 2.0303		$ 2.0303
Total proceeds			$ 0.1
Conversion of Series D Preferred
Related Party Transaction [Line Items]
Issued (in shares)	0		0		8,099,994
Share price (in dollars per share)							$ 4.50
Convertible preferred stock, par value (in dollars per share)	$ 0.0001		$ 0.0001		$ 0.0001
Conversion of Series D Preferred | Landlord
Related Party Transaction [Line Items]
Issued (in shares)	488,888		488,888
Share price (in dollars per share)	$ 4.50		$ 4.50
Total proceeds			$ 2.2
Conversion of Series E Preferred
Related Party Transaction [Line Items]
Issued (in shares)	0		0		8,888,888
Share price (in dollars per share)		$ 4.50		$ 4.50
Convertible preferred stock, par value (in dollars per share)	$ 0.0001		$ 0.0001		$ 0.0001
Conversion of Series E Preferred | Landlord
Related Party Transaction [Line Items]
Bridge loan						$ 0.2

Technology License Agreements -

Technology License Agreements - MIT (Narrative) (Details) - MIT	Nov. 25, 2008USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment amount upon achievement of defined clinical milestones	$ 1,500,000
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	10.00%
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sublicense income	30.00%

Technology License Agreements76

Technology License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	May 31, 2014	Sep. 30, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period for commercialization of product	48 months
Aggregate amount of upfront and milestone based payments		$ 1
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
Period for prior written notice to terminate license	60 days

Technology License Agreements77

Technology License Agreements - Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (Narrative) (Details) - MEE	1 Months Ended
	May 31, 2016USD ($)product
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Minimum number of products marketed and sold | product	1
Payment under development milestone method	$ 4,175,000
Payment under sales milestone method	50,000,000
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payment under development milestone method	37,025,000
Payment under sales milestone method	$ 70,000,000

401(k) Savings Plan (Details)

401(k) Savings Plan (Details) - 401(k) Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Contribution Plan Disclosure [Line Items]
Vesting period			4 years
Employer contribution made	$ 0.1	$ 0.1	$ 0.1	$ 0.1