Rubius Therapeutics (RUBY) 10-K 28 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 28, 2019	Jul. 18, 2018
Document and Entity Information
Entity Registrant Name	Rubius Therapeutics, Inc.
Entity Central Index Key	0001709401
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 718.9
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		79,529,965
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 307,064	$ 104,288
Investments	96,987
Prepaid expenses and other current assets	9,737	700
Restricted cash	622
Total current assets	414,410	104,988
Property, plant and equipment, net	62,796	2,415
Restricted cash	1,735	284
Other assets	168
Total assets	479,109	107,687
Current liabilities:
Accounts payable	7,886	2,033
Accrued expenses and other current liabilities	12,118	2,986
Current portion of long-term debt		2,139
Total current liabilities	20,004	7,158
Long-term debt, net of discount and current portion	24,347	3,302
Deferred rent	143	158
Preferred stock warrant liability		866
Liability for early exercise of stock options and restricted stock	166	100
Lease liability, net of current portion	41,441
Total liabilities	86,101	11,584
Commitments and contingencies (Note 12)
Convertible preferred stock (Series A, B and C), $0.001 par value; no shares and 44,070,808 shares authorized at December 31, 2018 and 2017, respectively; no shares and 43,933,006 shares issued and outstanding at December 31, 2018 and 2017, respectively		139,790
Stockholders' equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares and no shares authorized at December 31, 2018 and 2017, respectively; no shares issued or outstanding at December 31, 2018 and 2017
Common stock, $0.001 par value; 150,000,000 and 65,000,000 shares authorized at December 31, 2018 and 2017, respectively; 79,234,853 and 14,977,317 shares issued and outstanding at December 31, 2018 and 2017, respectively	79	15
Additional paid-in capital	543,040	17,277
Accumulated other comprehensive loss	(29)
Accumulated deficit	(150,082)	(60,979)
Total stockholders' equity (deficit)	393,008	(43,687)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 479,109	$ 107,687

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Convertible preferred stock, shares authorized ( in shares)		44,070,808
Convertible preferred stock, shars issued (in shares)		43,933,006
Convertible preferred stock, shars outstanding (in shares)	0	43,933,006
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, share authorized (in shares)	10,000,000	0
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, share authorized (in shares)	150,000,000	65,000,000
Common stock, shares issued (in shares)	79,234,853	14,977,317
Common stock, shares outstanding (in shares)	79,234,853	14,977,317
Convertible preferred stock
Convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized ( in shares)	0	44,070,808
Convertible preferred stock, shars issued (in shares)	0	43,933,006
Convertible preferred stock, shars outstanding (in shares)	0	43,933,006

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 51,769	$ 21,226	$ 8,403
General and administrative	39,894	22,038	2,449
Total operating expenses	91,663	43,264	10,852
Loss from operations	(91,663)	(43,264)	(10,852)
Other income (expense):
Change in fair value of preferred stock warrant liability	(2,187)	(785)	1
Interest expense	(464)	(309)	(149)
Interest income and other income (expense), net	5,119	511	(16)
Total other income (expense), net	2,468	(583)	(164)
Net loss	(89,195)	(43,847)	(11,016)
Accretion of Series A redeemable convertible preferred stock to redemption value		(656)	(748)
Net loss attributable to common stockholders	$ (89,195)	$ (44,503)	$ (11,764)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.27)	$ (5.55)	$ (1.63)
Weighted average common shares outstanding, basic and diluted	39,285,468	8,023,785	7,200,581
Comprehensive loss:
Net loss	$ (89,195)	$ (43,847)	$ (11,016)
Other comprehensive loss:
Unrealized losses on investments, net of tax of $0	(29)
Comprehensive loss	$ (89,224)	$ (43,847)	$ (11,016)

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Unrealized gains (losses) on investments, tax	$ 0

CONSOLIDATED STATEMENTS OF CONV

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFICIT - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit	Convertible preferred stock	Series A redeemable convertible preferred stock	Series B convertible preferred stock	Total
Balances at Beginning of period (in shares) at Dec. 31, 2015	7,505,000
Balances at Beginning of period at Dec. 31, 2015	$ 8			$ (5,570)				$ (5,562)
Issuance of common stock for technology license (in shares)	366,667
Issuance of common stock for technology license		$ 55						55
Issuance of common stock upon exercise of stock options (in shares)	14,625
Stock-based compensation expense		147						147
Accretion of Series A redeemable convertible preferred stock to redemption value		(202)		(546)				(748)
Net loss				(11,016)				(11,016)
Balances at End of period (in shares) at Dec. 31, 2016	7,886,292
Balances at End of period at Dec. 31, 2016	$ 8			(17,132)				(17,124)
Balances at Beginning of period (in shares) at Dec. 31, 2015					10,487,329
Balances at Beginning of period at Dec. 31, 2015					$ 6,882
Convertible preferred stock
Issuance of stock, net of issuance costs (in shares)						19,083,333
Issuance of stock, net of issuance costs						$ 11,437
Accretion of Series A redeemable convertible preferred stock to redemption value					$ 748
Balances at End of period (in shares) at Dec. 31, 2016					29,570,662
Balances at End of period at Dec. 31, 2016					$ 19,067
Issuance of common stock upon exercise of stock options (in shares)	250,572
Issuance of common stock upon exercise of stock options		37						37
Issuance of common stock for one‑time bonus payment (in shares)	213,439
Issuance of restricted common stock upon early exercise of stock options (in shares)	1,400,000
Issuance of restricted common stock upon early exercise of stock options	$ 1	(1)
Issuance of restricted common stock (in shares)	5,227,014
Issuance of restricted common stock	$ 6	(6)
Stock-based compensation expense		17,903						17,903
Accretion of Series A redeemable convertible preferred stock to redemption value		(656)						(656)
Net loss				(43,847)				(43,847)
Balances at End of period (in shares) at Dec. 31, 2017	14,977,317
Balances at End of period at Dec. 31, 2017	$ 15	17,277		(60,979)				$ (43,687)
Convertible preferred stock
Issuance of stock, net of issuance costs (in shares)							14,362,344
Issuance of stock, net of issuance costs							$ 120,067
Accretion of Series A redeemable convertible preferred stock to redemption value					$ 656
Balances at End of period (in shares) at Dec. 31, 2017					43,933,006	29,570,662	14,362,344	43,933,006
Balances at End of period at Dec. 31, 2017					$ 139,790
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering		3,053						$ 3,053
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438
Conversion of redeemable convertible preferred stock to common stock	$ 52	240,724						240,776
Issuance of common stock, initial public offering, net of issuance cost $3,548 (in shares)	12,055,450
Issuance of common stock, initial public offering, net of issuance cost $3,548	$ 12	254,306						254,318
Cashless exercise of warrants	131,273
Vesting of restricted common stock		180						180
Issuance of common stock upon exercise of stock options (in shares)	225,375
Issuance of common stock upon exercise of stock options		64						64
Stock-based compensation expense		27,528						27,528
Unrealized losses on investments			$ (29)					(29)
Net loss				(89,195)				(89,195)
Balances at End of period (in shares) at Dec. 31, 2018	79,234,853
Balances at End of period at Dec. 31, 2018	$ 79	543,040	$ (29)	(150,082)				$ 393,008
Convertible preferred stock
Issuance of stock, net of issuance costs (in shares)					7,912,432
Issuance of stock, net of issuance costs					$ 100,986
Conversion of redeemable convertible preferred stock to common stock (in shares)					(51,845,438)
Conversion of redeemable convertible preferred stock to common stock					$ (240,776)
Balances at End of period (in shares) at Dec. 31, 2018					0			0
Cumulative effect adjustment for adoption of ASU 2018-07		$ (92)		$ 92

CONSOLIDATED STATEMENTS OF CO_2

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFICIT (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Issuance cost	$ 3,548
Series A redeemable convertible preferred stock
Net of issuance cost			$ 13
Series B convertible preferred stock
Net of issuance cost		$ 433
Series C convertible preferred stock
Net of issuance cost	$ 214

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (89,195)	$ (43,847)	$ (11,016)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	27,528	17,903	147
Depreciation and amortization expense	1,263	447	118
Issuance of common stock for technology license			55
Change in fair value of preferred stock warrant liability	2,187	785	(1)
Accretion of discount on investments	(329)
Loss on disposal of property and equipment			29
Non-cash interest expense	86	42	27
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(9,037)	(647)	(36)
Other assets	(74)
Accounts payable	4,938	1,110	589
Accrued expenses and other current liabilities	4,307	2,247	618
Deferred rent	(15)	22	(32)
Net cash used in operating activities	(58,341)	(21,938)	(9,502)
Cash flows from investing activities:
Purchases of property, plant and equipment	(14,952)	(2,251)	(443)
Purchases of investments	(160,972)
Sales and maturities of investments	64,285
Net cash used in investing activities	(111,639)	(2,251)	(443)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	100,986	120,067	11,437
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	257,866
Payments of initial public offering costs	(3,548)
Proceeds from issuance of common stock upon exercise of stock options	64	37
Proceeds from the sale of restricted common stock		100
Proceeds from repayment of promissory note	246
Payments of debt issuance costs	(285)	(11)	(19)
Proceeds from borrowings under loan and security agreement	25,000	1,500	4,000
Payment of long-term debt	(5,500)
Net cash provided by financing activities	374,829	121,693	15,418
Net increase in cash, cash equivalents and restricted cash	204,849	97,504	5,473
Cash, cash equivalents and restricted cash at beginning of period	104,572	7,068	1,595
Cash, cash equivalents and restricted cash at end of period	309,421	104,572	7,068
Supplemental cash flow information:
Cash paid for interest	385	265	115
Supplemental disclosure of non-cash investing and financing information:
Purchases of property, plant and equipment included in accounts payable or accrued expenses	1,550	9	266
Landlord incentives for leasehold improvements recorded as deferred rent			100
Amounts capitalized under build-to-suit lease transaction	45,142
Debt issuance costs included in accounts payable and accrued expenses	489
Issuance of preferred stock warrant in connection with loan and security agreement		14
Reclassification of warrants to additional paid-in capital	3,053
Accretion of Series A redeemable convertible preferred stock to redemption value		$ 656	$ 748
Conversion of preferred stock to common stock upon closing of the initial public offering	$ 240,776

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Nature of the Business and Basis of Presentation
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Rubius Therapeutics, Inc. (“Rubius” or the “Company”) is a therapeutics company focused on using its platform to develop red cell therapeutics for the treatment of patients with severe diseases. Rubius was incorporated in April 2013 as VL26, Inc. under the laws of the State of Delaware. In January 2015, the Company changed its name to Rubius Therapeutics, Inc. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the ability to establish clinical- and commercial-scale manufacturing processes and the ability to secure additional capital to fund operations. In addition, the Company is subject to uncertainty regarding the performance and safety of red cell therapeutics in humans. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On July 20, 2018, the Company completed its initial public offering (“IPO”), pursuant to which it issued and sold 12,055,450 shares of common stock, inclusive of 1,572,450 shares sold by the Company pursuant to the full exercise of the underwriters’ option to purchase additional shares. The aggregate net proceeds received by the Company from the IPO were $254.3 million, after deducting underwriting discounts and commissions and other offering costs. Upon the closing of the IPO, all of the shares of the Company’s outstanding convertible preferred stock then outstanding automatically converted into 51,845,438 shares of common stock (see Note 7). The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. The Company has incurred recurring losses since inception, including net losses of $89.2 million, $43.8 million and $11.0 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, the Company had an accumulated deficit of $150.1 million. The Company expects to continue to generate operating losses in the foreseeable future. As of March 28, 2019, the issuance date of the consolidated financial statements, the Company expects that its cash, cash equivalents and investments will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance date of the consolidated financial statements. The Company may seek additional funding through private or public equity financings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborations or other arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company's stockholders. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. The Company’s consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany accounts and transactions have been eliminated in consolidation.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with GAAP requires 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 financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the valuation of common stock and the preferred stock warrant liability prior to the IPO and the valuation of stock-based awards . The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and investments. The Company’s cash equivalents and investments as of December 31, 2018 consisted of U.S. government money market funds, U.S government treasury bills, U.S. government agency bonds and U.S. government treasury notes. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of an equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the statements of operations and comprehensive loss. Deferred Financing Costs The Company capitalizes certain legal and other third‑party fees that are directly associated with obtaining access to capital under credit facilities. Deferred financing costs incurred in connection with obtaining access to capital are recorded in other assets and are amortized over the term of the credit facility. Deferred financing costs related to a recognized debt liability are recorded as a reduction of the carrying amount of the debt liability and amortized to interest expense using the effective interest method over the repayment term. Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Restricted Cash As of December 31, 2018 and 2017, the Company maintained restricted cash totaling $1.8 million and $0.3 million, respectively, held in the form of cash-secured letters of credit for the benefit of the landlords of its leased properties. As of December 31, 2018, the Company also maintained restricted cash of $0.5 million to collateralize its corporate credit card. The Company classified $1.7 million and $0.3 million of the restricted cash as a non-current asset in its consolidated balance sheet as of December 31, 2018 and 2017, respectively, and classified $0.6 million of the restricted cash as a current asset in its consolidated balance sheet as of December 31, 2018. Property, Plant and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight‑line method over the estimated useful life of each asset as follows: Estimated useful life Laboratory and office equipment 5 years Computers and software 3 years Furniture and fixtures 7 years Building 30 years Leasehold improvements Shorter of life of lease or 10 years Costs for capital assets not yet placed into service are capitalized as construction‑in‑progress and depreciated once placed into service. Interest costs incurred during the construction of major capital projects are capitalized until the underlying asset is ready for its intended use, at which point the interest costs are amortized as depreciation expense over the life of the underlying asset. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. Impairment of Long‑Lived Assets Long‑lived assets consist of property and equipment. Long‑lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long‑lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long‑lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment losses on long‑lived assets during the periods presented. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: · Level 1—Quoted prices in active markets for identical assets or liabilities. · Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. · Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s long‑term debt approximates its fair value due to its variable interest rate, which approximates a market interest rate. Investments The Company’s investments are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its investments with maturities beyond one year as short-term, based on their highly liquid nature and because such investments are available for current operations. The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Classification and Accretion of Convertible Preferred Stock The carrying value of the Company’s Series A redeemable convertible preferred stock was being accreted to its redemption value from the date of issuance of such shares through the earliest date of redemption. During the year ended December 31, 2017, the redemption rights were removed from the Series A redeemable convertible preferred stock (see Note 7), and as such, the Company no longer recorded adjustments to the carrying value of its outstanding convertible preferred stock for accretion to redemption value. The Company’s Series A, Series B and Series C convertible preferred stock were classified outside of stockholders’ equity (deficit) because the holders of such shares had liquidation rights in the event of a deemed liquidation that, in certain situations, were not solely within the control of the Company. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is developing red cell therapeutics for the treatment of patients with severe diseases. All of the Company’s tangible assets are held in the United States. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities and clinical trials, as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research and Manufacturing Contract Costs and Accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies or clinical trials and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Stock‑Based Compensation The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees and directors based on the fair value on the date of grant using the Black‑Scholes option‑pricing model. The Company measures restricted common stock awards using the difference between the purchase price per share of the award, if any, and the fair value of the Company’s common stock. Compensation expense for those awards is recognized over the requisite service period, which is generally the vesting period of the respective award. The Company uses the straight‑line method to record the expense of awards with only service‑based vesting conditions. The Company uses the graded‑vesting method to record the expense of awards with both service‑based and performance‑based vesting conditions, commencing once achievement of the performance condition becomes probable. Prior to the adoption of ASU 2018-07 effective January 1, 2018 discussed below, t he Company measured the fair value of stock-based awards granted to non-employees on the date that the related service was complete, which was generally the vesting date of the award. Prior to the service completion date, compensation expense was recognized over the period during which services were rendered by such non-employees. At the end of each financial reporting period prior to the service completion date, the fair value of these awards was remeasured using the then-current fair value of the Company's common stock and updated assumption inputs in the Black-Scholes option-pricing model for options or the difference between the purchase price per share of the award, if any, and the then-current fair value of the Company's common stock for restricted common stock awards. For stock-based awards with market-based vesting conditions, the Company measures the fair value on the date of grant using a Monte Carlo simulation model. When service-based vesting conditions also exist, the Company recognizes stock-based compensation expense using the graded-vesting method over the longer of the derived service period from the market condition or the required service period. In accordance with accounting guidance for awards with market conditions, the stock-based compensation expense will be recognized over the appropriate period regardless of whether the award achieves the market condition and will only be adjusted to the extent the service condition is not met. When an award contains a market-based vesting condition and a performance-based vesting condition where both must be achieved to earn the award, the Company recognizes stock-based compensation expense over the longer of the derived service period from the market condition or the period estimated for the performance-based vesting condition to be achieved. The Company begins recording stock based compensation expense for this type of award once the achievement of the performance-based vesting condition becomes probable regardless of whether the market condition has been achieved. For restricted stock awards under which restricted common stock is purchased by the holder with a promissory note treated as a nonrecourse note for accounting purposes, the Company measures the fair value of the award using the Black‑Scholes option‑pricing model. The Company classifies stock‑based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2018, the Company’s only element of other comprehensive loss was unrealized losses on marketable securities. For the years ended December 31, 2017 and 2016, there was no difference between net loss and comprehensive loss in the accompanying consolidated financial statements. Net Income (Loss) per Share Prior to the closing of its IPO, the Company followed the two-class method when computing net income (loss) per share, as the Company had issued shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reported a net loss, such losses were not allocated to such participating securities, and as a result, basic and diluted net loss per share were the same. Subsequent to the closing of its IPO, basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding common stock equivalents. Accordingly, in periods in which the Company reported a net loss, dilutive common shares were not assumed to have been issued as their effect was anti-dilutive, and as a result, diluted net loss per common share was the same as basic net loss per common share. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two‑step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more‑likely‑than‑not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board, (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Compensation — Stock Compensation (Topic 718), Improvements to Non-Employee Share-Based Payment Accounting (“ASU 2018-07”) . This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption was permitted for all entities but no earlier than the Company’s adoption of ASU 2014-09. The Company adopted ASU 2018-07 effective January 1, 2018 by remeasuring outstanding equity-classified awards issued to non-employees for which a measurement date had not been established as of January 1, 2018 through a cumulative-effect adjustment to accumulated deficit as of January 1, 2018. The Company has elected to estimate the expected term of options utilizing the “simplified” method for both employee and non-employee options that qualify as “plain-vanilla” options. The Company has elected to account for forfeitures for non-employee options as they occur rather than apply an estimated forfeiture rate to stock-based compensation expense . The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of ASU 2018-07 on January 1, 2018 (in thousands): Balance at Balance at December 31, 2017 Adjustments January 1, 2018 Additional paid-in capital $ 17,277 $ (92) $ 17,185 Accumulated deficit $ (60,979) $ 92 $ (60,887) The $0.1 million adjustment is the result of the change in fair value of the unvested awards, representing awards for which a measurement date had not been established, using an expected term rather than the contractual term of the awards. As of the adoption date of January 1, 2018, the Company had 330,917 outstanding options to non-employees for which a measurement date had not been established. As of the adoption date of January 1, 2018, the Company had 4,767,014 shares of restricted common stock held by non-employees that were being accounted for as stock options for which a measurement date had not been established (see Note 10). The weighted average fair value of these awards was $5.88 per share as of January 1, 2018. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of outstanding awards granted to non-employees for which a measurement date had not been established as of the adoption date of January 1, 2018: Risk-free interest rate 2.3 % Expected volatility 74 % Expected dividend yield — Expected term (in years) 6.1 Common stock value $ 6.28 Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. For public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. Early adoption is permitted. ASU 2016-02 initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842) , which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption. The Company has elected to adopt ASU 2016-02 effective January 1, 2019 through a cumulative-effect adjustment under ASU 2018-11. This standard provides a number of optional practical expedients in transition. The Company plans to apply the package of practical expedients to leases that commenced prior to the effective date whereby it will elect to not reassess the following: (i) whether any expired or existing contracts contain leases; (ii) the lease classification for any expired or existing leases; and (iii) initial direct costs for any existing leases. The Company expects to elect the short-term lease recognition exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for short term leases. The Company expects that the most significant effects of adoption will be the recognition of material new right-of-use assets and corresponding liabilities on its consolidated balance sheet related to its existing facility operating leases (see Note 12). In addition, the Company has a material lease where the Company was deemed the owner during the construction perio d and for which the construction was not complete as of January 1, 2019. The Company took control of the leased space during the first quarter of 2019 at which time the lease commenced. Under ASU 2016-02, as the commencement date of this material lease had not occurred, the new right-of-use assets and corresponding liabilities related to this lease would not be recognized on the consolidated balance sheet as of date of adoption, January 1, 2019, however, will be recognized upon commencement date. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”), which modifies the disclosure requirements for fair value measurements. For all entities, this guidance is required to be adopted for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted. The Company currently is evaluating the impact the adoption of ASU 2018-13 may have on its disclosures.

Investments and Fair Value of F

Investments and Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
Investments and Fair Value of Financial Assets and Liabilities
Investments and Fair Value of Financial Assets and Liabilities	3. Investments and Fair Value of Financial Assets and Liabilities Investments by security type consisted of the following (in thousands): December 31, 2018 Gross Gross Unrealized Unrealized Amortized Cost Gains Losses Fair Value U.S. treasury notes (due within one year) $ 79,312 $ — $ (26) $ 79,286 U.S. government agency bonds (due within one year) 17,704 — (3) 17,701 $ 97,016 $ — $ (29) $ 96,987 The Company did not have any investments as of December 31, 2017. The following tables present the Company’s fair value hierarchy for its assets and liabilities, which are measured at fair value on a recurring basis (in thousands): Fair value measurements at December 31, 2018 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 282,160 $ — $ — $ 282,160 U.S. treasury bills — 24,904 — 24,904 Marketable securities: U.S. government agency bonds — 17,701 — 17,701 U.S. treasury notes — 79,286 — 79,286 $ 282,160 $ 121,891 $ — $ 404,051 Fair value measurements at December 31, 2017 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 104,288 $ — $ — $ 104,288 Liabilities: Preferred stock warrant liability $ — $ — $ 866 $ 866 U.S. government money market funds were valued by the Company based on quoted market prices, which represent a Level 1 measurement within the fair value hierarchy. U.S. treasury notes and U.S. government agency bonds were valued by the Company using quoted prices in active markets for similar securities, which represent a Level 2 measurement within the fair value hierarchy. There have been no changes to the valuation methods during the year ended December 31, 2018. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers between Level 1, Level 2 or Level 3 during the year ended December 31, 2018 and 2017, respectively. The preferred stock warrant liability in the table above consisted of the fair value of warrants to purchase Series A and Series B convertible preferred stock (see Note 8) and was based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy. The Company’s valuation of the preferred stock warrants utilized the Black-Scholes option-pricing model, which incorporated assumptions and estimates to value the preferred stock warrants. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Changes in the fair value of the preferred stock warrants were recognized as other income (expense) in the consolidated statements of operations and comprehensive loss. Upon the closing of the IPO, the warrants for the purchase of preferred stock automatically became warrants for the purchase of common stock and the Company reclassified the carrying value of the warrants from a non-current liability to additional paid-in capital in its consolidated balance sheet. The following table provides a roll-forward of the aggregate fair values of the Company’s preferred stock warrants for which fair value was determined by Level 3 inputs (in thousands): Preferred stock warrant liability Fair value at December 31, 2016 $ 67 Issuance of warrants to purchase shares of Series B convertible preferred stock 14 Change in fair value 785 Fair value at December 31, 2017 866 Change in fair value through the exercise date 2,187 Reclassification to additional paid-in capital in connection with IPO (3,053) Fair value at December 31, 2018 $ —

Property, Plant and Equipment,

Property, Plant and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment, Net.
Property, Plant and Equipment, Net	4. Property, Plant and Equipment, net Property, plant and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Laboratory equipment $ 7,122 $ 2,751 Land 1,300 — Leasehold improvements 117 117 Computer equipment 276 57 Construction-in-progress 55,828 74 64,643 2,999 Less: Accumulated depreciation and amortization (1,847) (584) $ 62,796 $ 2,415 Manufacturing Facility On July 31, 2018, the Company completed its purchase of a 135,000 square foot manufacturing facility located in Smithfield, Rhode Island for a purchase price of $8.0 million. In August 2018, the Company began renovations to customize this facility to manufacture clinical supply of its product candidates. Of the total purchase price, $1.3 million was allocated to the value of land acquired based on the value of comparable assets, and $6.7 million was allocated to construction in progress, as the building was not ready for its intended use. During the year ended December 31, 2018, the Company capitalized approximately $2.1 million in construction-in-progress for design and demolition costs related to the renovation project. Construction-in-progress, as of December 31, 2018, also included $45.1 million capitalized in connection with the Company’s build-to-suit lease accounting (see Note 12), $1.0 million for the Company’s share of the costs of construction-in-progress for leasehold improvements related to the Company’s January 2018 lease for office and laboratory space in Cambridge, Massachusetts and $0.9 million of lab equipment received but not yet placed into service. Depreciation and amortization expense was $1.3 million, $0.4 million and $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2018 2017 Accrued employee compensation and benefits $ 3,377 $ 1,339 Accrued external research and development expenses 2,252 1,230 Accrued lease liability, current portion 4,502 — Other 1,987 417 $ 12,118 $ 2,986

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt
Debt	6. Debt Long‑term debt consisted of the following (in thousands): December 31, 2018 2017 Principal amount of long‑term debt $ 25,000 $ 5,500 Less: Current portion of long‑term debt — (2,139) Long‑term debt, net of current portion 25,000 3,361 Debt discount (653) (59) Long‑term debt, net of discount and current portion $ 24,347 $ 3,302 2015 Credit Facility The Company was party to a loan and security agreement, as amended (the “2015 Credit Facility”), under which the Company had borrowed an aggregate of $5.5 million. Until May 2018, borrowings under the 2015 Credit Facility bore interest at an annual rate equal to the bank’s prime rate plus 1.25%, subject to a floor of 4.5%, and were repayable in monthly interest-only payments through May 2018 and in equal monthly payments of principal plus accrued interest from June 2018 until the maturity date in November 2019. In May 2018, the Company further amended the 2015 Credit Facility to modify the interest rate and extend the interest-only payment period and the maturity date. Subsequent to this amendment, outstanding borrowings under the 2015 Credit Facility bear interest at an annual rate equal to the bank’s prime rate plus 0.75%, subject to a floor of 5.5%, and were repayable in monthly interest-only payments through May 2019 and in equal monthly payments of principal plus accrued interest from June 2019 until the maturity date in November 2020. The May 2018 amendment to the 2015 Credit Facility was accounted for as a debt modification, rather than a debt extinguishment, based on a comparison of the present value of the cash flows under the terms of the debt immediately before and after the amendment, which resulted in a change of less than 10%. As a result, issuance costs paid to the lender in connection with the amendment were recorded as a reduction of the carrying amount of the debt liability and were not significant. Unamortized issuance costs as of the date of the modification were amortized to interest expense using the effective interest method over the revised repayment term. Issuance costs paid to third parties were recorded as expense and were not significant. Borrowings under the 2015 Credit Facility were collateralized by substantially all of the Company’s personal property, other than its intellectual property. There were no financial covenants associated with the 2015 Credit Facility; however, the Company was subject to certain affirmative and negative covenants restricting the Company’s activities, including limitations on dispositions, mergers or acquisitions; encumbering its intellectual property; incurring indebtedness or liens; paying dividends; making certain investments; and engaging in certain other business transactions. The obligations under the 2015 Credit Facility were subject to acceleration upon the occurrence of specified events of default, including a material adverse change in the Company’s business, operations or financial or other condition. In December 2018, the Company repaid all borrowings under the 2015 Credit Facility. The aggregate principal amount of the loan outstanding at the time of repayment was $5.5 million and the Company did not incur any penalties as a result of the repayment. The Company recognized a loss on the extinguishment of the 2015 Credit Facility of less than $0.1 million related to the unamortized debt discount at the time of repayment. The loss on extinguishment was recorded as additional interest expense. 2018 Credit Facility On December 21, 2018 (the “Closing Date”), the Company entered into a loan and security agreement (the “Loan Agreement”) with Solar Capital Ltd. as collateral agent for the lenders party thereto for an aggregate principal amount of $75.0 million. The aggregate principal amount will be funded in three tranches of term loans of $25.0 million each. On the Closing Date, the Company made an initial borrowing of $25.0 million. The second tranche will be available to the Company through June 30, 2019, subject to certain conditions including the satisfaction of certain financial covenants. The third tranche will be available to the Company through June 30, 2020, subject to certain conditions including the Food and Drug Administration’s acceptance of at least one of the Company’s investigational new drug applications and the satisfaction of certain financial covenants. Interest on the outstanding loan balance will accrue at a rate of the one-month U.S. LIBOR rate plus 5.50%. Monthly principal payments will commence 36 months after the Closing Date and will be amortized over the following 24 months. Certain backend fees are due to the lender at the time of final repayment based on the total funded term loans. The Company accrues the backend fees that will be due at final repayment to outstanding debt by charges to interest expense over the term of the loans using the effective-interest method. The term loans are subject to a prepayment fee of 1.00% in the first year, 0.50% in the second year and 0.25% in the third year. In conjunction with 2018 Credit Facility, the Company incurred issuance costs of $0.8 million. The Loan Agreement contains financial covenants that require the Company to maintain either a certain minimum cash balance or a minimum market capitalization threshold. The Company was in compliance with all such covenants as of December 31, 2018. The Loan Agreement contains customary representations, warranties and covenants and also includes customary events of default, including payment defaults, breaches of covenants, change of control and a material adverse change default. Upon the occurrence of an event of default, a default interest rate of an additional 4.00% per annum may be applied to the outstanding loan balances, and the lenders may declare all outstanding obligations immediately due and payable. Borrowings under the Loan Agreement are collateralized by substantially all of the Company’s assets, other than its intellectual property. As of December 31, 2018, the estimated future principal payments due were as follows (in thousands): Year ending December 31, 2019 $ — 2020 — 2021 — 2022 12,500 Thereafter 12,500 $ 25,000

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Convertible Preferred Stock.
Convertible Preferred Stock	7. Convertible Preferred Stock The Company had issued Series A redeemable convertible preferred stock (the “Series A Preferred Stock”), Series B convertible preferred stock (the “Series B Preferred Stock”) and Series C convertible preferred stock (the “Series C Preferred Stock”). The Series A Preferred Stock, the Series B Preferred Stock and the Series C Preferred Stock are referred to collectively as the “Preferred Stock”. Upon issuance of the Series A Preferred Stock, the holders of such shares were entitled to receive cumulative dividends of 8.0% per year, compounding annually, and such shares were redeemable at the option of the holder after five years from issuance date of the Series A Preferred Stock. In connection with the issuance and sale of Series B Preferred Stock in June 2017, the holders of Series A Preferred Stock agreed to remove the cumulative dividend rights and redemption features of the Series A Preferred Stock. The change to the terms of the Series A Preferred Stock was accounted for as a modification, rather than an extinguishment, of the Series A Preferred Stock based on a comparison of the fair value of the stock immediately before and after the change in terms, which resulted in a fair value change of less than 10%. This modification did not have any impact on the Company’s consolidated financial statements. For periods after the June 2017 date of the modification of the Series A Preferred Stock, the Company no longer accreted the carrying value of the Series A Preferred Stock to redemption value as such shares were no longer redeemable. In June 2017, the Company issued and sold 14,362,344 shares of Series B Preferred Stock at a price of $8.39 per share for gross proceeds of $120.5 million. The Company incurred issuance costs in connection with the Series B Preferred Stock of $0.4 million. In February 2018, the Company issued and sold 7,912,432 shares of Series C Preferred Stock at a price of $12.79 per share for gross proceeds of $101.2 million. The Company incurred issuance costs in connection with the Series C Preferred Stock of $0.2 million. Upon issuance of each class of Preferred Stock, the Company assessed the embedded conversion and liquidation features of the shares and determined that such features did not require the Company to separately account for these features. The Company also concluded that no beneficial conversion feature existed on the issuance date of each class of Preferred Stock. As of December 31, 2017, Preferred Stock consisted of the following (in thousands, except share amounts): December 31, 2017 Preferred stock Common stock Preferred stock issued and Carrying Liquidation issuable upon authorized outstanding value preference conversion Series A Preferred Stock 29,703,995 29,570,662 $ 19,723 $ 17,742 29,570,662 Series B Preferred Stock 14,366,813 14,362,344 120,067 120,500 14,362,344 44,070,808 43,933,006 $ 139,790 $ 138,242 43,933,006 Upon the closing of the IPO in July 2018, all 51,845,438 shares of the Company’s outstanding convertible preferred stock automatically converted into shares of common stock and, therefore, there was no outstanding Preferred Stock at December 31, 2018.

Warrants to Purchase Convertibl

Warrants to Purchase Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Warrants to Purchase Convertible Preferred Stock.
Warrants to Purchase Convertible Preferred Stock	8. Warrants to Purchase Convertible Preferred Stock During 2015, the Company issued warrants to purchase up to 133,333 shares of Series A Preferred Stock in connection with the 2015 Credit Facility (see Note 6). The warrants were exercisable at a price of $0.60 per share and had a contractual term of ten years from issuance. The fair value of the warrants on the issuance date of $0.1 million was recorded as a deferred financing cost and as preferred stock warrant liability. In May 2017, the Company issued warrants to purchase up to 2,234 shares of Series B Preferred Stock in connection with an amendment to the 2015 Credit Facility (see Note 6). The warrants were exercisable at a price of $8.39 per share and had a contractual term of ten years from issuance. The fair value of the warrants on the issuance date of less than $0.1 million was recorded as a debt discount and as a preferred stock warrant liability. The Company remeasured the fair value of the liability for these preferred stock warrants at each reporting date and recorded any adjustments as other income (expense). The warrants outstanding at each reporting date were remeasured using the Black-Scholes option-pricing model (see Note 3), and the resulting change in fair value was recorded in other income (expense) in the Company’s consolidated statements of operations and comprehensive loss. For the years ended December 31, 2018, 2017 and 2016, the Company recorded other expense of $2.2 million, other expense of $0.8 million and other income of less than $0.1 million, respectively, to reflect the change in fair value of these preferred stock warrants. Upon the closing of the IPO in July 2018, the Company’s outstanding warrants to purchase Preferred Stock automatically became warrants to purchase an aggregate of 135,567 shares of common stock. In July 2018, the holders of such warrants completed a cashless exercise of the warrants, resulting in the Company’s issuance of 131,273 shares of common stock, whereby 4,294 shares of common stock were withheld by the Company to pay for the exercise price of the warrants.

Equity

Equity	12 Months Ended
	Dec. 31, 2018
Equity
Equity	9. Equity Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends, unless declared by the board of directors. In February 2018, the Company increased the number of authorized shares of common stock from 65,000,000 shares to 75,000,000 shares. In April 2018, the Company increased the number of authorized shares of common stock from 75,000,000 shares to 78,800,000 shares. In June 2018, the Company increased the number of authorized shares of common stock from 78,800,000 shares to 79,000,000 shares. On July 20 2018, the Company filed a restated certificate of incorporation in the State of Delaware, which, among other things, restated the number of shares of all classes of stock that the Company has authority to issue to 160,000,000 shares, consisting of (i) 150,000,000 shares of common stock, $0.001 par value per share, and (ii) 10,000,000 shares of preferred stock, $0.001 par value per share. The shares of preferred stock are undesignated. Also on July 20, 2018, the Company completed its initial public offering (“IPO”), pursuant to which it issued and sold 12,055,450 shares of common stock, inclusive of 1,572,450 shares sold by the Company pursuant to the full exercise of the underwriters’ option to purchase additional shares. The aggregate net proceeds received by the Company from the IPO were $254.3 million, after deducting underwriting discounts and commissions and other offering costs. Upon the closing of the IPO, all of the shares of the Company’s outstanding convertible preferred stock then outstanding automatically converted into 51,845,438 shares of common stock (see Note 7).

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Stock-Based Compensation
Stock-Based Compensation	10. Stock-Based Compensation 2014 Stock Incentive Plan The Company’s 2014 Stock Incentive Plan (the “2014 Plan”) provided for the Company to sell or issue incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2014 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated. Stock options granted under the 2014 Plan with service‑based vesting conditions generally vested over three or four years and expired after ten years. The 2014 Plan allowed for the early exercise of unvested stock options, subject to certain restrictions, including the ability of the Company to repurchase such options upon an option holder’s termination of employment with the Company if such options had not yet vested. Restricted stock granted under the 2014 Plan with service‑based vesting conditions generally vested over three or four years. The exercise price for stock options granted was not less than the fair value of common shares as determined by the board of directors as of the date of grant. The Company’s board of directors valued the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant. Stock options were only granted under the 2014 Plan during the period that the Company was privately held. The total number of shares of common stock that could have been issued under the 2014 Plan was 19,152,328 shares, of which 47,447 shares remained available for future issuance prior to the effectiveness of the Company’s 2018 Stock Option and Incentive Plan (the “2018 Plan”). Upon effectiveness of the 2018 Plan in July 2018, the remaining shares available under the 2014 Plan ceased to be available for issuance and no future issuances will be made under the 2014 Plan. The shares of common stock underlying outstanding awards under the 2014 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) will be added to the shares of common stock available for issuance under the 2018 Plan. 2018 Equity Incentive Plan On July 6, 2018, the Company’s board of directors adopted, and its stockholders approved, the 2018 Plan, which became effective on July 16, 2018. The 2018 Plan provides for the grant of incentive stock options, non-qualified options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2018 Plan is 5,708,931, which shall be cumulatively increased on January 1, 2019 and each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without the issuance of stock, expire or are otherwise terminated (other than by exercise) under the 2018 Plan and the 2014 Plan will be added back to the shares of common stock available for issuance under the 2018 Plan. As of December 31, 2018, 2,335,217 shares remained available for issuance under the 2018 Plan. The number of authorized shares reserved for issuance under the 2018 Plan was increased by 2,377,045 shares effective as of January 1, 2019. 2018 Employee Stock Purchase Plan On July 6, 2018, the Company’s board of directors adopted and its stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”), which became effective on July 16, 2018. A total of 951,488 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on January 1, 2019, and each January 1 thereafter through January 1, 2028, by the least of (i) 951,488 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the administrator of the Company’s ESPP. As of December 31, 2018, all 951,488 shares remained available for issuance under the 2018 ESPP. The number of authorized shares reserved for issuance under the ESPP was increased by 792,348 shares effective as of January 1, 2019. Stock Option Valuation Service-Based and Performance-Based Stock Options The fair value of stock option grants with service-based and performance-based vesting conditions is estimated using the Black-Scholes option-pricing model. The Company estimates expected volatility based on the historical volatility of publicly traded peer companies. The Company expects to continue to do so until such time as it has adequate historical data regarding the volatility of its traded stock price following our July 2018 IPO. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. For periods prior to the adoption of ASU 2018-07 on January 1, 2018, the expected term of stock options granted to non-employees was equal to the contractual term of the option award. Upon the adoption of ASU 2018-07 on January 1, 2018, the expected term of stock options granted to non-employees has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk‑free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees: Year ended December 31, 2018 2017 2016 Risk-free interest rate 2.71 % 2.05 % 1.81 % Expected volatility 74.0 % 75.6 % 77.2 % Expected dividend yield — — — Expected term (in years) 6.21 6.39 6.25 The following table presents the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock awards granted to non-employees, prior to the adoption of ASU 2018-07: Year ended December 31, 2017 2016 Risk-free interest rate 2.02% - 2.31% 2.35% - 2.45% Expected volatility 74% - 85% 81% - 85% Expected dividend yield — — Expected term (in years) 6 - 10 8 - 10 Fair value of common stock $0.19 - $6.28 $0.18 - $0.19 The following table summarizes the Company’s service-based and performance-based option activity since December 31, 2017: Weighted Weighted average average Aggregate Number of exercise contractual intrinsic shares price term value (in years) (in thousands) Outstanding as of December 31, 2017 4,661,635 $ 1.29 8.64 $ 23,264 Granted 10,405,323 12.12 Exercised (225,375) 0.29 Forfeited (56,813) 1.44 Outstanding as of December 31, 2018 14,784,770 $ 8.93 8.89 $ 126,367 Vested and expected to vest as of December 31, 2018 14,784,770 $ 8.93 8.89 $ 126,367 Options exercisable as of December 31, 2018 2,622,981 $ 1.74 7.59 $ 37,726 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2018, 2017 and 2016 was $3.7 million, $3.7 million and less than $0.1 million, respectively. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2018, 2017 and 2016 was $8.71 per share, $2.74 per share and $0.12 per share, respectively. In April 2017, an executive officer early exercised an option to purchase 1,400,000 shares of common stock, at an exercise price of $0.18 per share, for cash proceeds of $0.1 million and a promissory note for $0.2 million (see Note 14). The employee received shares of restricted common stock upon such exercise. The unvested shares of restricted common stock issued upon exercise are subject to the Company’s repurchase right at the lesser of the original exercise price per share or the fair value of such shares on the repurchase date. The $0.1 million of cash proceeds from the early exercise of this stock option was recorded as a liability in the Company’s consolidated balance sheet and will be reclassified to stockholders’ equity (deficit) as the shares vest and the Company’s repurchase rights related to such shares lapse. The promissory note was partial-recourse, but was treated as nonrecourse for accounting purposes. As a result, (i) this early exercise of common stock with a promissory note continued to be accounted for as an outstanding stock option and (ii) no receivable for amounts due under the promissory note was recorded on the Company’s consolidated balance sheet. Stock-based compensation expense related to this award is being recognized over the requisite service period of the award based on the grant-date fair value of the award, which was determined using the Black-Scholes option-pricing model. On June 21, 2018, the principal balance of $0.2 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory note was terminated (see Note 14). The portion of the repayment that was associated with vested shares for which the Company’s repurchase obligations had lapsed was recorded to stockholders’ equity (deficit) and the remaining amount was recorded as a liability in the consolidated balance sheet and will be recorded to stockholders’ equity (deficit) as the shares vest and the Company’s repurchase rights related to such shares lapse . Market-Based Stock Options The fair value of stock option grants with market-based vesting conditions is estimated using a Monte Carlo simulation model . In October 2018, the Company granted to an executive officer an option to purchase 164,400 shares of common stock (“Option A”) at an exercise price of $16.43 per share, vesting upon the achievement of a specified thirty-day average closing price of its common stock and the satisfaction of service-based vesting conditions, and an option to purchase 193,400 shares of common stock (“Option B”) at an exercise price of $16.43 per share, vesting upon the achievement of a specified thirty-day average closing price of its common stock and the achievement of certain other performance-based vesting conditions. The Company used a Monte Carlo simulation model to estimate the grant-date fair value of the awards. Assumptions and estimates utilized in the model include the risk-free interest rate, dividend yield, expected stock volatility based on a combination of the Company’s historical stock volatility since its July 2018 IPO and the historical volatility of a publicly traded set of peer companies and the estimated period to achievement of the market condition. Stock-based compensation expense for Option A is being recognized using the graded-vesting method over the longer of the derived service period from the market condition or the explicit service period required to be completed for each vesting trenche. Stock-based compensation expense for Option B will be recognized when the achievement of the performance-based vesting conditions become probable regardless of whether the market condition has been achieved. The aggregate grant date fair value of these options was $4.3 million. During the year ended December 31, 2018, the Company recorded stock-based compensation expense on Option A of $0.2 million and no stock-based compensation expense on Option B, as the performance-based vesting conditions have not yet been determined to be probable. The following table presents, on a weighted average basis, the assumptions used in the Monte Carlo simulation model to determine the fair value of stock-based awards granted to employees : Year ended December 31, 2018 2017 2016 Risk-free interest rate 3.15 % — % — % Expected volatility 69.0 % — % — % Expected dividend yield — — — Derived service period (in years) 2.30 — — The weighted average grant-date fair value of stock options with market-based vesting conditions granted during the year ended December 31, 2018 was $11.88 per share. During the year ended December 31, 2018, none of the outstanding stock awards with market-based vesting conditions were exercised, forfeited or vested and they had no intrinsic value at December 31, 2018. Restricted Common Stock The Company has granted restricted common stock with service-based vesting conditions. Shares of unvested restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The following table summarizes the Company’s restricted common stock award activity since December 31, 2017: Weighted average grant‑date Shares fair value Unvested restricted common stock as of December 31, 2017 5,227,014 $ 0.514 Issued — — Vested (3,667,613) 0.519 Forfeited — — Unvested restricted common stock as of December 31, 2018 1,559,401 $ 0.502 In the table above, the number of shares of unvested restricted common stock outstanding as of December 31, 2018 and December 31, 2017 excludes 670,834 shares and 1,050,000 shares, respectively, of restricted common stock that remained unvested as of those dates related to the early exercise of a stock option during the year ended December 31, 2017 in exchange for 1,400,000 shares of restricted common stock. As of December 31, 2018, shares of unvested restricted common stock totaled 2,230,235 shares, consisting of 1,559,401 shares from unvested restricted common stock awards and 670,834 shares from the early exercise of a stock option. The aggregate intrinsic value of restricted stock awards is calculated as the positive difference between the prices paid, if any, of the restricted stock awards and the fair value of the Company’s common stock. The aggregate intrinsic value of restricted stock awards that vested during the years ended December 31 2018, 2017 and 2016 was $34.0 million, $0.9 million and $0.1 million, respectively. In April 2017, the Company issued 460,000 shares of restricted common stock, at a price of $0.19 per share, to an executive officer in exchange for a promissory note in the principal amount of $0.1 million. The promissory note was partial-recourse, but was treated as nonrecourse for accounting purposes and, as such, (i) this purchase of common stock with a promissory note was accounted for as if it were a stock option grant and (ii) no receivable for amounts due under the promissory note was recorded on the Company’s consolidated balance sheet. Stock-based compensation expense related to this award is being recognized over the requisite service period of the award based on the grant-date fair value of the award, which was determined using the Black-Scholes option-pricing model. On June 21, 2018, the principal balance of $0.1 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory note was terminated (see Note 14). In January 2017 and May 2017, the Company issued 3,667,014 shares and 1,100,000 shares, respectively, of restricted common stock at prices of $0.19 per share and $1.65 per share, respectively, to the chairman of the Company’s board of directors in exchange for two promissory notes totaling $2.5 million. The promissory notes are partial-recourse, but were treated as nonrecourse for accounting purposes and, as such, (i) each of these purchases of common stock with a promissory note was accounted for as if it were a stock option grant and (ii) no receivable for amounts due under the promissory note was recorded on the Company’s consolidated balance sheet. All of the stock-based awards issued to the chairman of the Company’s board of directors were issued for his services as a consultant and prior to the adoption of ASU 2018-07, which was effective January 1, 2018, were being accounted for as non-employee stock-based awards. As a result, stock-based compensation expense related to the awards was being recognized over the requisite service period of the award based on the remeasured fair value of the award at each reporting period until the award vested, which was determined using the Black-Scholes option-pricing model. Upon the adoption of ASU 2018-07, the Company valued the remaining unvested options issued to non-employees as of January 1, 2018 and is recognizing stock-based compensation over the remaining vesting period. Effective January 1, 2018, the Company no longer remeasures the fair value of options granted to non-employees at each reporting period end (see Note 2). On June 21, 2018, the aggregate principal balance of both promissory notes of $2.5 million and all interest that had accrued thereon, totaling $0.1 million, was forgiven by the Company and the promissory notes were terminated (see Note 14). The forgiveness of these promissory notes by the Company was treated as an option modification and resulted in the recognition of incremental stock-based compensation expense of $1.5 million during the year ended December 31, 2018, which represents the change in the fair value of the award on the modification date. The aggregate amount of stock-based compensation expense related to these restricted stock awards recognized during the year ended December 31, 2018 was $7.3 million. Stock-based compensation expense related to these awards will continue to be recognized over the requisite service period of the awards. Stock-Based Compensation The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year ended December 31, 2018 2017 2016 Research and development expenses $ 3,787 $ 1,756 $ 107 General and administrative expenses 23,741 16,147 40 $ 27,528 $ 17,903 $ 147 In October 2017, the Company issued 213,439 shares of common stock to the Company’s chairman of its board of directors as payment of a one‑time bonus that was payable, at his election, in cash or shares of common stock. The shares were issued out of the 2014 Plan. In connection with this issuance, the Company recorded $1.0 million of stock‑based compensation expense, equal to the aggregate fair value of this common stock on the date of issuance. Stock-based compensation expense for the year ended December 31, 2018 includes $2.2 million of stock-based compensation expense related to options for the purchase of an aggregate of 447,000 shares of common stock that have non-market, performance-based vesting conditions for which the performance condition was achieved during the year ended December 31, 2018. As of December 31, 2018, the Company has outstanding options for the purchase of an aggregate of 232,500 shares of common stock with non-market, performance-based vesting conditions whereby the achievement of the conditions has not yet been determined to be probable and, therefore, the Company has not recorded any compensation expense related to these stock options. As of December 31, 2018 , total unrecognized compensation cost related to unvested stock‑based awards was $89.6 million, which is expected to be recognized over a weighted average period of 2.7 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Taxes
Income Taxes	11. Income Taxes 2017 U.S. Tax Reform On December 22, 2017, the Tax Cuts and Jobs Act of 2017 (the “TCJA Act”) was signed into United States law, making significant changes to the Internal Revenue Code. Changes included, but were not limited to, a corporate tax rate decrease from 35% to 21% effective for tax years beginning after December 31, 2017, the transition of U.S international taxation from a worldwide tax system to a territorial system, and a one‑time transition tax on the mandatory deemed repatriation of cumulative deferred foreign earnings as of December 31, 2017. On December 22, 2017, the Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 (“SAB 118”), which directed taxpayers to consider the impact of the TCJA as “provisional” when a registrant did not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the TCJA. During the fourth quarter of 2018 the Company completed its accounting for the tax effects of the TCJA with no material changes. During the years ended December 31, 2018 and 2017, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits generated in each year, due to its uncertainty of realizing a benefit from those items. All of the Company’s operating losses since inception have been generated in the United States. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year ended December 31, 2018 2017 Federal statutory income tax rate (21.0) % (34.0) % State taxes, net of federal benefit (4.8) (3.2) Federal and state research and development tax credits (2.9) (1.4) Stock‑based compensation expense 4.5 12.8 Other 0.5 0.7 Remeasurement of deferred taxes due to the Tax Cuts and Jobs Act — 11.1 Increase in deferred tax asset valuation allowance 23.7 14.0 Effective income tax rate — % — % Net deferred tax assets consisted of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 25,508 $ 10,422 Research and development tax credit carryforwards 3,824 1,241 Accrued expenses 1,139 413 Capitalized intellectual property costs 764 367 Capitalized research and development expense 120 131 Stock‑based compensation expense 2,652 375 Total deferred tax assets 34,007 12,949 Deferred tax liabilities: Depreciation and other (341) (374) Total deferred tax liabilities (341) (374) Valuation allowance (33,666) (12,575) Net deferred tax assets $ — $ — As of December 31, 2018, the Company had U.S. federal and state net operating loss (“NOL”) carryforwards of $93.2 million and $93.9 million, respectively, which may be available to offset future taxable income. The federal NOLs include $37.2 million which expire at various dates through 2037 and $56.0 million which carryforward indefinitely. The state NOLs expire at various dates through 2038. As of December 31, 2018, the Company also had U.S. federal and state research and development tax credit carryforwards of $3.0 million and $1.1 million, respectively, which may be available to offset future tax liabilities and begin to expire in 2034 and 2031, respectively. During the year ended December 31, 2018, deferred tax assets, before valuation allowance, increased by approximately $21.1 million mainly due to the operating loss incurred by the Company during that period. Utilization of the U.S. federal and state net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Sections 382 and 383 of the Internal Revenue Code of 1986, and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income or tax liabilities. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three‑year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long‑term tax‑exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. Further, until a study is completed by the Company and any limitation is known, no amounts are being presented as an uncertain tax position. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the net deferred tax assets as of December 31, 2018 and 2017. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018 and 2017 related primarily to the increase in net operating loss carryforwards and research and development tax credit carryforwards in 2018 and 2017, and the impact of the TCJA in 2017, and were as follows (in thousands): Year ended December 31, 2018 2017 Valuation allowance as of beginning of year $ 12,575 $ 6,454 Decreases recorded as benefit to income tax provision — (4,887) Increases recorded to income tax provision 21,091 11,008 Valuation allowance as of end of year $ 33,666 $ 12,575 As of December 31, 2018 and 2017, the Company had not recorded any amounts for unrecognized tax benefits. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision. As of December 31, 2018 and 2017, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts had been recognized in the Company’s consolidated statements of operations and comprehensive loss. The Company files income tax returns in the U.S. and Massachusetts, as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company is open to future tax examination under statute from 2015 to the present; however, carryforward attributes that were generated prior to January 1, 2015 may still be adjusted upon examination by federal, state or local tax authorities if they either have been or will be used in a future period.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies
Commitments and Contingencies	12. Commitments and Contingencies Operating Leases The Company leases its office and laboratory facilities in Cambridge, Massachusetts under three noncancelable operating leases that expire in December 2018, February 2019 and September 2021. The Company continued to occupy the facility covered by the lease expiring in December 2018 until February 2019 in accordance with the holdover provisions in the lease. The lease agreements include lease incentives, payment escalations and rent holidays, which are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the terms of occupancy. Rent expense for the years ended December 31, 2018, 2017 and 2016 was $2.4 million, $1.0 million and $0.4 million, respectively. Future minimum lease payments under operating leases as of December 31, 2018 are as follows (in thousands): Year ended December 31, 2019 $ 956 2020 713 2021 547 $ 2,216 Build-To-Suit Lease In January 2018, the Company entered into a lease for office and laboratory space in Cambridge, Massachusetts ( the “Initial Space”) . The lease term commenced on January 28, 2019 and expires eight years from the commencement date. The Company is entitled to one five-year option to extend. The initial annual base rent is approximately $3.8 million, and such amount will increase during the initial term by 3% annually on the anniversary of the commencement date. The Company is obligated to pay its portion of real estate taxes and costs related to the premises, including costs of operations, maintenance, repair, replacement and management of the new leased premises. In connection with the lease, the Company maintains a letter of credit for the benefit of the landlord in the amount of $0.9 million, which is secured by a cash deposit of the same amount. The lease agreement allows for a landlord-provided tenant improvement allowance of $9.9 million to be applied to the costs of the construction of the leasehold improvements, of which $0.5 million is repayable to the landlord over the term of the lease. In November 2018, the Company amended its January 2018 lease to lease additional office and laboratory space in the same building (the “Expansion Space”). The term for the Expansion Space is expected to commence in August 2019 and expires nine years from the commencement date. The initial annual base rent for the Expansion Space is approximately $2.5 million and such amount will increase by 3% annually on the anniversary of the commencement date. The Company is obligated to pay its portion of real estate taxes and costs related to the Expansion Space, including costs of operations, maintenance, repair, replacement and property management. In connection with the lease amendment, the Company increased the letter of credit held for the benefit of the landlord by $0.6 million, which is secured by a cash deposit of the same amount. The lease amendment increased the landlord-provided tenant improvement allowance by $7.2 million. The Company is not the legal owner of the leased space. However, in accordance with ASC 840, Leases , the Company is deemed to be the owner of the leased space during the construction period because of certain indemnification provisions within the lease agreement. As a result, as of December 31, 2018, the Company capitalized approximately $45.1 million (equal to the estimated fair value of its leased portion of the premises) as construction-in-progress within property, plant and equipment and recorded a corresponding build-to-suit facility lease financing obligation. As of December 31, 2018, the current portion of the lease financing obligation of $4.5 million was classified within accrued expenses and other current liabilities and the remaining $41.4 million was classified as a lease liability, net of current portion, on its consolidated balance sheet. The Company took control of the Initial Space during the first quarter of 2019 at which time the lease commenced. Upon the commencement date of the Initial Space, in the first quarter of 2019, the Company assessed and determined the accounting treatment for the asset and corresponding liability under ASC 842, Leases , which was adopted as of January 1, 2019 according to ASU No. 2016-02 (see Note 2). As of December 31, 2018, minimum commitments under this lease are as follows (in thousands): Year ending December 31, 2019 $ 4,502 2020 6,428 2021 6,621 2022 6,820 2023 7,024 Thereafter 27,777 $ 59,172 License Agreement with the Whitehead Institute for Biomedical Research The Company has a license agreement with the Whitehead Institute for Biomedical Research (“WIBR”), as amended, under which the Company has been granted an exclusive, sublicensable, nontransferable license under certain patent families related to the development of the Company’s red cell therapies (the “WIBR License”). The Company is obligated to pay WIBR annual license maintenance fees of less than $0.1 million, as well as patent-related costs, including legal fees and low single-digit royalties based on annual net sales of licensed products and licensed services by the Company and its sublicensees. Based on the progress the Company makes in the advancement of products covered by the licensed patent rights, the Company is required to make aggregate milestone payments of up to $1.6 million upon the achievement of specified preclinical, clinical and regulatory milestones. In addition, the Company is required to pay to WIBR a percentage of the non-royalty payments that it receives from sublicensees of the patent rights licensed by WIBR. This percentage varies from low single-digit to low double-digit percentages and will be based upon the clinical stage of the product that is the subject of the sublicense. Royalties shall be paid by the Company on a licensed product-by-licensed product and country-by-country basis, beginning on the first commercial sale of such licensed product in such country until expiration of the last valid patent claim covering such licensed product in such country. The Company has the right to terminate the WIBR License in its entirety, on a patent-by-patent or country-by-country basis, at will upon three months’ notice to WIBR. WIBR may terminate the agreement upon breach of contract or in the event of the Company’s bankruptcy, liquidation, insolvency or cessation of business related to the license. 401(k) Plan In January 2018, the Company established a defined-contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. The Company will make matching contributions at a rate of 50% of each employee’s contribution up to a maximum employee contribution of 6% of eligible plan compensation. For the year ended December 31, 2018, the Company made matching contributions of $0.2 million. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, contract research organizations, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and certain of its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. The Company has not incurred any material costs as a result of such indemnifications and is not currently aware of any indemnification claims. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to such legal proceedings.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2018
Net Loss Per Share
Net Loss Per Share	13. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Year ended December 31, 2018 2017 2016 Numerator: Net loss $ (89,195) $ (43,847) $ (11,016) Accretion of Series A redeemable convertible preferred stock to redemption value — (656) (748) Net loss attributable to common stockholders $ (89,195) $ (44,503) $ (11,764) Denominator: Weighted average common shares outstanding, basic and diluted 39,285,468 8,023,785 7,200,581 Net loss per share attributable to common stockholders, basic and diluted $ (2.27) $ (5.55) $ (1.63) Upon the issuance of Series A Preferred Stock, the holders of such shares were entitled to cumulative dividends of 8.0% per year, compounding annually. In connection with the issuance and sale of Series B Preferred Stock in June 2017, the holders of Series A Preferred Stock agreed to remove the cumulative dividend and redemption rights associated with the Series A Preferred Stock. Accordingly, during the year ended December 31, 2017, the calculation of net loss attributable to common stockholders included the accretion of Series A redeemable convertible preferred stock to redemption value. The Company’s potential dilutive securities have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares from the periods in the table above, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year ended December 31, 2018 2017 2016 Convertible preferred stock (as converted to common stock) — 43,933,006 29,570,662 Warrants to purchase convertible preferred stock (as converted to common stock) — 135,567 133,333 Restricted common stock(1) 2,230,235 6,277,014 348,120 Stock options to purchase common stock 14,784,770 4,661,635 4,159,165 17,015,005 55,007,222 34,211,280 (1) Includes unvested restricted stock and vested restricted stock issued for promissory notes.

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2018
Related Parties
Related Parties	14. Related Parties In April 2013, the Company entered into a services agreement with Flagship Ventures Management, Inc. (“Flagship”), an affiliate of one of its principal stockholders, to provide general and administrative services to the Company, including certain consulting services and the provision of employee health and dental benefit plans for the Company’s employees. The Company recorded general and administrative expense and made cash payments for services received under this agreement of $1.3 million, $0.9 million and $0.8 million during the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, 2017 and 2016, the Company had no amounts payable to Flagship for costs related to the services agreement. In January 2017, the Company loaned $0.7 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). In May 2017, the Company loaned $1.8 million to the chairman of its board of directors to purchase shares of common stock pursuant to a promissory note and a restricted stock agreement (see Note 10). The January 2017 promissory note provided that the unpaid principal amount of the loan bore interest at 1.97% annually, and the May 2017 promissory note provided that the unpaid principal amount of the loan bore interest at 2.04% annually. Interest was payable annually or was converted to principal and payable at the maturity date. The maturity date of the promissory notes occurred on the earliest of (i) seven years from the issuance date of the notes, (ii) 60 days following the date of termination of services of the borrower, and (iii) immediately prior to an initial filing of a registration statement by the Company. The promissory notes were partial‑recourse and secured by a pledge of the shares of common stock purchased with the promissory notes. As of December 31, 2017, no amounts were due to the Company and no amounts had been received by the Company as repayment of these promissory notes. On June 21, 2018, the aggregate principal balance of both promissory notes of $2.5 million and all interest that had accrued thereon, totaling $0.1 million, was forgiven by the Company and the promissory notes were terminated. In April 2017, the Company loaned $0.2 million to an executive officer of the Company to purchase shares of common stock pursuant to two promissory notes and two restricted stock agreements (see Note 10). The promissory notes provided that the unpaid principal amount of the loans bore interest at 2.05% annually, and interest was payable annually or was converted to principal and payable at the maturity date. The maturity date of the promissory notes occurred on the earliest of (i) seven years from the issuance date of the notes, (ii) 60 days following the date of termination of employment of the borrower, and (iii) immediately prior to an initial filing of a registration statement by the Company. The promissory notes were partial‑recourse and secured by a pledge of the shares of common stock purchased with the promissory notes. As of December 31, 2017, no amounts were due to the Company and no amounts had been received by the Company as repayment of these promissory notes. On June 21, 2018, the aggregate principal balance of both promissory notes of $0.2 million and all interest that had accrued thereon, totaling less than $0.1 million, was repaid in full by the executive officer and the promissory notes were terminated.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Data (Unaudited)
Selected Quarterly Financial Data (Unaudited)	15. Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2018 and 2017. The information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information (in thousands, except per share data): Three Months Ended March 31, June 30, September 30, December 31, 2018(1) 2018(1) 2018 2018 Consolidated Statements of Operations Data: Revenue $ — $ — $ — $ — Total operating expenses 14,603 20,384 27,554 29,122 Loss from operations (14,603) (20,384) (27,554) (29,122) Net loss attributable to common stockholders (14,411) (21,239) (26,362) (27,183) Net loss per share attributable to common stockholders, basic and diluted $ (1.72) $ (2.43) $ (0.42) $ (0.35) Three Months Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Consolidated Statements of Operations Data: Revenue $ — $ — $ — $ — Total operating expenses 4,783 9,033 11,975 17,473 Loss from operations (4,783) (9,033) (11,975) (17,473) Net loss attributable to common stockholders (5,233) (9,834) (11,919) (17,517) Net loss per share attributable to common stockholders, basic and diluted $ (0.68) $ (1.25) $ (1.48) $ (2.07) During the three months ended September 30, 2018, the Company adopted ASU 2018-07 (see Note 2) effective January 1, 2018. The Company has revised the results for the three months ended March 31, 2018 and the three months ended June 30, 2018 to reflect the adoption of ASU 2018-07. The following tables summarize the impact of adoption to the Company’s previously issued consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended Three Months Ended March 31, 2018 June 30, 2018 As previously As previously reported As revised reported As revised Operating expenses: Research and development $ 9,650 $ 9,506 $ 11,965 $ 11,361 General and administrative 5,797 5,097 16,279 9,023 Total operating expenses 15,447 14,603 28,244 20,384 Loss from operations (15,447) (14,603) (28,244) (20,384) Net loss attributable to common stockholders $ (15,255) $ (14,411) $ (29,099) $ (21,239) Net loss per share attributable to common stockholders, basic and diluted $ (1.83) $ (1.72) $ (3.33) $ (2.43)

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Summary of Significant Accounting Policies
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires 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 financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the valuation of common stock and the preferred stock warrant liability prior to the IPO and the valuation of stock-based awards . The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, and investments. The Company’s cash equivalents and investments as of December 31, 2018 consisted of U.S. government money market funds, U.S government treasury bills, U.S. government agency bonds and U.S. government treasury notes. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of an equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the offering. Should the in-process equity financing be abandoned, the deferred offering costs will be expensed immediately as a charge to operating expenses in the statements of operations and comprehensive loss
Deferred Financing Costs	Deferred Financing Costs The Company capitalizes certain legal and other third‑party fees that are directly associated with obtaining access to capital under credit facilities. Deferred financing costs incurred in connection with obtaining access to capital are recorded in other assets and are amortized over the term of the credit facility. Deferred financing costs related to a recognized debt liability are recorded as a reduction of the carrying amount of the debt liability and amortized to interest expense using the effective interest method over the repayment term.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.
Restricted Cash	Restricted Cash As of December 31, 2018 and 2017, the Company maintained restricted cash totaling $1.8 million and $0.3 million, respectively, held in the form of cash-secured letters of credit for the benefit of the landlords of its leased properties. As of December 31, 2018, the Company also maintained restricted cash of $0.5 million to collateralize its corporate credit card. The Company classified $1.7 million and $0.3 million of the restricted cash as a non-current asset in its consolidated balance sheet as of December 31, 2018 and 2017, respectively, and classified $0.6 million of the restricted cash as a current asset in its consolidated balance sheet as of December 31, 2018.
Property, Plant and Equipment	Property, Plant and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight‑line method over the estimated useful life of each asset as follows: Estimated useful life Laboratory and office equipment 5 years Computers and software 3 years Furniture and fixtures 7 years Building 30 years Leasehold improvements Shorter of life of lease or 10 years Costs for capital assets not yet placed into service are capitalized as construction‑in‑progress and depreciated once placed into service. Interest costs incurred during the construction of major capital projects are capitalized until the underlying asset is ready for its intended use, at which point the interest costs are amortized as depreciation expense over the life of the underlying asset. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.
Impairment of Long Lived Assets	Impairment of Long‑Lived Assets Long‑lived assets consist of property and equipment. Long‑lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long‑lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long‑lived asset group to its carrying value. An impairment loss would be recognized in loss from operations when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment losses on long‑lived assets during the periods presented.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: · Level 1—Quoted prices in active markets for identical assets or liabilities. · Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. · Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these liabilities. The carrying value of the Company’s long‑term debt approximates its fair value due to its variable interest rate, which approximates a market interest rate.
Investments	Investments The Company’s investments are classified as available-for-sale and are carried at fair value, with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value determined to be other than temporary are based on the specific identification method and are included as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company classifies its investments with maturities beyond one year as short-term, based on their highly liquid nature and because such investments are available for current operations. The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Classification and Accretion of Convertible Preferred Stock	Classification and Accretion of Convertible Preferred Stock The carrying value of the Company’s Series A redeemable convertible preferred stock was being accreted to its redemption value from the date of issuance of such shares through the earliest date of redemption. During the year ended December 31, 2017, the redemption rights were removed from the Series A redeemable convertible preferred stock (see Note 7), and as such, the Company no longer recorded adjustments to the carrying value of its outstanding convertible preferred stock for accretion to redemption value. The Company’s Series A, Series B and Series C convertible preferred stock were classified outside of stockholders’ equity (deficit) because the holders of such shares had liquidation rights in the event of a deemed liquidation that, in certain situations, were not solely within the control of the Company.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is developing red cell therapeutics for the treatment of patients with severe diseases. All of the Company’s tangible assets are held in the United States.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities and clinical trials, as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research and Manufacturing Contract Costs and Accruals	Research and Manufacturing Contract Costs and Accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies or clinical trials and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock Based Compensation	Stock‑Based Compensation The Company measures stock options with service-based vesting or performance-based vesting granted to employees, non-employees and directors based on the fair value on the date of grant using the Black‑Scholes option‑pricing model. The Company measures restricted common stock awards using the difference between the purchase price per share of the award, if any, and the fair value of the Company’s common stock. Compensation expense for those awards is recognized over the requisite service period, which is generally the vesting period of the respective award. The Company uses the straight‑line method to record the expense of awards with only service‑based vesting conditions. The Company uses the graded‑vesting method to record the expense of awards with both service‑based and performance‑based vesting conditions, commencing once achievement of the performance condition becomes probable. Prior to the adoption of ASU 2018-07 effective January 1, 2018 discussed below, t he Company measured the fair value of stock-based awards granted to non-employees on the date that the related service was complete, which was generally the vesting date of the award. Prior to the service completion date, compensation expense was recognized over the period during which services were rendered by such non-employees. At the end of each financial reporting period prior to the service completion date, the fair value of these awards was remeasured using the then-current fair value of the Company's common stock and updated assumption inputs in the Black-Scholes option-pricing model for options or the difference between the purchase price per share of the award, if any, and the then-current fair value of the Company's common stock for restricted common stock awards. For stock-based awards with market-based vesting conditions, the Company measures the fair value on the date of grant using a Monte Carlo simulation model. When service-based vesting conditions also exist, the Company recognizes stock-based compensation expense using the graded-vesting method over the longer of the derived service period from the market condition or the required service period. In accordance with accounting guidance for awards with market conditions, the stock-based compensation expense will be recognized over the appropriate period regardless of whether the award achieves the market condition and will only be adjusted to the extent the service condition is not met. When an award contains a market-based vesting condition and a performance-based vesting condition where both must be achieved to earn the award, the Company recognizes stock-based compensation expense over the longer of the derived service period from the market condition or the period estimated for the performance-based vesting condition to be achieved. The Company begins recording stock based compensation expense for this type of award once the achievement of the performance-based vesting condition becomes probable regardless of whether the market condition has been achieved. For restricted stock awards under which restricted common stock is purchased by the holder with a promissory note treated as a nonrecourse note for accounting purposes, the Company measures the fair value of the award using the Black‑Scholes option‑pricing model. The Company classifies stock‑based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2018, the Company’s only element of other comprehensive loss was unrealized losses on marketable securities. For the years ended December 31, 2017 and 2016, there was no difference between net loss and comprehensive loss in the accompanying consolidated financial statements.
Net Income (Loss) per Share	Net Income (Loss) per Share Prior to the closing of its IPO, the Company followed the two-class method when computing net income (loss) per share, as the Company had issued shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reported a net loss, such losses were not allocated to such participating securities, and as a result, basic and diluted net loss per share were the same. Subsequent to the closing of its IPO, basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding common stock equivalents. Accordingly, in periods in which the Company reported a net loss, dilutive common shares were not assumed to have been issued as their effect was anti-dilutive, and as a result, diluted net loss per common share was the same as basic net loss per common share.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two‑step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more‑likely‑than‑not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board, (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Compensation — Stock Compensation (Topic 718), Improvements to Non-Employee Share-Based Payment Accounting (“ASU 2018-07”) . This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption was permitted for all entities but no earlier than the Company’s adoption of ASU 2014-09. The Company adopted ASU 2018-07 effective January 1, 2018 by remeasuring outstanding equity-classified awards issued to non-employees for which a measurement date had not been established as of January 1, 2018 through a cumulative-effect adjustment to accumulated deficit as of January 1, 2018. The Company has elected to estimate the expected term of options utilizing the “simplified” method for both employee and non-employee options that qualify as “plain-vanilla” options. The Company has elected to account for forfeitures for non-employee options as they occur rather than apply an estimated forfeiture rate to stock-based compensation expense . The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of ASU 2018-07 on January 1, 2018 (in thousands): Balance at Balance at December 31, 2017 Adjustments January 1, 2018 Additional paid-in capital $ 17,277 $ (92) $ 17,185 Accumulated deficit $ (60,979) $ 92 $ (60,887) The $0.1 million adjustment is the result of the change in fair value of the unvested awards, representing awards for which a measurement date had not been established, using an expected term rather than the contractual term of the awards. As of the adoption date of January 1, 2018, the Company had 330,917 outstanding options to non-employees for which a measurement date had not been established. As of the adoption date of January 1, 2018, the Company had 4,767,014 shares of restricted common stock held by non-employees that were being accounted for as stock options for which a measurement date had not been established (see Note 10). The weighted average fair value of these awards was $5.88 per share as of January 1, 2018. The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of outstanding awards granted to non-employees for which a measurement date had not been established as of the adoption date of January 1, 2018: Risk-free interest rate 2.3 % Expected volatility 74 % Expected dividend yield — Expected term (in years) 6.1 Common stock value $ 6.28 Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less may be accounted for similar to existing guidance for operating leases today. For public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. Early adoption is permitted. ASU 2016-02 initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842) , which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption. The Company has elected to adopt ASU 2016-02 effective January 1, 2019 through a cumulative-effect adjustment under ASU 2018-11. This standard provides a number of optional practical expedients in transition. The Company plans to apply the package of practical expedients to leases that commenced prior to the effective date whereby it will elect to not reassess the following: (i) whether any expired or existing contracts contain leases; (ii) the lease classification for any expired or existing leases; and (iii) initial direct costs for any existing leases. The Company expects to elect the short-term lease recognition exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for short term leases. The Company expects that the most significant effects of adoption will be the recognition of material new right-of-use assets and corresponding liabilities on its consolidated balance sheet related to its existing facility operating leases (see Note 12). In addition, the Company has a material lease where the Company was deemed the owner during the construction perio d and for which the construction was not complete as of January 1, 2019. The Company took control of the leased space during the first quarter of 2019 at which time the lease commenced. Under ASU 2016-02, as the commencement date of this material lease had not occurred, the new right-of-use assets and corresponding liabilities related to this lease would not be recognized on the consolidated balance sheet as of date of adoption, January 1, 2019, however, will be recognized upon commencement date. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”), which modifies the disclosure requirements for fair value measurements. For all entities, this guidance is required to be adopted for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted. The Company currently is evaluating the impact the adoption of ASU 2018-13 may have on its disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Data
Summary of property plant and equipment	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight‑line method over the estimated useful life of each asset as follows: Estimated useful life Laboratory and office equipment 5 years Computers and software 3 years Furniture and fixtures 7 years Building 30 years Leasehold improvements Shorter of life of lease or 10 years
Non-employees
Selected Quarterly Financial Data
Schedule of assumptions used to determine the fair value of stock awards granted	Risk-free interest rate 2.3 % Expected volatility 74 % Expected dividend yield — Expected term (in years) 6.1 Common stock value $ 6.28
ASU 2018-07
Selected Quarterly Financial Data
Summary of cumulative effect of adopting new accounting principal	The following table summarizes the cumulative effect to the Company’s consolidated balance sheet upon the adoption of ASU 2018-07 on January 1, 2018 (in thousands): Balance at Balance at December 31, 2017 Adjustments January 1, 2018 Additional paid-in capital $ 17,277 $ (92) $ 17,185 Accumulated deficit $ (60,979) $ 92 $ (60,887)
Schedule of assumptions used to determine the fair value of stock awards granted	Year ended December 31, 2017 2016 Risk-free interest rate 2.02% - 2.31% 2.35% - 2.45% Expected volatility 74% - 85% 81% - 85% Expected dividend yield — — Expected term (in years) 6 - 10 8 - 10 Fair value of common stock $0.19 - $6.28 $0.18 - $0.19

Investments and Fair Value of_2

Investments and Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments and Fair Value of Financial Assets and Liabilities
Schedule of investments by security type	Investments by security type consisted of the following (in thousands): December 31, 2018 Gross Gross Unrealized Unrealized Amortized Cost Gains Losses Fair Value U.S. treasury notes (due within one year) $ 79,312 $ — $ (26) $ 79,286 U.S. government agency bonds (due within one year) 17,704 — (3) 17,701 $ 97,016 $ — $ (29) $ 96,987
Schedule of financial assets and liabilities measured at fair value on a recurring basis	The following tables present the Company’s fair value hierarchy for its assets and liabilities, which are measured at fair value on a recurring basis (in thousands): Fair value measurements at December 31, 2018 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 282,160 $ — $ — $ 282,160 U.S. treasury bills — 24,904 — 24,904 Marketable securities: U.S. government agency bonds — 17,701 — 17,701 U.S. treasury notes — 79,286 — 79,286 $ 282,160 $ 121,891 $ — $ 404,051 Fair value measurements at December 31, 2017 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 104,288 $ — $ — $ 104,288 Liabilities: Preferred stock warrant liability $ — $ — $ 866 $ 866
Roll-forward of aggregate fair values of preferred stock warrants for which fair value determined by Level 3 inputs	The following table provides a roll-forward of the aggregate fair values of the Company’s preferred stock warrants for which fair value was determined by Level 3 inputs (in thousands): Preferred stock warrant liability Fair value at December 31, 2016 $ 67 Issuance of warrants to purchase shares of Series B convertible preferred stock 14 Change in fair value 785 Fair value at December 31, 2017 866 Change in fair value through the exercise date 2,187 Reclassification to additional paid-in capital in connection with IPO (3,053) Fair value at December 31, 2018 $ —

Property, Plant and Equipment_2

Property, Plant and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment, Net.
Schedule of property, plant and equipment, Net	Property, plant and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Laboratory equipment $ 7,122 $ 2,751 Land 1,300 — Leasehold improvements 117 117 Computer equipment 276 57 Construction-in-progress 55,828 74 64,643 2,999 Less: Accumulated depreciation and amortization (1,847) (584) $ 62,796 $ 2,415

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities
Schedule of accrued expenses and other current liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2018 2017 Accrued employee compensation and benefits $ 3,377 $ 1,339 Accrued external research and development expenses 2,252 1,230 Accrued lease liability, current portion 4,502 — Other 1,987 417 $ 12,118 $ 2,986

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt
Schedule of long term debt	Long‑term debt consisted of the following (in thousands): December 31, 2018 2017 Principal amount of long‑term debt $ 25,000 $ 5,500 Less: Current portion of long‑term debt — (2,139) Long‑term debt, net of current portion 25,000 3,361 Debt discount (653) (59) Long‑term debt, net of discount and current portion $ 24,347 $ 3,302
Schedule of estimated future principal payments due	As of December 31, 2018, the estimated future principal payments due were as follows (in thousands): Year ending December 31, 2019 $ — 2020 — 2021 — 2022 12,500 Thereafter 12,500 $ 25,000

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Convertible Preferred Stock.
Schedule of convertible preferred stock	As of December 31, 2017, Preferred Stock consisted of the following (in thousands, except share amounts): December 31, 2017 Preferred stock Common stock Preferred stock issued and Carrying Liquidation issuable upon authorized outstanding value preference conversion Series A Preferred Stock 29,703,995 29,570,662 $ 19,723 $ 17,742 29,570,662 Series B Preferred Stock 14,366,813 14,362,344 120,067 120,500 14,362,344 44,070,808 43,933,006 $ 139,790 $ 138,242 43,933,006

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Schedule of stock option activity	Weighted Weighted average average Aggregate Number of exercise contractual intrinsic shares price term value (in years) (in thousands) Outstanding as of December 31, 2017 4,661,635 $ 1.29 8.64 $ 23,264 Granted 10,405,323 12.12 Exercised (225,375) 0.29 Forfeited (56,813) 1.44 Outstanding as of December 31, 2018 14,784,770 $ 8.93 8.89 $ 126,367 Vested and expected to vest as of December 31, 2018 14,784,770 $ 8.93 8.89 $ 126,367 Options exercisable as of December 31, 2018 2,622,981 $ 1.74 7.59 $ 37,726
Schedule of restricted common stock award activity	Weighted average grant‑date Shares fair value Unvested restricted common stock as of December 31, 2017 5,227,014 $ 0.514 Issued — — Vested (3,667,613) 0.519 Forfeited — — Unvested restricted common stock as of December 31, 2018 1,559,401 $ 0.502
Schedule of allocation of share based compensation	The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): Year ended December 31, 2018 2017 2016 Research and development expenses $ 3,787 $ 1,756 $ 107 General and administrative expenses 23,741 16,147 40 $ 27,528 $ 17,903 $ 147
Employees, directors and non-employees
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Schedule of assumptions used to determine the fair value of stock awards granted	Year ended December 31, 2018 2017 2016 Risk-free interest rate 2.71 % 2.05 % 1.81 % Expected volatility 74.0 % 75.6 % 77.2 % Expected dividend yield — — — Expected term (in years) 6.21 6.39 6.25
Market-Based Stock Options
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Schedule of assumptions used to determine the fair value of stock awards granted	Year ended December 31, 2018 2017 2016 Risk-free interest rate 3.15 % — % — % Expected volatility 69.0 % — % — % Expected dividend yield — — — Derived service period (in years) 2.30 — —
ASU 2018-07
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Schedule of assumptions used to determine the fair value of stock awards granted	Year ended December 31, 2017 2016 Risk-free interest rate 2.02% - 2.31% 2.35% - 2.45% Expected volatility 74% - 85% 81% - 85% Expected dividend yield — — Expected term (in years) 6 - 10 8 - 10 Fair value of common stock $0.19 - $6.28 $0.18 - $0.19

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Taxes
Schedule of reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax	Year ended December 31, 2018 2017 Federal statutory income tax rate (21.0) % (34.0) % State taxes, net of federal benefit (4.8) (3.2) Federal and state research and development tax credits (2.9) (1.4) Stock‑based compensation expense 4.5 12.8 Other 0.5 0.7 Remeasurement of deferred taxes due to the Tax Cuts and Jobs Act — 11.1 Increase in deferred tax asset valuation allowance 23.7 14.0 Effective income tax rate — % — %
Summary of changes in the valuation allowance for deferred tax assets	December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 25,508 $ 10,422 Research and development tax credit carryforwards 3,824 1,241 Accrued expenses 1,139 413 Capitalized intellectual property costs 764 367 Capitalized research and development expense 120 131 Stock‑based compensation expense 2,652 375 Total deferred tax assets 34,007 12,949 Deferred tax liabilities: Depreciation and other (341) (374) Total deferred tax liabilities (341) (374) Valuation allowance (33,666) (12,575) Net deferred tax assets $ — $ —
Net deferred tax assets	Year ended December 31, 2018 2017 Valuation allowance as of beginning of year $ 12,575 $ 6,454 Decreases recorded as benefit to income tax provision — (4,887) Increases recorded to income tax provision 21,091 11,008 Valuation allowance as of end of year $ 33,666 $ 12,575

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies
Schedule future minimum payments due under the operating leases	Future minimum lease payments under operating leases as of December 31, 2018 are as follows (in thousands): Year ended December 31, 2019 $ 956 2020 713 2021 547 $ 2,216
Schedule of minimum commitments under the Companys facilities lease	As of December 31, 2018, minimum commitments under this lease are as follows (in thousands): Year ending December 31, 2019 $ 4,502 2020 6,428 2021 6,621 2022 6,820 2023 7,024 Thereafter 27,777 $ 59,172

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Net Loss Per Share
Schedule of calculation of basic and diluted net loss per share attributable to common stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Year ended December 31, 2018 2017 2016 Numerator: Net loss $ (89,195) $ (43,847) $ (11,016) Accretion of Series A redeemable convertible preferred stock to redemption value — (656) (748) Net loss attributable to common stockholders $ (89,195) $ (44,503) $ (11,764) Denominator: Weighted average common shares outstanding, basic and diluted 39,285,468 8,023,785 7,200,581 Net loss per share attributable to common stockholders, basic and diluted $ (2.27) $ (5.55) $ (1.63)
Schedule of dilutive securities excluded from computations of diluted weighted average shares outstanding	Year ended December 31, 2018 2017 2016 Convertible preferred stock (as converted to common stock) — 43,933,006 29,570,662 Warrants to purchase convertible preferred stock (as converted to common stock) — 135,567 133,333 Restricted common stock(1) 2,230,235 6,277,014 348,120 Stock options to purchase common stock 14,784,770 4,661,635 4,159,165 17,015,005 55,007,222 34,211,280 (1) Includes unvested restricted stock and vested restricted stock issued for promissory notes.

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Data
Quarterly financial information	The following table contains quarterly financial information for 2018 and 2017. The information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information (in thousands, except per share data): Three Months Ended March 31, June 30, September 30, December 31, 2018(1) 2018(1) 2018 2018 Consolidated Statements of Operations Data: Revenue $ — $ — $ — $ — Total operating expenses 14,603 20,384 27,554 29,122 Loss from operations (14,603) (20,384) (27,554) (29,122) Net loss attributable to common stockholders (14,411) (21,239) (26,362) (27,183) Net loss per share attributable to common stockholders, basic and diluted $ (1.72) $ (2.43) $ (0.42) $ (0.35) Three Months Ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Consolidated Statements of Operations Data: Revenue $ — $ — $ — $ — Total operating expenses 4,783 9,033 11,975 17,473 Loss from operations (4,783) (9,033) (11,975) (17,473) Net loss attributable to common stockholders (5,233) (9,834) (11,919) (17,517) Net loss per share attributable to common stockholders, basic and diluted $ (0.68) $ (1.25) $ (1.48) $ (2.07)
ASU 2018-07
Selected Quarterly Financial Data
Quarterly financial information	The following tables summarize the impact of adoption to the Company’s previously issued consolidated statements of operations and comprehensive loss (in thousands): Three Months Ended Three Months Ended March 31, 2018 June 30, 2018 As previously As previously reported As revised reported As revised Operating expenses: Research and development $ 9,650 $ 9,506 $ 11,965 $ 11,361 General and administrative 5,797 5,097 16,279 9,023 Total operating expenses 15,447 14,603 28,244 20,384 Loss from operations (15,447) (14,603) (28,244) (20,384) Net loss attributable to common stockholders $ (15,255) $ (14,411) $ (29,099) $ (21,239) Net loss per share attributable to common stockholders, basic and diluted $ (1.83) $ (1.72) $ (3.33) $ (2.43)

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands	Jul. 20, 2018	Jul. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Sale of Equity
Net proceeds after deducting underwriting discounts and commissions and other offering costs											$ 257,866
Recurring losses			$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (17,517)	$ (11,919)	$ (9,834)	$ (5,233)	(89,195)	$ (44,503)	$ (11,764)
Accumulated deficit			$ (150,082)				$ (60,979)				$ (150,082)	$ (60,979)
Common stock
Sale of Equity
Shares issued (in shares)											12,055,450
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438	51,845,438									51,845,438
IPO
Sale of Equity
Shares issued (in shares)	12,055,450
Net proceeds after deducting underwriting discounts and commissions and other offering costs	$ 254,300
Conversion of redeemable convertible preferred stock to common stock (in shares)		131,273
Overallotment
Sale of Equity
Shares issued (in shares)	1,572,450

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($)	36 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Restricted cash for the benefit of the landlords of leased properties	$ 1,800,000	$ 300,000
Restricted cash to collateralize corporate credit card	500,000
Restricted cash (non-current)	1,735,000	$ 284,000
Restricted cash (current)	622,000
Other than Temporary Impairment Losses, Investments	$ 0

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Estimated useful life (Details)	12 Months Ended
	Dec. 31, 2018
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computers and software
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Building
Property, Plant and Equipment [Line Items]
Estimated useful life	30 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Data
Additional paid-in capital		$ 543,040,000	$ 17,277,000
Accumulated deficit		$ (150,082,000)	$ (60,979,000)
Change in the fair value of the unvested awards	$ 100,000
Assumptions used to determine the fair value of outstanding awards granted
Expected volatility		74.00%	75.60%	77.20%
Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Weighted average grant-date fair value (per share)	$ 5.88
Risk-free interest rate	2.30%
Expected volatility	74.00%
Expected term (in years)	6 years 1 month 6 days
Common stock value	$ 6.28
Restricted Common Stock | Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Number of outstanding options	4,767,014
Stock options | Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Number of outstanding options	330,917
ASU 2018-07
Selected Quarterly Financial Data
Additional paid-in capital	$ 17,185,000
Accumulated deficit	(60,887,000)
Adjustments for New Accounting Principle, Early Adoption [Member] | ASU 2018-07
Selected Quarterly Financial Data
Additional paid-in capital	(92,000)
Accumulated deficit	$ 92,000

Investments and Fair Value of_3

Investments and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Marketable securities
Amortized Cost	$ 97,016
Gross Unrealized Losses	(29)
Fair Value	96,987
Assets:
Investments	96,987
Transfers between Level 1, Level 2 or Level 3
Transfer from Level 1 to Level 2, assets	0	$ 0
Transfer from Level 2 to Level 1, assets	0	0
Transfers into Level 3, assets	0	0
Transfer out of Level 3, assets	0	0
Transfer from Level 1 to Level 2, liabilities	0	0
Transfer from Level 2 to Level 1, liabilities	0	0
Transfers into Level 3, liabilities	0
U.S. government agency bonds
Marketable securities
Amortized Cost	17,704
Gross Unrealized Losses	(3)
Fair Value	17,701
Assets:
Investments	17,701
U.S. treasury notes
Marketable securities
Amortized Cost	79,312
Gross Unrealized Losses	(26)
Fair Value	79,286
Assets:
Investments	79,286
Preferred stock warrant liability
Roll forward of aggregate fair values of preferred stock warrants
Fair value at beginning of period	866	67
Issuance of warrants to purchase shares of Series B convertible preferred stock		14
Change in fair value through the exercise date	2,187	785
Reclassification to additional paid-in capital in connection with IPO	(3,053)
Fair value at end of period		866
Recurring
Assets:
Assets	404,051
Recurring | Money market funds
Assets:
Cash equivalents	282,160	104,288
Recurring | U.S. treasury bills
Assets:
Cash equivalents	24,904
Recurring | U.S. government agency bonds
Marketable securities
Fair Value	17,701
Assets:
Investments	17,701
Recurring | U.S. treasury notes
Marketable securities
Fair Value	79,286
Assets:
Investments	79,286
Recurring | Preferred stock warrant liability
Liabilities:
Liabilities		866
Recurring | Level 1
Assets:
Assets	282,160
Recurring | Level 1 | Money market funds
Assets:
Cash equivalents	282,160	104,288
Recurring | Level 2
Assets:
Assets	121,891
Recurring | Level 2 | U.S. treasury bills
Assets:
Cash equivalents	24,904
Recurring | Level 2 | U.S. government agency bonds
Marketable securities
Fair Value	17,701
Assets:
Investments	17,701
Recurring | Level 2 | U.S. treasury notes
Marketable securities
Fair Value	79,286
Assets:
Investments	$ 79,286
Recurring | Level 3 | Preferred stock warrant liability
Liabilities:
Liabilities		$ 866

Property, Plant and Equipment_3

Property, Plant and Equipment, Net (Details) $ in Thousands	Jul. 31, 2018USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		$ 64,643	$ 2,999
Less: Accumulated depreciation and amortization		(1,847)	(584)
Property, plant and equipment, net		62,796	2,415
Depreciation and amortization expense		1,300	400	$ 100
Laboratory equipment
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		7,122	2,751
Construction-in-progress		900
Land
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		1,300
Purchases and additions	$ 1,300
Leasehold improvements
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		117	117
Construction-in-progress		1,000
Computers and software
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		276	57
Construction-in-progress
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		55,828	$ 74
Purchases and additions	$ 6,700
Build-to-suit lease
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		45,100
Manufacturing facility
Property, Plant and Equipment, Net
Square footing of facility acquired | ft²	135,000
Purchases and additions	$ 8,000
Technology Service | Construction-in-progress
Property, Plant and Equipment, Net
Purchases and additions		$ 2,100

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses and Other Current Liabilities
Accrued employee compensation and benefits	$ 3,377	$ 1,339
Accrued external research and development expenses	2,252	1,230
Accrued lease liability, current portion	4,502
Other	1,987	417
Accrued expenses and other current liabilities	$ 12,118	$ 2,986

Debt_current_Non current (Detai

Debt_current_Non current (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt
Principal amount of long-term debt	$ 25,000	$ 5,500
Less: Current portion of long-term debt		(2,139)
Long term debt net of current maturities	25,000	3,361
Debt discount	(653)	(59)
Long‑term debt, net of current portion	$ 24,347	$ 3,302

Debt (Details)

Debt (Details) $ in Thousands	Dec. 21, 2018USD ($)item	Dec. 31, 2018USD ($)	May 31, 2018	May 31, 2017	Dec. 31, 2018USD ($)
Debt
Repayment					$ 5,500
Estimated future principal payments due
2022		$ 12,500			12,500
Thereafter		12,500			12,500
Total debt		25,000			25,000
2015 Credit Facility
Debt
Principal amount		5,500			$ 5,500
Floor rate			5.50%	4.50%
Percentage of change in debt modification from debt extinguishment			10.00%
Repayment		5,500
2015 Credit Facility | Maximum
Debt
Loss on extinguishment		$ (100)
2015 Credit Facility | Prime rate
Debt
Variable interest rate			0.75%	1.25%
2018 Credit Facility
Debt
Maximum amount outstanding	$ 75,000
Number of tranche | item	3
Initial borrowing	$ 25,000
Period for commencing monthly principal payments	36 years
Amortization period	24 years
Prepayment fee in first year	1.00%
Prepayment fee in second year	0.50%
Prepayment fee in third year	0.25%
Issuance costs	$ 800
Debt default interest rate addition (as a percent)	4
2018 Credit Facility | One-month U.S. LIBOR
Debt
Variable interest rate	5.50%
2018 Credit Facility | Term loan
Debt
Principal amount of each tranche of term loans	$ 25,000

Convertible Preferred Stock (De

Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 20, 2018	Jul. 31, 2018	Feb. 28, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Convertible Preferred Stock
Proceeds from issuance of convertible preferred stock					$ 100,986	$ 120,067	$ 11,437
Issuance cost					$ 3,548
Preferred stock authorized						44,070,808
Preferred stock issued						43,933,006
Preferred stock outstanding					0	43,933,006
Preferred stock carrying value						$ 139,790
Preferred stock, liquidation preference						$ 138,242
Common stock issuable upon conversion						43,933,006
Series A redeemable convertible preferred stock
Convertible Preferred Stock
Percentage of cumulative dividends per year					8.00%
Convertible preferred stock redemption period					5 years
Maximum percentage of change in fair value of preferred stock					10.00%
Preferred stock authorized						29,703,995
Preferred stock issued						29,570,662
Preferred stock outstanding						29,570,662
Preferred stock carrying value						$ 19,723
Preferred stock, liquidation preference						$ 17,742
Common stock issuable upon conversion						29,570,662
Series B convertible preferred stock
Convertible Preferred Stock
Convertible preferred stock issue price (dollars per share)				$ 8.39
Proceeds from issuance of convertible preferred stock				$ 120,500
Issuance cost				$ 400
Preferred stock authorized						14,366,813
Preferred stock issued				14,362,344		14,362,344
Preferred stock outstanding						14,362,344
Preferred stock carrying value						$ 120,067
Preferred stock, liquidation preference						$ 120,500
Common stock issuable upon conversion						14,362,344
Series C convertible preferred stock
Convertible Preferred Stock
Convertible preferred stock issue price (dollars per share)			$ 12.79
Proceeds from issuance of convertible preferred stock			$ 101,200
Issuance cost			$ 200
Preferred stock issued			7,912,432
Common stock
Convertible Preferred Stock
Number of common stock issued on conversion	51,845,438	51,845,438			51,845,438

Warrants to Purchase Converti_2

Warrants to Purchase Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Jul. 31, 2018	May 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Warrants to purchase convertible preferred stock
Other expenses			$ 2.2	$ 0.8
IPO
Warrants to purchase convertible preferred stock
Warrants to purchase aggregate	135,567
Issuance of warrants exercise	131,273
Shares withheld to pay exercise price	4,294
Maximum
Warrants to purchase convertible preferred stock
Other income					$ 0.1
Series A redeemable convertible preferred stock
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants						133,333
Warrants exercisable price						$ 0.60
Warrants contractual term (in years)						10 years
Fair value of warrants						$ 0.1
Series B convertible preferred stock
Warrants to purchase convertible preferred stock
Maximum number of preferred shares can purchase from outstanding warrants		2,234
Warrants exercisable price		$ 8.39
Warrants contractual term (in years)		10 years
Series B convertible preferred stock | Maximum
Warrants to purchase convertible preferred stock
Fair value of warrants		$ 0.1

Equity (Details)

Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 20, 2018	Dec. 31, 2018	Jun. 30, 2018	Apr. 30, 2018	Feb. 28, 2018	Jan. 31, 2018	Dec. 31, 2017
Equity
Common stock, authorized (in shares)	150,000,000	150,000,000	79,000,000	78,800,000	75,000,000	65,000,000	65,000,000
Shares authorized	160,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001					$ 0.001
Preferred stock, share authorized (in shares)	10,000,000	10,000,000					0
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001					$ 0.001
Net proceeds after deducting underwriting discounts and commissions and other offering costs		$ 257,866
Common stock
Equity
Stock Issued During Period, Shares, New Issues		12,055,450
Number of shares converted into number of common stock	51,845,438
IPO
Equity
Stock Issued During Period, Shares, New Issues	12,055,450
Net proceeds after deducting underwriting discounts and commissions and other offering costs	$ 254,300
Overallotment
Equity
Stock Issued During Period, Shares, New Issues	1,572,450

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - shares	Jan. 01, 2019	Jul. 06, 2018	Dec. 31, 2018
2014 Stock Incentive Plan
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Expiration period			10 years
Total number of shares of common stock authorized to issue			19,152,328
Number of shares remained available for future issuance			47,447
2014 Stock Incentive Plan | Minimum
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Vesting period			3 years
2014 Stock Incentive Plan | Maximum
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Vesting period			4 years
2014 Stock Incentive Plan | Restricted Common Stock | Minimum
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Vesting period			3 years
2014 Stock Incentive Plan | Restricted Common Stock | Maximum
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Vesting period			4 years
2018 Equity Incentive Plan
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Total number of shares of common stock authorized to issue		5,708,931
Number of shares remained available for future issuance			2,335,217
Annual increase of shares reserved for awards (as a percent)		4.00%
Increase in number of authorized shares reserved for issuance	2,377,045
2018 Employee Stock Purchase Plan
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Total number of shares of common stock authorized to issue		951,488
Number of shares remained available for future issuance			951,488
Annual increase of shares reserved for awards (as a percent)		1.00%
Annual increase of shares reserved for awards if less than 1% of the outstanding stock		951,488
Increase in number of authorized shares reserved for issuance	792,348

Stock-Based Compensation - Empl

Stock-Based Compensation - Employees, directors and non-employees (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Expected volatility	74.00%	75.60%	77.20%
Employees, directors and non-employees
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Risk-free interest rate	2.71%	2.05%	1.81%
Expected term (in years)	6 years 2 months 16 days	6 years 4 months 21 days	6 years 3 months
Market-Based Stock Options
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Risk-free interest rate	3.15%
Expected volatility	69.00%
Expected term (in years)	2 years 3 months 18 days

Stock-Based Compensation - Gran

Stock-Based Compensation - Granted to non-employees, prior to the adoption of ASU 2018-07 (Details) - Non-employees - ASU 2018-07 - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Minimum
Fair value of stock awards granted prior to the adoption of ASU 2018-07
Risk-free interest rate	2.02%	2.35%
Expected volatility	74.00%	81.00%
Expected term (in years)	6 years	8 years
Fair value of common stock	$ 0.19	$ 0.18
Maximum
Fair value of stock awards granted prior to the adoption of ASU 2018-07
Risk-free interest rate	2.31%	2.45%
Expected volatility	85.00%	85.00%
Expected term (in years)	10 years	10 years
Fair value of common stock	$ 6.28	$ 0.19

Stock-Based Compensation - Opti

Stock-Based Compensation - Option activity (Details) - Stock options. - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Number of shares
Outstanding at beginning of period	4,661,635
Granted	10,405,323
Exercised	(225,375)
Forfeited	(56,813)
Outstanding at end of period	14,784,770	4,661,635
Vested and expected to vest as of December 31, 2018	14,784,770
Options exercisable as of December 31, 2018	2,622,981
Weighted average exercise price
Outstanding at the beginning (in dollars per share)	$ 1.29
Granted (in dollars per share)	12.12
Exercised (in dollars per share)	0.29
Forfeited (in dollars per share)	1.44
Outstanding at the end (in dollars per share)	8.93	$ 1.29
Vested and expected to vest (in dollars per share)	8.93
Options exercisable (in dollars per share	$ 1.74
Weighted average contractual term and aggregate intrinsic value
Outstanding at the beginning (in years)	8 years 10 months 21 days	8 years 7 months 21 days
Outstanding at the end (in years)	8 years 10 months 21 days	8 years 7 months 21 days
Vested and expected to vest (in years)	8 years 10 months 21 days
Options exercisable (in years)	7 years 7 months 2 days
Outstanding at the beginning (in dollars)	$ 23,264
Outstanding at the end (in dollars)	126,367	$ 23,264
Vested and expected to vest (in dollars)	126,367
Options exercisable (in dollars)	$ 37,726

Stock-Based Compensation - Perf

Stock-Based Compensation - Performance based stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 21, 2018	Oct. 31, 2018	Oct. 31, 2017	Apr. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Proceeds from issuance of common stock upon exercise of stock options					$ 64	$ 37
Receivables from promissory note recorded in balance sheet				$ 0
Stock-based compensation expense			$ 1,000		27,528	17,903	$ 147
Executive officer
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				1,400,000
Options exercise price				$ 0.18
Proceeds from issuance of common stock upon exercise of stock options				$ 100
Amount of promissory note received from early exercise of stock options				200
Principal balance of promissory note repaid	$ 200
Interest on promissory note repaid	$ 100
Stock-based compensation, fair value					$ 4,300
Stock options.
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross					10,405,323
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period					225,375
Aggregate intrinsic value of stock options exercised					$ 3,700	$ 3,700	$ 100
Weighted average grant-date fair value (per share)					$ 8.71	$ 2.74	$ 0.12
Aggregate Intrinsic value					$ 126,367	$ 23,264
Options exercise price					$ 0.29
Liability from early exercise of stock options				$ 100
Stock-based compensation expense					$ 2,200
Stock options. | Performance-based vesting
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross					447,000
Market-Based Stock Options
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period					0
Weighted average grant-date fair value (per share)					$ 11.88
Market-Based Stock Options | Vesting based on service period | Executive officer
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		164,400
Options exercise price		$ 16.43
Stock-based compensation expense					$ 200
Market-Based Stock Options | Performance-based vesting | Executive officer
Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		193,400
Options exercise price		$ 16.43

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Common Stock (Details) $ / shares in Units, $ in Thousands	Jun. 21, 2018USD ($)	Oct. 31, 2018$ / shares	May 31, 2017USD ($)item$ / sharesshares	Apr. 30, 2017USD ($)$ / sharesshares	Jan. 31, 2017$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)	Dec. 31, 2018shares	Dec. 31, 2017shares	May 31, 2017USD ($)shares	Apr. 30, 2017USD ($)shares	Jan. 31, 2017USD ($)shares
Weighted Average Grant Date Fair Value
The aggregate intrinsic value of restricted stock awards vested | $						$ 34,000	$ 900	$ 100
Executive officer
Weighted Average Grant Date Fair Value
Exercised				1,400,000
Restricted stock exercise price | $ / shares				$ 0.18
Principal balance of promissory note repaid | $	$ 200
Interest on promissory note repaid | $	100
Restricted Common Stock
Number of shares
Unvested restricted common stock beginning of period						5,227,014
Vested						(3,667,613)
Unvested restricted common stock end of period						1,559,401	5,227,014
Weighted Average Grant Date Fair Value
Outstanding at the ending (in dollars per share) | $ / shares						$ 0.514
Weighted Average Grant Date Fair Value- vested (in dollars per share) | $ / shares						0.519
Ending at the beginning (in dollars per share) | $ / shares						$ 0.502	$ 0.514
Total unvested restricted common stock including early exercise of stock options						670,834
Shares issued						5,227,014	5,227,014		1,559,401	5,227,014
Number of shares exercised							1,400,000
Unvested shares of restricted common stock, including shares from the early exercise of a stock option									2,230,235
Amount recorded in balance sheet due to promissory notes received from sale of common stock | $											$ 0	$ 0	$ 0
Principal amount of promissory note forgiven | $	2,500
Interest on promissory note forgiven | $	100
Unvested restricted common stock									670,834	1,050,000
Shares exchanged						5,227,014	5,227,014		1,559,401	5,227,014
Common stock Award									1,559,401
Incremental share based compensation expense | $						$ 1,500
Restricted Common Stock | Executive officer
Number of shares
Unvested restricted common stock beginning of period			460,000
Unvested restricted common stock end of period				460,000
Weighted Average Grant Date Fair Value
Shares issued			460,000	460,000								460,000
Restricted common stock exercise price | $ / shares				$ 0.19
Principal amount of promissory note received in exchange for restricted common stock | $				$ 100
Principal balance of promissory note repaid | $	100
Interest on promissory note repaid | $	$ 100
Shares exchanged			460,000	460,000								460,000
Restricted Common Stock | Board of Directors Chairman
Number of shares
Unvested restricted common stock end of period			1,100,000		3,667,014
Weighted Average Grant Date Fair Value
Shares issued			1,100,000		3,667,014						1,100,000		3,667,014
Restricted common stock exercise price | $ / shares			$ 1.65		$ 0.19
Principal amount of promissory note received in exchange for restricted common stock | $			$ 2,500
Number of promissory notes | item			2
Shares exchanged			1,100,000		3,667,014						1,100,000		3,667,014
Market-Based Stock Options
Weighted Average Grant Date Fair Value
Weighted average grant-date fair value (per share) | $ / shares						$ 11.88
Exercised						0
Forfeited						0
Market-Based Stock Options | Vesting based on service period | Executive officer
Weighted Average Grant Date Fair Value
Restricted stock exercise price | $ / shares		$ 16.43

Stock-Based Compensation - Comp

Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Stock-based compensation expense
Stock-based compensation expense	$ 1,000	$ 27,528	$ 17,903	$ 147
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense		3,787	1,756	107
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense		$ 23,741	$ 16,147	$ 40
Employees, directors and non-employees
Stock-based compensation expense
Unrecognized compensation cost					$ 89,600
Unrecognized compensation cost expected to be recognized over a weighted average period		2 years 8 months 12 days
Chairman
Stock-based compensation expense
Issuance of common stock for one‑time bonus payment (in shares)	213,439
Stock options.
Stock-based compensation expense
Stock-based compensation expense		$ 2,200
Shares issued					232,500
Purchase of an aggregate		10,405,323
Stock options. | Performance-based vesting
Stock-based compensation expense
Purchase of an aggregate		447,000
Restricted Common Stock
Stock-based compensation expense
Stock-based compensation expense		$ 7,300
Shares issued		5,227,014	5,227,014		1,559,401
Unvested restricted common stock beginning of period		5,227,014

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate	21.00%	34.00%
Federal Operating loss carryforwards	$ 93,200
State Operating loss carryforwards	93,900
Federal Operating loss carryforwards subject to expiration	37,200
Federal Operating loss carryforwards subject to expiration indefinitely	56,000
Research and development tax credit carryforwards	3,824	$ 1,241
Increase in gross deferred tax assets, before valuation allowance	21,100
Accrued interest or penalties	0	0
Uncertain tax positions	0	$ 0
Maximum
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate		35.00%
U.S. federal
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	3,000
State
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	$ 1,100

Income Taxes - U S federal stat

Income Taxes - U S federal statutory income tax rate (Details) - USD ($) $ in Millions	12 Months Ended		24 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Income Taxes
Income tax benefits - operating losses incurred			$ 0
Income tax benefits - research and development tax credits			$ 0
Federal statutory income tax rate	(21.00%)	(34.00%)
State taxes, net of federal benefit	(4.80%)	(3.20%)
Federal and state research and development tax credits	(2.90%)	(1.40%)
Stock-based compensation expense	4.50%	12.80%
Other	0.50%	0.70%
Remeasurement of deferred taxes due to the Tax Cuts and Jobs Act		11.10%
Increase in deferred tax asset valuation allowance	23.70%	14.00%

Income Taxes - Net deferred tax

Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 25,508	$ 10,422
Research and development tax credit carryforwards	3,824	1,241
Accrued expenses	1,139	413
Capitalized intellectual property costs	764	367
Capitalized research and development expense	120	131
Stock-based compensation expense	2,652	375
Total deferred tax assets	34,007	12,949
Deferred tax liabilities:
Depreciation and other	(341)	(374)
Total deferred tax liabilities	(341)	(374)
Valuation allowance	$ (33,666)	$ (12,575)	$ (6,454)

Income Taxes - Valuation allowa

Income Taxes - Valuation allowance for deferred tax assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Valuation allowance as of beginning of year	$ 12,575	$ 6,454
Decreases recorded as benefit to income tax provision		(4,887)
Increases recorded to income tax provision	21,091	11,008
Valuation allowance as of end of year	$ 33,666	$ 12,575

Commitments and Contingencies -

Commitments and Contingencies - Operating Lease (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)lease	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Commitments and Contingencies
Number of leases | lease	3
Rent expense	$ 2,400	$ 1,000	$ 400
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	956
2020	713
2021	547
Total	$ 2,216

Commitments and Contingencies_2

Commitments and Contingencies - Capital Lease (Details) - USD ($) $ in Thousands	1 Months Ended
	Nov. 30, 2018	Jan. 31, 2018	Dec. 31, 2018
Commitments and Contingencies
Lease term	9 years	8 years
Renewal option term		5 years
Initial base rent		$ 3,800
Expansion Rent	$ 2,500
Base rent incremental (as a percentage)	3.00%	3.00%
Benefit of the landlord secured by cash deposit	$ 600	$ 900
Tenant improvement allowance	$ 7,200	9,900
Repayable to landlord		$ 500
Construction-in-progress within property and equipment			$ 45,100
Current portion of the lease financing obligation			4,500
Lease liability, net of current portion			41,441
Minimum commitments under this lease
2019			4,502
2020			6,428
2021			6,621
2022			6,820
2023			7,024
Thereafter			27,777
Total			$ 59,172

Commitments and Contingencies_3

Commitments and Contingencies - Collaborative Arrangements And Non-collaborative Arrangement (Details) - WIBR $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Collaborative Arrangements and Non-collaborative Arrangement
Aggregate milestone payments	$ 1.6
Maximum | License maintenance fees
Collaborative Arrangements and Non-collaborative Arrangement
License costs	$ 0.1

Commitments and Contingencies_4

Commitments and Contingencies - Defined Contribution Plan (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2018
Commitments and Contingencies
Employer matching contribution (as a percent)	50.00%
Percentage of employee's gross pay match	6.00%
Company matching contribution		$ 0.2

Net Loss Per Share - Weighted A

Net Loss Per Share - Weighted Average Shares (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss									$ (89,195)	$ (43,847)	$ (11,016)
Accretion of Series A redeemable convertible preferred stock to redemption value										(656)	(748)
Net loss attributable to common stockholders	$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (17,517)	$ (11,919)	$ (9,834)	$ (5,233)	$ (89,195)	$ (44,503)	$ (11,764)
Denominator:
Weighted average common shares outstanding basic and diluted									39,285,468	8,023,785	7,200,581
Net loss per share attributable to common stockholders, basic and diluted	$ (0.35)	$ (0.42)	$ (2.43)	$ (1.72)	$ (2.07)	$ (1.48)	$ (1.25)	$ (0.68)	$ (2.27)	$ (5.55)	$ (1.63)

Net Loss Per Share - Antidiluti

Net Loss Per Share - Antidilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	17,015,005	55,007,222	34,211,280
Convertible preferred stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding		43,933,006	29,570,662
Warrants to purchase convertible preferred stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding		135,567	133,333
Restricted Common Stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	2,230,235	6,277,014	348,120
Stock options
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	14,784,770	4,661,635	4,159,165
Series A redeemable convertible preferred stock
Diluted net loss per share attributable to common stockholders
Percentage of cumulative dividends per year	8.00%

Related Parties (Details)

Related Parties (Details) $ in Thousands	Jun. 21, 2018USD ($)	May 31, 2017USD ($)	Apr. 30, 2017USD ($)item	Jan. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Related Parties
Due to related parties						$ 0
Maturity term from issuance date of the notes		7 years	7 years
Maturity date of termination		60 days	60 days
Interest rate (as a percent)			2.05%
Aggregate principal balance						0
Amount from related party						0
Amount due to related party						0
January 2017 promissory note
Related Parties
Due to related parties						0
Interest rate (as a percent)				1.97%
Aggregate principal balance						0
Amount from related party						0
Amount due to related party						0
May 2017 promissory note
Related Parties
Interest rate (as a percent)		2.04%
Aggregate principal balance						0
Amount from related party						0
Flagship | Services agreement
Related Parties
Payment of general and administrative expenses					$ 1,300	900	$ 800
Cash payments					$ 1,300	$ 900	$ 800
Board of Directors Chairman
Related Parties
Amount loaned or remitted to related parties		$ 1,800		$ 700
Principal amount of promissory note forgiven	$ 2,500
Interest on promissory note forgiven	100
Executive officer
Related Parties
Amount loaned or remitted to related parties			$ 200
Number of promissory notes | item			2
Number of restricted stock agreements | item			2
Aggregate principal balance	200
Interest income form related parties	100
Amount from related party	$ 200

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statements of Operations Data:
Total operating expenses	$ 29,122	$ 27,554	$ 20,384	$ 14,603	$ 17,473	$ 11,975	$ 9,033	$ 4,783	$ 91,663	$ 43,264	$ 10,852
Loss from operations	(29,122)	(27,554)	(20,384)	(14,603)	(17,473)	(11,975)	(9,033)	(4,783)	(91,663)	(43,264)	(10,852)
Net loss attributable to common stockholders	$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (17,517)	$ (11,919)	$ (9,834)	$ (5,233)	$ (89,195)	$ (44,503)	$ (11,764)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.35)	$ (0.42)	$ (2.43)	$ (1.72)	$ (2.07)	$ (1.48)	$ (1.25)	$ (0.68)	$ (2.27)	$ (5.55)	$ (1.63)

Selected Quarterly Financial _4

Selected Quarterly Financial Data (Unaudited) - Impact of adoption (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development									$ 51,769	$ 21,226	$ 8,403
General and administrative									39,894	22,038	2,449
Total operating expenses	$ 29,122	$ 27,554	$ 20,384	$ 14,603	$ 17,473	$ 11,975	$ 9,033	$ 4,783	91,663	43,264	10,852
Loss from operations	(29,122)	(27,554)	(20,384)	(14,603)	(17,473)	(11,975)	(9,033)	(4,783)	(91,663)	(43,264)	(10,852)
Net loss attributable to common stockholders	$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (17,517)	$ (11,919)	$ (9,834)	$ (5,233)	$ (89,195)	$ (44,503)	$ (11,764)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.35)	$ (0.42)	$ (2.43)	$ (1.72)	$ (2.07)	$ (1.48)	$ (1.25)	$ (0.68)	$ (2.27)	$ (5.55)	$ (1.63)
Comprehensive loss:
Net loss									$ (89,195)	$ (43,847)	$ (11,016)
Other comprehensive loss:
Unrealized losses on investments, net of tax of $0									(29)
Comprehensive loss									$ (89,224)	$ (43,847)	$ (11,016)
ASU 2018-07
Operating expenses:
Research and development			$ 11,361	$ 9,506
General and administrative			9,023	5,097
Total operating expenses			20,384	14,603
Loss from operations			(20,384)	(14,603)
Net loss attributable to common stockholders			$ (21,239)	$ (14,411)
Net loss per share attributable to common stockholders, basic and diluted			$ (2.43)	$ (1.72)
As previously reported | ASU 2018-07
Operating expenses:
Research and development			$ 11,965	$ 9,650
General and administrative			16,279	5,797
Total operating expenses			28,244	15,447
Loss from operations			(28,244)	(15,447)
Net loss attributable to common stockholders			$ (29,099)	$ (15,255)
Net loss per share attributable to common stockholders, basic and diluted			$ (3.33)	$ (1.83)