Rubius Therapeutics (RUBY) 10-K 12 Mar 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 02, 2020	Jun. 28, 2019
Document and Entity Information
Document Type	10-K
Document Period End Date	Dec. 31, 2019
Entity Registrant Name	RUBIUS THERAPEUTICS, INC.
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Public Float			$ 548.7
Entity Common Stock, Shares Outstanding		80,321,978
Entity Central Index Key	0001709401
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 91,898	$ 307,064
Investments	191,389	96,987
Prepaid expenses and other current assets	5,979	9,737
Restricted cash		622
Total current assets	289,266	414,410
Operating lease, right-of-use-asset	46,559
Property, plant and equipment, net	56,924	62,796
Restricted cash	1,735	1,735
Other assets	357	168
Total assets	394,841	479,109
Current liabilities:
Accounts payable	7,178	7,886
Accrued expenses and other current liabilities	16,042	12,118
Operating lease liabilities	10,540
Total current liabilities	33,760	20,004
Long-term debt, net of discount	49,596	24,347
Other long-term liabilities	405	309
Operating lease liabilities, net of current portion	36,867
Lease financing obligation		41,441
Total liabilities	120,628	86,101
Commitments and contingencies (see Note 12)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2019 and December 31, 2018; no shares issued or outstanding at December 31, 2019 and December 31, 2018
Common stock, $0.001 par value; 150,000,000 shares authorized at December 31, 2019 and December 31, 2018; 80,016,245 and 79,234,853 shares issued and outstanding at December 31, 2019 and December 31, 2018, respectively	80	79
Additional paid-in capital	586,798	543,040
Accumulated other comprehensive income (loss)	75	(29)
Accumulated deficit	(312,740)	(150,082)
Total stockholders' equity	274,213	393,008
Total liabilities and stockholders' equity	$ 394,841	$ 479,109

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
CONSOLIDATED BALANCE SHEETS
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, share authorized (in shares)	10,000,000	10,000,000
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	150,000,000
Common stock, shares issued (in shares)	80,016,245	79,234,853
Common stock, shares outstanding (in shares)	80,016,245	79,234,853

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 112,419	$ 51,769	$ 21,226
General and administrative	57,182	39,894	22,038
Total operating expenses	169,601	91,663	43,264
Loss from operations	(169,601)	(91,663)	(43,264)
Other income (expense):
Interest income	7,994	5,078	584
Interest expense	(2,590)	(464)	(309)
Change in fair value of preferred stock warrant liability		(2,187)	(785)
Other income (expense), net	739	41	(73)
Total other income (expense), net	6,143	2,468	(583)
Net loss	(163,458)	(89,195)	(43,847)
Accretion of Series A redeemable convertible preferred stock to redemption value			(656)
Net loss attributable to common stockholders	$ (163,458)	$ (89,195)	$ (44,503)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.08)	$ (2.27)	$ (5.55)
Weighted average common shares outstanding, basic and diluted	78,688,878	39,285,468	8,023,785
Comprehensive loss:
Net loss	$ (163,458)	$ (89,195)	$ (43,847)
Other comprehensive income (loss):
Unrealized gains (losses) on investments, net of tax of $0	104	(29)
Comprehensive loss	$ (163,354)	$ (89,224)	$ (43,847)

CONSOLIDATED STATEMENTS OF OP_2

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Unrealized gains (losses) on investments, tax	$ 0	$ 0	$ 0

CONSOLIDATED STATEMENTS OF CONV

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit	Convertible preferred stock	Series B convertible preferred stock	Series C convertible preferred stock	Total
Balances at Beginning of period (in shares) at Dec. 31, 2016	7,886,292
Balances at Beginning of period at Dec. 31, 2016	$ 8			$ (17,132)				$ (17,124)
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)
Balances at Beginning of period (in shares) at Dec. 31, 2016					29,570,662
Balances at Beginning of period at Dec. 31, 2016					$ 19,067
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
Balances at End of period at Dec. 31, 2017					$ 139,790
Shares issued	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
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
Vesting of restricted common stock		180						180
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
Unrealized gains (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
Cumulative effect adjustment for adoption		(92)		92
Issuance of common stock upon exercise of stock options (in shares)	1,449,309
Issuance of common stock upon exercise of stock options	$ 2	2,443						$ 2,445
Stock-based compensation expense		41,271						41,271
Repurchase of unvested restricted common stock (in shares)	(667,917)
Repurchase of unvested restricted common stock	$ (1)							(1)
Vesting of restricted common stock		44						44
Unrealized gains (losses) on investments			104					104
Net loss				(163,458)				(163,458)
Balances at End of period (in shares) at Dec. 31, 2019	80,016,245
Balances at End of period at Dec. 31, 2019	$ 80	$ 586,798	$ 75	(312,740)				$ 274,213
Balances at End of period (in shares) at Dec. 31, 2019								0
Cumulative effect adjustment for adoption				$ 800				$ 800

CONSOLIDATED STATEMENTS OF CO_2

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

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (163,458)	$ (89,195)	$ (43,847)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	41,271	27,528	17,903
Depreciation and amortization expense	2,995	1,263	447
Change in fair value of preferred stock warrant liability		2,187	785
Accretion of discount on investments	(2,320)	(329)
Loss on disposal of property and equipment	335
Non-cash interest expense	289	86	42
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,655	(9,037)	(647)
Operating lease, right-of-use-asset	4,991
Other assets	2	(74)
Accounts payable	(1,037)	4,938	1,110
Accrued expenses and other current liabilities	6,503	4,307	2,247
Deferred rent		(15)	22
Operating lease liabilities	(2,670)
Net cash used in operating activities	(110,444)	(58,341)	(21,938)
Cash flows from investing activities:
Purchases of property, plant and equipment	(40,657)	(14,952)	(2,251)
Purchases of investments	(319,133)	(160,972)
Sales and maturities of investments	227,155	64,285
Net cash used in investing activities	(132,635)	(111,639)	(2,251)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		100,986	120,067
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 the sale of restricted common stock			100
Proceeds from repayment of promissory note		246
Payments of debt issuance costs		(285)	(11)
Repurchase of unvested restricted common stock	(122)
Proceeds from borrowings under loan and security agreement	25,000	25,000	1,500
Payment of long-term debt		(5,500)
Proceeds from issuance of common stock upon exercise of stock options	2,413	64	37
Net cash provided by financing activities	27,291	374,829	121,693
Net increase (decrease) in cash, cash equivalents and restricted cash	(215,788)	204,849	97,504
Cash, cash equivalents and restricted cash at beginning of period	309,421	104,572	7,068
Cash, cash equivalents and restricted cash at end of period	93,633	309,421	104,572
Supplemental cash flow information:
Cash paid for interest, net of interest capitalized	2,961	385	265
Cash paid for leases	5,375
Supplemental disclosure of non-cash investing and financing information:
Lease assets obtained in exchange for new operating lease liabilities	49,799
Purchases of property, plant and equipment included in accounts payable or accrued expenses	3,095	1,550	9
Amounts capitalized under build-to-suit transaction		45,142
Proceeds from issuance of common stock upon exercise of stock options in other current assets	$ 32
Reclassification of warrant liability to additional paid-in capital		3,053
Conversion of preferred stock to common stock upon closing of the initial public offering		240,776
Issuance of preferred stock warrant in connection with loan and security agreement			14
Accretion of Series A redeemable convertible preferred stock to redemption value			$ 656
Debt issuance costs included in accounts payable and accrued expenses		$ 489

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2019
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 $163.5 million, $89.2 million and $43.8 million for the years ended December 31, 2019, 2018 and 2017, respectively. As of December 31, 2019, the Company had an accumulated deficit of $312.7 million. The Company expects to continue to generate operating losses in the foreseeable future. As of March 12, 2020, 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 will 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. Certain prior period amounts have been reclassified to conform to the current year presentation.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
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, 2019 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, 2019 and 2018, the Company maintained letters of credit for the benefit of the landlords of its leased properties totaling $1.7 million and $1.7 million, respectively. As of December 31, 2018, the Company also maintained restricted cash of $0.5 million to collateralize its corporate credit card. The Company was required to maintain separate cash balances of these amounts to secure the letters of credit. Related to these separate cash balances, the Company classified $1.7 million and $1.7 million as restricted cash (non-current) in its consolidated balance sheet as of December 31, 2019 and 2018, respectively. The Company also classified $0.6 million as restricted cash (current) in its consolidated balance sheet as of December 31, 2018. The Company did not have any restricted cash (current) as of December 31, 2019. Cash, cash equivalents and restricted cash was $93.6 million, $309.4 million and $104.6 million for the years ended December 31, 2019, 2018 and 2017, respectively, of which $1.7 million, $2.3 million and $0.3 million was restricted cash, respectively. 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 major renewals and improvements which extend the life or usefulness of the asset are capitalized. Items of an ordinary repair or maintenance nature are charged directly to operating 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 are carried at fair value, 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. Leases In February 2016, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which was codified as Accounting Standards Codification (“ASC”), 842, Leases, to enhance the transparency and comparability of financial reporting related to leasing arrangements. The Company adopted the standard effective January 1, 2019 using the modified retrospective transition method. The prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. The adoption of the new standard resulted in the recognition of a cumulative effect adjustment of $0.8 million to accumulated deficit due to the derecognition of the Company’s build-to-suit lease. At the inception of an arrangement as lessee or lessor, the Company determines whether the arrangement is or contains a lease. Operating lease cost is recognized over the lease term on a straight-line basis. Variable lease cost and short-term leases (lease terms less than 12 months) are recognized as incurred. For both lessee and lessor arrangements, variable lease payments are the amounts owed by the Company to a lessor that are not fixed, such as reimbursement for common area maintenance and utilities costs, and are expensed when incurred. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that it will exercise that option. For lessee arrangements, operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes its incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received. Operating leases are recognized on the balance sheet as right-of-use assets, operating lease liabilities current and operating lease liabilities non-current. The Company has elected to adopt the following lease policies in conjunction with the adoption of ASU 2016-02: (1) for lessee and lessor arrangements within all asset classes, combine lease and non-lease components as a single component, with the lease expense recognized over the expected term on a straight-line basis and (2) for lessee arrangements, apply short-term lease exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for leases with terms of one year or less. 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 with research institutions and other companies both inside and outside of the U.S. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations as of period end with those third parties to record accruals for estimated ongoing research and development costs. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research and development activities, invoicing to date under the contracts, communication from the research institution or other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. 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 measures restricted stock units with service-based vesting as the market value of the Company’s stock on the date of grant. 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. The Company accounts for forfeitures as they occur and records compensation cost assuming all option holders will complete the requisite service period. If an award is forfeited, the Company reverses compensation expense previously recognized in the period the award is forfeited. 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 years ended December 31, 2019 and 2018, the Company’s only element of other comprehensive loss was unrealized gains (losses) on investments. For the year ended December 31, 2017, 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 ASU No. 2018-07, Compensation — Stock Compensation 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 was 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. Effective January 1, 2018, the Company adopted ASU 2018-07 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 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 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 was 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 ASU No. 2016-02, Leases 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 classify 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 determines whether lease expense is recognized over the lease term based on an effective interest method for financing leases or on a straight-line basis for operating leases. 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 previous guidance for operating leases under ASC 840. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. 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 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 adopted ASC 842 using the modified retrospective approach with an effective date of January 1, 2019 for leases that existed on that date. Prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. This standard provides a number of optional practical expedients in transition. The Company applied the package of practical expedients to leases that commenced prior to the effective date, whereby it elected not to 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 elected 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 that have terms of one year or less. The most significant effects of adoption were 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 13). In addition, the Company has a material lease where the Company was deemed the owner during the construction period 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 ASC 842, as the commencement date of this material lease had not occurred, the new right-of-use assets and corresponding liabilities related to this lease were not recognized on the consolidated balance sheet as of date of adoption, January 1, 2019, however, were recognized upon the commencement date of January 28, 2019. The adoption of this standard has had a material impact on the Company’s financial position did not significantly affect the Company’s results of operations. The following table summarizes the financial impact on the Company’s consolidated balance sheet upon the adoption of ASU 2016-02 and the cumulative effect adjustment on January 1, 2019 (in thousands): Balance at Balance at December 31, 2018 Adjustments January 1, 2019 Operating lease, right-of-use-asset $ — $ 1,751 $ 1,751 Property, plant and equipment, net 62,796 (45,142) 17,654 Deferred rent 143 (143) — Accrued expenses and other current liabilities 12,118 (4,451) 7,667 Lease financing obligation 41,441 (41,441) — Operating lease liabilities — 616 616 Operating lease liabilities, net of current portion — 1,226 1,226 Accumulated deficit (150,082) 800 (149,282) ASU No. 2018-15, Intangibles–Goodwill and Other–Internal-Use Software In August 2018, the

Investments and Fair Value of F

Investments and Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2019
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, 2019 Amortized Cost Gross Gross Fair Value U.S. treasury bills and notes (due within one year) $ 189,816 $ 91 $ (20) $ 189,887 U.S. government agency bonds (due within one year) 1,498 4 — 1,502 $ 191,314 $ 95 $ (20) $ 191,389 December 31, 2018 Amortized Cost Gross Gross 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 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, 2019 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 91,898 $ — $ — $ 91,898 Investments: U.S. government agency bonds — 1,502 — 1,502 U.S. treasury bills and notes — 189,887 — 189,887 $ 91,898 $ 191,389 $ — $ 283,287 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 Investments: U.S. government agency bonds — 17,701 — 17,701 U.S. treasury notes — 79,286 — 79,286 $ 282,160 $ 121,891 $ — $ 404,051 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, 2019. 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, 2019 and 2018, respectively.

Property, Plant and Equipment,

Property, Plant and Equipment, Net	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Laboratory equipment $ 16,079 $ 7,122 Land 1,300 1,300 Leasehold improvements 445 117 Computer equipment 1,051 276 Furniture and fixtures 1,228 — Construction-in-progress 41,262 55,828 61,365 64,643 Less: Accumulated depreciation and amortization (4,441) (1,847) $ 56,924 $ 62,796 On January 1, 2019, in connection with the adoption of ASU 2016-02, the existing construction-in-progress balance within property and equipment, and the corresponding build-to-suit facility lease financing obligation balance were derecognized, resulting in a reduction to construction-in-progress of $45.1 million (see Note 2). 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, 2019, the Company capitalized approximately $26.3 million in construction-in-progress for design, demolition and construction costs related to the renovation project and $28.4 million has been capitalized to construction-in-progress to date. During the year ended December 31, 2019 and to date, the Company capitalized $5.9 million in construction-in-progress for manufacturing equipment to be used in the facility. In addition, the Company capitalized interest of $0.7 million during the year ended December 31, 2019 and to date during the construction period. As of December 31, 2019, the manufacturing facility was not ready for its intended use and continued to be included in construction in progress. In January 2020, after achieving the qualifications required to bring it to its intended use, the Company placed the manufacturing facility into service. Depreciation and amortization expense was $3.0 million, $1.3 million and $0.4 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Accrued employee compensation and benefits $ 5,045 $ 3,377 Accrued external research and development expenses 6,715 2,252 Accrued manufacturing facility expenses 2,824 — Accrued general and administrative expenses 1,136 1,676 Accrued lease liability, current portion — 4,502 Other 322 311 $ 16,042 $ 12,118

Debt

Debt	12 Months Ended
	Dec. 31, 2019
Debt
Debt	6. Debt Long‑term debt consisted of the following (in thousands): December 31, 2019 2018 Principal amount of long‑term debt $ 50,000 $ 25,000 Less: Current portion of long‑term debt — — Long‑term debt, net of current portion 50,000 25,000 Accrued final interest payment 213 16 Debt discount (617) (669) Long‑term debt, net of discount and current portion $ 49,596 $ 24,347 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 bore 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 scheduled maturity date in November 2020. 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. In June 2019, the Company made a second borrowing of $25.0 million. The third tranche will be available to the Company through June 30, 2020, subject to 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, 2019. 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, 2019, the estimated future principal payments due were as follows (in thousands): Year ending December 31, 2020 $ — 2021 — 2022 25,000 2023 25,000 2024 and thereafter — $ 50,000

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2019
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. 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, 2019 and 2018.

Warrants to Purchase Convertibl

Warrants to Purchase Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2019
Warrants to Purchase Convertible Preferred Stock.
Warrants to Purchase Convertible Preferred Stock	8. Warrants to Purchase Convertible Preferred Stock During 2015 and 2017, the Company issued warrants to purchase up to 133,333 shares of Series A Preferred Stock and 2,234 shares of Series B Preferred Stock, respectively, in connection with the 2015 Credit Facility, as amended (see Note 6). The warrants were exercisable at a price of $0.60 per share and $8.39 per share and had a contractual term of ten years from issuance. The fair value of the warrants on the issuance date 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 and 2017, the Company recorded other expense of $2.2 million and $0.8 million, respectively. 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, 2019
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 preferred stock will have such rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences, as shall be determined by the Company’s board of directors upon issuance. The shares of preferred stock are currently undesignated. Also on July 20, 2018, the Company completed its 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). On August 1, 2019, the Company entered into a Distribution Agreement (the “Distribution Agreement”) with J.P. Morgan Securities LLC, Jefferies LLC and SVB Leerink LLC (the “Sales Agents”), pursuant to which the Company may issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $100.0 million through the Sales Agents. The Company’s registration statement on Form S-3 filed on August 1, 2019 was declared effective on August 21, 2019. The Sales Agents may sell common stock by any method permitted by law deemed to be an “at the market offering” as defined in Rule 415(a)(4) of the Securities Act of 1933, as amended, including sales made directly on or through the Nasdaq Global Select Market or any other existing trade market for the common stock, in negotiated transactions at market prices prevailing at the time of sale or at prices related to prevailing market prices, or any other method permitted by law. The Sales Agents will be entitled to receive 3.0% of the gross sales price per share of common stock sold pursuant to the Distribution Agreement. As of December 31, 2019, no shares of common stock have been issued and sold pursuant to the Distribution Agreement.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
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, 2019, 3,334,533 shares remained available for issuance under the 2018 Plan. The number of authorized shares reserved for issuance under the 2018 Plan was increased by 3,200,649 shares effective as of January 1, 2020. 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, 2019, all 1,751,650 shares remained available for issuance under the 2018 ESPP. The number of authorized shares reserved for issuance under the ESPP was not increased on January 1, 2020. 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 2019 and 2018, non-employees: Year ended December 31, 2019 2018 2017 Risk-free interest rate 2.07 % 2.71 % 2.05 % Expected volatility 76.9 % 74.0 % 75.6 % Expected dividend yield — — — Expected term (in years) 6.06 6.21 6.39 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 Risk-free interest rate 2.02% - 2.31% Expected volatility 74% - 85% Expected dividend yield — Expected term (in years) 6 - 10 Fair value of common stock $0.19 - $6.28 The following table summarizes the Company’s service-based and performance-based option activity since December 31, 2018: Weighted Weighted average average Aggregate Number of exercise contractual intrinsic shares price term value (in years) (in thousands) Outstanding as of December 31, 2018 14,784,770 $ 8.93 8.89 $ 126,367 Granted 3,059,004 12.67 Exercised (1,449,309) 1.68 Forfeited (1,290,558) 8.58 Outstanding as of December 31, 2019 15,103,907 $ 10.41 8.22 $ 36,006 Vested and expected to vest as of December 31, 2019 15,103,907 $ 10.41 8.22 $ 36,006 Options exercisable as of December 31, 2019 5,182,808 $ 6.33 7.49 $ 21,743 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, 2019, 2018 and 2017 was $17.7 million, $3.7 million and $3.7 million, respectively. The weighted average grant-date fair value of stock options granted during the years ended December 31, 2019, 2018 and 2017 was $8.58 per share, $8.71 per share and $2.74 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 15). 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 15). 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 tranche. 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 years ended December 31, 2019 and 2018 the Company recorded stock-based compensation expense on Option A of $0.9 million and $0.2 million, respectively. No stock-based compensation expense has been recorded 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 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. The Company did not grant market-based stock options during the year ended December 31, 2019. During the years ended December 31, 2019, 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, 2019. Restricted Common Stock Awards 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, 2018: Weighted average grant‑date Shares fair value Unvested restricted common stock as of December 31, 2018 1,559,401 $ 0.502 Issued — — Vested (1,258,837) 0.519 Forfeited (201,250) 0.387 Unvested restricted common stock as of December 31, 2019 99,314 $ 0.526 In the table above, the number of shares of unvested restricted common stock outstanding as of December 31, 2018 excludes 670,834 shares of restricted common stock that remained unvested as of that date 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. During 2019, 466,667 shares of unvested restricted common stock remaining from that early exercise were repurchased by the Company. As of December 31, 2019, shares of unvested restricted common stock awards totaled 99,314 shares. 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, 2019, 2018 and 2017 was $16.3 million, $34.0 million and $0.9 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 15). 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 15). 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 years ended December 31, 2019 and 2018 was $7.6 million and $7.3 million, respectively. Stock-based compensation expense related to these awards will continue to be recognized over the requisite service period of the awards. Restricted Stock Units The Company has also granted restricted stock units to its employees. During the year ended December 31, 2019, the Company granted restricted stock units to employees that were subject to time-based vesting conditions that lapse over three years from date of grant. Restricted stock units with time-based vesting conditions are valued on the grant date using the grant date market price of the underlying shares. The following table summarizes the Company’s restricted stock unit activity since December 31, 2018: Weighted average grant‑date Shares fair value Unvested restricted common stock as of December 31, 2018 — $ — Issued 277,200 10.660 Vested — — Forfeited — — Unvested restricted common stock as of December 31, 2019 277,200 $ 10.660 No outstanding restricted stock units vested or were forfeited during the year ended December 31, 2019. 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, 2019 2018 2017 Research and development expenses $ 9,011 $ 3,787 $ 1,756 General and administrative expenses 32,260 23,741 16,147 $ 41,271 $ 27,528 $ 17,903 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. During 2019, options for the purchase of 257,854 shares of common stock were canceled before the performance conditions were met. As of December 31, 2019, the Company has no outstanding options with non-market, performance-based vesting conditions. As of December 31, 2019 , total unrecognized compensation cost related to unvested stock‑based awards was $70.6 million, which is expected to be recognized over a weighted average period of 2.2 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Taxes
Income Taxes	11. Income Taxes During the years ended December 31, 2019 and 2018, 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, 2019 2018 Federal statutory income tax rate (21.0) % (21.0) % State taxes, net of federal benefit (6.2) (4.8) Federal and state research and development tax credits (6.0) (2.9) Stock‑based compensation expense 0.7 4.5 Other 0.5 0.5 Increase in deferred tax asset valuation allowance 32.0 23.7 Effective income tax rate — % — % Net deferred tax assets consisted of the following (in thousands): December 31, 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 61,160 $ 25,508 Research and development tax credit carryforwards 13,525 3,824 Accrued expenses 1,142 1,139 Capitalized intellectual property costs 1,073 764 Capitalized research and development expense 109 120 Operating lease liabilities 12,952 — Stock‑based compensation expense 9,156 2,652 Total deferred tax assets 99,117 34,007 Deferred tax liabilities: Operating lease assets (12,720) — Depreciation and other (513) (341) Total deferred tax liabilities (13,233) (341) Valuation allowance (85,884) (33,666) Net deferred tax assets $ — $ — As of December 31, 2019, the Company had U.S. federal and state net operating loss (“NOL”) carryforwards of $222.9 million and $227.0 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 $185.7 million which carryforward indefinitely. The state NOLs expire at various dates through 2039. As of December 31, 2019, the Company also had U.S. federal and state research and development tax credit carryforwards of $9.5 million and $5.1 million, respectively, which may be available to offset future tax liabilities and begin to expire in 2034 and 2026, respectively. During the year ended December 31, 2019, deferred tax assets, before valuation allowance, increased by approximately $52.2 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, 2019 and 2018. 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, 2019 and 2018 related primarily to the increase in net operating loss carryforwards and research and development tax credit carryforwards in 2019 and 2018, and were as follows (in thousands): Year ended December 31, 2019 2018 Valuation allowance as of beginning of year $ 33,666 $ 12,575 Decreases recorded as benefit to income tax provision — — Increases recorded to income tax provision 52,218 21,091 Valuation allowance as of end of year $ 85,884 $ 33,666 As of December 31, 2019 and 2018, 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, 2019 and 2018, 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 2016 to the present; however, carryforward attributes that were generated prior to January 1, 2016 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, 2019
Commitments and Contingencies
Commitments and Contingencies	12. Commitments and Contingencies 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 years ended December 31, 2019 and 2018, the Company made matching contributions of $0.6 million and $0.2 million, respectively. 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 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.

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Leases
Leases	13. Leases Operating Leases The Company leases its office and laboratory facilities in Cambridge, Massachusetts under two noncancelable operating leases. The first lease expires in September 2021. The second lease is subject to two expiration dates based on two distinct leased spaces, expiring in January 2027 and August 2028. The lease agreements include lease incentives, payment escalations and rent holidays. 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, which is not included in the lease term. 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 collateralized by a cash deposit of the same amount. The lease agreement allows for a landlord-provided tenant improvement allowance of $9.4 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 entered into a lease amendment for office and laboratory space in the same building (the “Expansion Space”). The lease term for the Expansion Space commenced on August 8, 2019 and expires approximately 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 collateralized by a cash deposit of the same amount. The lease amendment increased the landlord-provided tenant improvement allowance by $9.2 million, of which $2.0 million is repayable to the landlord over the term of the lease. During 2018, in accordance with ASC 840, Leases, the Company was deemed to be the owner of the lease for the Initial and Expansion Space during the construction period due to 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. Under ASC 842, the Company is no longer considered the accounting owner due to (1) the Company not having the right to obtain or control the leased premises during the construction period, (2) the lessor having no right of payment for the partially constructed assets, and the leased premises are not of a specialized nature and, thus, could be potentially leased to another tenant, and (3) the Company not legally owning or controlling the land on which the property improvements will be constructed. As such, upon adoption of ASU 2016-02 (Topic 842), the existing construction-in-progress balance within property and equipment, and the corresponding build-to-suit facility lease financing obligation balance were derecognized. Subsequently, the Company took control of the Initial Space on January 28, 2019 at which time the lease commenced and the Company assessed and determined the accounting treatment for the asset and corresponding liability under ASC 842. The Company evaluated its vendor contracts to identify embedded leases, if any, and noted that an agreement with a contract manufacturing supplier constituted a lease under ASC 842 as the Company has the right to substantially all the economic benefits from the use of the asset and can direct the use of the asset. The Company entered into the agreement during the first quarter of 2019. The lease commenced during March 2019 and expires 22 months from commencement date with no stated option to extend the term. As the Company’s leases do not provide an implicit rate, the Company utilized its incremental borrowing rate based on information available at the lease commencement date, which represents an internally developed rate that would be incurred to borrow, on a collateralized basis, over a similar term, an amount equal to the lease payments in a similar economic environment. The Company has elected to account for each lease component and its associated non-lease components as a single lease component and, therefore, has allocated all the contract consideration across lease components only. This may result in the initial and subsequent measurement of the balances of the right-of-use asset and lease liability for leases being greater than if the policy election was not applied. Assets under operating lease at December 31, 2019 were $46.6 million. The leases do not include any restrictions or covenants that had to be accounted for under the lease guidance. As of December 31, 2019, minimum lease payments under the Company’s operating leases are as follows (in thousands): Year ending December 31, 2020 $ 10,540 2021 7,574 2022 7,186 2023 7,390 2024 7,601 Thereafter 22,058 62,349 Less: imputed interest (14,942) $ 47,407 The Company has not entered any material financing leases as of December 31, 2019. Additional Lease Information Related to the Application of ASC 840 As of December 31, 2018, minimum commitments under the Company’s operating leases, as required under prior lease guidance in ASC 840, were as follows (in thousands): Year ending December 31, 2019 $ 5,458 2020 7,141 2021 7,168 2022 6,820 2023 7,024 Thereafter 27,777 $ 61,388 Rent expense for the years ended December 31, 2018 and 2017 was $2.4 million and $1.0 million, respectively. Sublease In February 2019, the Company entered into a sublease agreement with a related party to sublease all office and laboratory space under one of the Company’s operating leases in Cambridge, Massachusetts (see Note 15). The term of the sublease agreement commenced in February 2019 and expires at the end of the Company’s lease agreement with the landlord in September 2021, with no option to extend. The annual rent for the subleased premises is equal to the annual rent owed by the Company to the landlord for the leased premises. The sublessee is obligated to pay all real estate taxes and costs related to the subleased premises, including cost of operations, maintenance, repair, replacement and property management. The Company concluded that the sublease is an operating lease. Consistent with the Company’s policy election for lessor operating leases, each lease component and its associated non-lease components is accounted for as a single lease component. As of December 31, 2019, future undiscounted cash inflows under the sublease are as follows (in thousands): Year ending December 31, 2020 $ 735 2021 541 $ 1,276 Lease Portfolio The components of lease cost and supplemental balance sheet information for the Company’s lease portfolio were as follows (in thousands, except term and discount rate amounts): Year ended December 31, 2019 Lease Cost Operating lease cost $ 7,208 Short-term lease cost 127 Variable lease cost 1,976 Sublease income (908) Total lease cost $ 8,403 Operating Leases Operating lease, right-of-use-asset $ 46,559 Operating lease liabilities $ 10,540 Operating lease liabilities, net of current portion $ 36,867 Other information: Weighted average remaining lease term - operating leases 7.47 years Weighted-average discount rate - operating leases

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2019
Net Loss Per Share
Net Loss Per Share	14. 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, 2019 2018 2017 Numerator: Net Loss $ (163,458) (89,195) (43,847) Accretion of Series A redeemable convertible preferred stock to redemption value — — (656) Net loss attributable to common stockholders $ (163,458) $ (89,195) $ (44,503) Denominator: Weighted average common shares outstanding, basic and diluted 78,688,878 39,285,468 8,023,785 Net loss per share, basic and diluted $ (2.08) $ (2.27) $ (5.55) 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, 2019 2018 2017 Convertible preferred stock (as converted to common stock) — — 43,933,006 Warrants to purchase convertible preferred stock (as converted to common stock) — — 135,567 Restricted common stock (1) 376,514 2,230,235 6,277,014 Stock options to purchase common stock 15,103,907 14,784,770 4,661,635 15,480,421 17,015,005 55,007,222 (1) Includes unvested restricted stock and, in 2018 and 2017, vested restricted stock issued for promissory notes.

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2019
Related Parties
Related Parties	15. 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 and $0.9 million during the years ended December 31, 2018 and 2017, respectively. The Company did not receive services and did not pay any amounts under this agreement during the year ended December 31, 2019. As of December 31, 2019 and 2018, 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. In February 2019, the Company entered into a sublease agreement with an affiliate of one of its principal stockholders to sublease all office and laboratory space under one of the Company’s operating leases in Cambridge, Massachusetts. The term of the sublease agreement commenced in February 2019 and expires at the end of the Company’s lease agreement with the landlord in September 2021, with no option to extend (see Note 13). Under this agreement, the Company recorded other income of $0.9 million during the year ended December 31, 2019. The Company received cash payments of $0.5 million during the year ended December 31, 2019. As of December 31, 2019, the Company recorded accounts receivable of $0.1 million under this agreement.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Selected Quarterly Financial Data (Unaudited)
Selected Quarterly Financial Data (Unaudited)	16. Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2019 and 2018. 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, 2019 2019 2019 2019 Consolidated Statements of Operations Data: Revenue $ — $ — $ — $ — Total operating expenses 34,406 41,285 48,482 45,428 Loss from operations (34,406) (41,285) (48,482) (45,428) Net loss attributable to common stockholders (32,581) (39,390) (47,015) (44,472) Net loss per share attributable to common stockholders, basic and diluted $ (0.42) $ (0.50) $ (0.59) $ (0.56) Three Months Ended March 31, June 30, September 30, December 31, 2018 2018 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)

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
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, 2019 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, 2019 and 2018, the Company maintained letters of credit for the benefit of the landlords of its leased properties totaling $1.7 million and $1.7 million, respectively. As of December 31, 2018, the Company also maintained restricted cash of $0.5 million to collateralize its corporate credit card. The Company was required to maintain separate cash balances of these amounts to secure the letters of credit. Related to these separate cash balances, the Company classified $1.7 million and $1.7 million as restricted cash (non-current) in its consolidated balance sheet as of December 31, 2019 and 2018, respectively. The Company also classified $0.6 million as restricted cash (current) in its consolidated balance sheet as of December 31, 2018. The Company did not have any restricted cash (current) as of December 31, 2019. Cash, cash equivalents and restricted cash was $93.6 million, $309.4 million and $104.6 million for the years ended December 31, 2019, 2018 and 2017, respectively, of which $1.7 million, $2.3 million and $0.3 million was restricted cash, respectively.
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 major renewals and improvements which extend the life or usefulness of the asset are capitalized. Items of an ordinary repair or maintenance nature are charged directly to operating 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 are carried at fair value, 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.
Leases	Leases In February 2016, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which was codified as Accounting Standards Codification (“ASC”), 842, Leases, to enhance the transparency and comparability of financial reporting related to leasing arrangements. The Company adopted the standard effective January 1, 2019 using the modified retrospective transition method. The prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. The adoption of the new standard resulted in the recognition of a cumulative effect adjustment of $0.8 million to accumulated deficit due to the derecognition of the Company’s build-to-suit lease. At the inception of an arrangement as lessee or lessor, the Company determines whether the arrangement is or contains a lease. Operating lease cost is recognized over the lease term on a straight-line basis. Variable lease cost and short-term leases (lease terms less than 12 months) are recognized as incurred. For both lessee and lessor arrangements, variable lease payments are the amounts owed by the Company to a lessor that are not fixed, such as reimbursement for common area maintenance and utilities costs, and are expensed when incurred. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that it will exercise that option. For lessee arrangements, operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes its incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received. Operating leases are recognized on the balance sheet as right-of-use assets, operating lease liabilities current and operating lease liabilities non-current. The Company has elected to adopt the following lease policies in conjunction with the adoption of ASU 2016-02: (1) for lessee and lessor arrangements within all asset classes, combine lease and non-lease components as a single component, with the lease expense recognized over the expected term on a straight-line basis and (2) for lessee arrangements, apply short-term lease exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for leases with terms of one year or less.
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 with research institutions and other companies both inside and outside of the U.S. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations as of period end with those third parties to record accruals for estimated ongoing research and development costs. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research and development activities, invoicing to date under the contracts, communication from the research institution or other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. 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 measures restricted stock units with service-based vesting as the market value of the Company’s stock on the date of grant. 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. The Company accounts for forfeitures as they occur and records compensation cost assuming all option holders will complete the requisite service period. If an award is forfeited, the Company reverses compensation expense previously recognized in the period the award is forfeited. 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 years ended December 31, 2019 and 2018, the Company’s only element of other comprehensive loss was unrealized gains (losses) on investments. For the year ended December 31, 2017, 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 ASU No. 2018-07, Compensation — Stock Compensation 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 was 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. Effective January 1, 2018, the Company adopted ASU 2018-07 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 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 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 was 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 ASU No. 2016-02, Leases 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 classify 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 determines whether lease expense is recognized over the lease term based on an effective interest method for financing leases or on a straight-line basis for operating leases. 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 previous guidance for operating leases under ASC 840. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. 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 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 adopted ASC 842 using the modified retrospective approach with an effective date of January 1, 2019 for leases that existed on that date. Prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. This standard provides a number of optional practical expedients in transition. The Company applied the package of practical expedients to leases that commenced prior to the effective date, whereby it elected not to 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 elected 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 that have terms of one year or less. The most significant effects of adoption were 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 13). In addition, the Company has a material lease where the Company was deemed the owner during the construction period 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 ASC 842, as the commencement date of this material lease had not occurred, the new right-of-use assets and corresponding liabilities related to this lease were not recognized on the consolidated balance sheet as of date of adoption, January 1, 2019, however, were recognized upon the commencement date of January 28, 2019. The adoption of this standard has had a material impact on the Company’s financial position did not significantly affect the Company’s results of operations. The following table summarizes the financial impact on the Company’s consolidated balance sheet upon the adoption of ASU 2016-02 and the cumulative effect adjustment on January 1, 2019 (in thousands): Balance at Balance at December 31, 2018 Adjustments January 1, 2019 Operating lease, right-of-use-asset $ — $ 1,751 $ 1,751 Property, plant and equipment, net 62,796 (45,142) 17,654 Deferred rent 143 (143) — Accrued expenses and other current liabilities 12,118 (4,451) 7,667 Lease financing obligation 41,441 (41,441) — Operating lease liabilities — 616 616 Operating lease liabilities, net of current portion — 1,226 1,226 Accumulated deficit (150,082) 800 (149,282) ASU No. 2018-15, Intangibles–Goodwill and Other–Internal-Use Software In August 2018, the FASB issued ASU No. 2018-15, Intangibles–Goodwill and Other–Internal-Use Software (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract. This new guidance aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2019 and for interim periods within those fiscal years. Early adoption was permitted. The Company early-adopted this standard on January 1, 2019 on a prospective basis. The adoption did not have a material impact on the Company’s consolidated financial statements. Recently Issued Accounting Pronouncements 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, 2019
Recently Adopted Accounting Pronouncements
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
Recently Adopted Accounting Pronouncements
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
Recently Adopted Accounting Pronouncements
Summary of impact of accounting standard adoption	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 Risk-free interest rate 2.02% - 2.31% Expected volatility 74% - 85% Expected dividend yield — Expected term (in years) 6 - 10 Fair value of common stock $0.19 - $6.28
ASU 2016-02
Recently Adopted Accounting Pronouncements
Summary of impact of accounting standard adoption	The following table summarizes the financial impact on the Company’s consolidated balance sheet upon the adoption of ASU 2016-02 and the cumulative effect adjustment on January 1, 2019 (in thousands): Balance at Balance at December 31, 2018 Adjustments January 1, 2019 Operating lease, right-of-use-asset $ — $ 1,751 $ 1,751 Property, plant and equipment, net 62,796 (45,142) 17,654 Deferred rent 143 (143) — Accrued expenses and other current liabilities 12,118 (4,451) 7,667 Lease financing obligation 41,441 (41,441) — Operating lease liabilities — 616 616 Operating lease liabilities, net of current portion — 1,226 1,226 Accumulated deficit (150,082) 800 (149,282)

Investments and Fair Value of_2

Investments and Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Amortized Cost Gross Gross Fair Value U.S. treasury bills and notes (due within one year) $ 189,816 $ 91 $ (20) $ 189,887 U.S. government agency bonds (due within one year) 1,498 4 — 1,502 $ 191,314 $ 95 $ (20) $ 191,389 December 31, 2018 Amortized Cost Gross Gross 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, 2019 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents: Money market funds $ 91,898 $ — $ — $ 91,898 Investments: U.S. government agency bonds — 1,502 — 1,502 U.S. treasury bills and notes — 189,887 — 189,887 $ 91,898 $ 191,389 $ — $ 283,287 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 Investments: U.S. government agency bonds — 17,701 — 17,701 U.S. treasury notes — 79,286 — 79,286 $ 282,160 $ 121,891 $ — $ 404,051

Property, Plant and Equipment_2

Property, Plant and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Laboratory equipment $ 16,079 $ 7,122 Land 1,300 1,300 Leasehold improvements 445 117 Computer equipment 1,051 276 Furniture and fixtures 1,228 — Construction-in-progress 41,262 55,828 61,365 64,643 Less: Accumulated depreciation and amortization (4,441) (1,847) $ 56,924 $ 62,796

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Accrued employee compensation and benefits $ 5,045 $ 3,377 Accrued external research and development expenses 6,715 2,252 Accrued manufacturing facility expenses 2,824 — Accrued general and administrative expenses 1,136 1,676 Accrued lease liability, current portion — 4,502 Other 322 311 $ 16,042 $ 12,118

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt
Schedule of long term debt	Long‑term debt consisted of the following (in thousands): December 31, 2019 2018 Principal amount of long‑term debt $ 50,000 $ 25,000 Less: Current portion of long‑term debt — — Long‑term debt, net of current portion 50,000 25,000 Accrued final interest payment 213 16 Debt discount (617) (669) Long‑term debt, net of discount and current portion $ 49,596 $ 24,347
Schedule of estimated future principal payments due	As of December 31, 2019, the estimated future principal payments due were as follows (in thousands): Year ending December 31, 2020 $ — 2021 — 2022 25,000 2023 25,000 2024 and thereafter — $ 50,000

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2018 14,784,770 $ 8.93 8.89 $ 126,367 Granted 3,059,004 12.67 Exercised (1,449,309) 1.68 Forfeited (1,290,558) 8.58 Outstanding as of December 31, 2019 15,103,907 $ 10.41 8.22 $ 36,006 Vested and expected to vest as of December 31, 2019 15,103,907 $ 10.41 8.22 $ 36,006 Options exercisable as of December 31, 2019 5,182,808 $ 6.33 7.49 $ 21,743
Schedule of restricted common stock award activity	Weighted average grant‑date Shares fair value Unvested restricted common stock as of December 31, 2018 1,559,401 $ 0.502 Issued — — Vested (1,258,837) 0.519 Forfeited (201,250) 0.387 Unvested restricted common stock as of December 31, 2019 99,314 $ 0.526
Schedule of restricted stock unit activity	Weighted average grant‑date Shares fair value Unvested restricted common stock as of December 31, 2018 — $ — Issued 277,200 10.660 Vested — — Forfeited — — Unvested restricted common stock as of December 31, 2019 277,200 $ 10.660
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, 2019 2018 2017 Research and development expenses $ 9,011 $ 3,787 $ 1,756 General and administrative expenses 32,260 23,741 16,147 $ 41,271 $ 27,528 $ 17,903
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, 2019 2018 2017 Risk-free interest rate 2.07 % 2.71 % 2.05 % Expected volatility 76.9 % 74.0 % 75.6 % Expected dividend yield — — — Expected term (in years) 6.06 6.21 6.39
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 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 Risk-free interest rate 2.02% - 2.31% Expected volatility 74% - 85% Expected dividend yield — Expected term (in years) 6 - 10 Fair value of common stock $0.19 - $6.28

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Federal statutory income tax rate (21.0) % (21.0) % State taxes, net of federal benefit (6.2) (4.8) Federal and state research and development tax credits (6.0) (2.9) Stock‑based compensation expense 0.7 4.5 Other 0.5 0.5 Increase in deferred tax asset valuation allowance 32.0 23.7 Effective income tax rate — % — %
Summary of changes in the valuation allowance for deferred tax assets	December 31, 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 61,160 $ 25,508 Research and development tax credit carryforwards 13,525 3,824 Accrued expenses 1,142 1,139 Capitalized intellectual property costs 1,073 764 Capitalized research and development expense 109 120 Operating lease liabilities 12,952 — Stock‑based compensation expense 9,156 2,652 Total deferred tax assets 99,117 34,007 Deferred tax liabilities: Operating lease assets (12,720) — Depreciation and other (513) (341) Total deferred tax liabilities (13,233) (341) Valuation allowance (85,884) (33,666) Net deferred tax assets $ — $ —
Net deferred tax assets	Year ended December 31, 2019 2018 Valuation allowance as of beginning of year $ 33,666 $ 12,575 Decreases recorded as benefit to income tax provision — — Increases recorded to income tax provision 52,218 21,091 Valuation allowance as of end of year $ 85,884 $ 33,666

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases
Schedule of minimum lease payments under the Company’s operating leases	As of December 31, 2019, minimum lease payments under the Company’s operating leases are as follows (in thousands): Year ending December 31, 2020 $ 10,540 2021 7,574 2022 7,186 2023 7,390 2024 7,601 Thereafter 22,058 62,349 Less: imputed interest (14,942) $ 47,407
Schedule of minimum commitments under the Company’s operating leases, as required under prior lease guidance in ASC 840	As of December 31, 2018, minimum commitments under the Company’s operating leases, as required under prior lease guidance in ASC 840, were as follows (in thousands): Year ending December 31, 2019 $ 5,458 2020 7,141 2021 7,168 2022 6,820 2023 7,024 Thereafter 27,777 $ 61,388
Schedule of future undiscounted cash inflows under the sublease	As of December 31, 2019, future undiscounted cash inflows under the sublease are as follows (in thousands): Year ending December 31, 2020 $ 735 2021 541 $ 1,276
Schedule of lease cost and lease liabilities for the Company’s lease portfolio	The components of lease cost and supplemental balance sheet information for the Company’s lease portfolio were as follows (in thousands, except term and discount rate amounts): Year ended December 31, 2019 Lease Cost Operating lease cost $ 7,208 Short-term lease cost 127 Variable lease cost 1,976 Sublease income (908) Total lease cost $ 8,403 Operating Leases Operating lease, right-of-use-asset $ 46,559 Operating lease liabilities $ 10,540 Operating lease liabilities, net of current portion $ 36,867 Other information: Weighted average remaining lease term - operating leases 7.47 years Weighted-average discount rate - operating leases

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Numerator: Net Loss $ (163,458) (89,195) (43,847) Accretion of Series A redeemable convertible preferred stock to redemption value — — (656) Net loss attributable to common stockholders $ (163,458) $ (89,195) $ (44,503) Denominator: Weighted average common shares outstanding, basic and diluted 78,688,878 39,285,468 8,023,785 Net loss per share, basic and diluted $ (2.08) $ (2.27) $ (5.55)
Schedule of dilutive securities excluded from computations of diluted weighted average shares outstanding	Year ended December 31, 2019 2018 2017 Convertible preferred stock (as converted to common stock) — — 43,933,006 Warrants to purchase convertible preferred stock (as converted to common stock) — — 135,567 Restricted common stock (1) 376,514 2,230,235 6,277,014 Stock options to purchase common stock 15,103,907 14,784,770 4,661,635 15,480,421 17,015,005 55,007,222 Includes unvested restricted stock and, in 2018 and 2017, vested restricted stock issued for promissory notes.

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Selected Quarterly Financial Data (Unaudited)
Quarterly financial information	The following table contains quarterly financial information for 2019 and 2018. 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, 2019 2019 2019 2019 Consolidated Statements of Operations Data: Revenue $ — $ — $ — $ — Total operating expenses 34,406 41,285 48,482 45,428 Loss from operations (34,406) (41,285) (48,482) (45,428) Net loss attributable to common stockholders (32,581) (39,390) (47,015) (44,472) Net loss per share attributable to common stockholders, basic and diluted $ (0.42) $ (0.50) $ (0.59) $ (0.56) Three Months Ended March 31, June 30, September 30, December 31, 2018 2018 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)

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, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Sale of Equity
Net proceeds after deducting underwriting discounts and commissions and other offering costs				$ 257,866
Net loss			$ (163,458)	(89,195)	$ (43,847)
Accumulated deficit			$ (312,740)	$ (150,082)		$ (60,887)
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 ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018	Dec. 31, 2016
Restricted cash for the benefit of its leased properties	$ 1,700	$ 1,700
Restricted cash to collateralize corporate credit card		500
Restricted cash	1,700	2,300	$ 300
Restricted cash (non-current)	1,735	1,735
Restricted cash (current)		622
Cash, Cash Equivalents and Restricted Cash	93,633	309,421	104,572			$ 7,068
Change in fair value of the unvested awards	800				$ 100
Other than Temporary Impairment Losses, Investments	0	0	$ 0
Accumulated deficit
Change in fair value of the unvested awards	$ 800	$ 92
Accumulated deficit | ASU 2016-02
Change in fair value of the unvested awards				$ 800

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Estimated useful life (Details)	12 Months Ended
	Dec. 31, 2019
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, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Recently Adopted Accounting Pronouncements
Additional paid-in capital	$ 17,185,000	$ 586,798,000	$ 543,040,000
Accumulated deficit	(60,887,000)	(312,740,000)	(150,082,000)
Change in fair value of the unvested awards	$ 100,000	$ 800,000
Lease, Practical Expedients, Package [true false]		true
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment
Operating lease, right-of-use-asset		$ 46,559,000			$ 1,751,000
Property, plant and equipment, net		56,924,000	62,796,000		17,654,000
Accrued expenses and other current liabilities					7,667,000
Accrued expenses and other current liabilities		16,042,000	12,118,000
Lease financing obligation			41,441,000
Operating lease liabilities, current portion		10,540,000			616,000
Operating lease liabilities, net of current portion		36,867,000			1,226,000
Stockholders Equity		$ 274,213,000	393,008,000	$ (43,687,000)	(149,282,000)	$ (17,124,000)
Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Weighted average grant-date fair value (per share)	$ 5.88
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 | As previously reported
Recently Adopted Accounting Pronouncements
Additional paid-in capital				17,277,000
Accumulated deficit				$ (60,979,000)
ASU 2018-07 | Adjustments
Recently Adopted Accounting Pronouncements
Additional paid-in capital	$ (92,000)
Accumulated deficit	$ 92,000
ASU 2016-02 | As previously reported
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment
Property, plant and equipment, net			62,796,000
Deferred rent			143,000
Accrued expenses and other current liabilities			12,118,000
Lease financing obligation			41,441,000
Stockholders Equity			$ (150,082,000)
ASU 2016-02 | Adjustments
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment
Operating lease, right-of-use-asset					1,751,000
Property, plant and equipment, net					(45,142,000)
Deferred rent					(143,000)
Accrued expenses and other current liabilities					(4,451,000)
Lease financing obligation					(41,441,000)
Operating lease liabilities, current portion					616,000
Operating lease liabilities, net of current portion					1,226,000
Stockholders Equity					$ 800,000
Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Risk-free interest rate	2.30%	2.07%	2.71%	2.05%
Expected volatility	74.00%	76.90%	74.00%	75.60%
Expected term (in years)	6 years 1 month 6 days	6 years 22 days	6 years 2 months 16 days	6 years 4 months 21 days
Common stock value	$ 6.28

Investments and Fair Value of_3

Investments and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Marketable securities
Amortized Cost	$ 191,314	$ 97,016
Gross Unrealized Gains	95
Gross Unrealized Losses	(20)	(29)
Fair Value	191,389	96,987
Assets:
Investments	191,389	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	0
Transfer out of Level 3, liabilities	0	0
U.S. treasury bills and notes
Marketable securities
Amortized Cost	189,816
Gross Unrealized Gains	91
Gross Unrealized Losses	(20)
Fair Value	189,887
Assets:
Investments	189,887
U.S. government agency bonds
Marketable securities
Amortized Cost	1,498	17,704
Gross Unrealized Gains	4
Gross Unrealized Losses		(3)
Fair Value	1,502	17,701
Assets:
Investments	1,502	17,701
U.S. treasury notes
Marketable securities
Amortized Cost		79,312
Gross Unrealized Losses		(26)
Fair Value		79,286
Assets:
Investments		79,286
Recurring
Assets:
Assets	283,287	404,051
Recurring | Money market funds
Assets:
Cash equivalents	91,898	282,160
Recurring | U.S. treasury bills and notes
Marketable securities
Fair Value	189,887
Assets:
Investments	189,887
Recurring | U.S. treasury bills
Assets:
Cash equivalents		24,904
Recurring | U.S. government agency bonds
Marketable securities
Fair Value	1,502	17,701
Assets:
Investments	1,502	17,701
Recurring | U.S. treasury notes
Marketable securities
Fair Value		79,286
Assets:
Investments		79,286
Recurring | Level 1
Assets:
Assets	91,898	282,160
Recurring | Level 1 | Money market funds
Assets:
Cash equivalents	91,898	282,160
Recurring | Level 2
Assets:
Assets	191,389	121,891
Recurring | Level 2 | U.S. treasury bills and notes
Marketable securities
Fair Value	189,887
Assets:
Investments	189,887
Recurring | Level 2 | U.S. treasury bills
Assets:
Cash equivalents		24,904
Recurring | Level 2 | U.S. government agency bonds
Marketable securities
Fair Value	1,502	17,701
Assets:
Investments	$ 1,502	17,701
Recurring | Level 2 | U.S. treasury notes
Marketable securities
Fair Value		79,286
Assets:
Investments		$ 79,286

Property, Plant and Equipment_3

Property, Plant and Equipment, Net (Details) $ in Thousands	Jul. 31, 2018USD ($)ft²	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2019USD ($)	Jan. 01, 2019USD ($)
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		$ 61,365	$ 64,643		$ 61,365
Less: Accumulated depreciation and amortization		(4,441)	(1,847)		(4,441)
Property, plant and equipment, net		56,924	62,796		56,924	$ 17,654
Depreciation and amortization expense		3,000	1,300	$ 400
Maximum
Property, Plant and Equipment, Net
Capitalized interest cost		700
Laboratory equipment
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		16,079	7,122		16,079
Land
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		1,300	1,300		1,300
Purchases and additions	$ 1,300
Leasehold improvements
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		445	117		445
Computers and software
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		1,051	276		1,051
Furniture and fixtures
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		1,228			1,228
Construction-in-progress
Property, Plant and Equipment, Net
Property, Plant and Equipment, Gross		41,262	$ 55,828		41,262
Purchases and additions	$ 6,700	5,900
Build-to-suit lease
Property, Plant and Equipment, Net
Derecognition of property and equipment						$ 45,100
Manufacturing facility
Property, Plant and Equipment, Net
Square footing of facility acquired | ft²	135,000
Purchases and additions	$ 8,000
Clinical Supply | Construction-in-progress
Property, Plant and Equipment, Net
Purchases and additions		$ 26,300			$ 28,400

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses and Other Current Liabilities
Accrued employee compensation and benefits	$ 5,045	$ 3,377
Accrued external research and development expenses	6,715	2,252
Accrued manufacturing facility expenses	2,824
Accrued general and administrative expenses	1,136	1,676
Accrued lease liability, current portion		4,502
Other	322	311
Accrued expenses and other current liabilities	$ 16,042	$ 12,118

Debt (Details)

Debt (Details) $ in Thousands	Dec. 21, 2018USD ($)item	Jun. 30, 2019USD ($)	May 31, 2018	May 31, 2017	Dec. 31, 2018USD ($)	Dec. 31, 2019USD ($)
Debt
Repayment					$ 5,500
Estimated future principal payments due
2022						$ 25,000
2023						25,000
Total debt						$ 50,000
2015 Credit Facility
Debt
Principal amount					5,500
Floor rate			5.50%	4.50%
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
Borrowing	$ 25,000	$ 25,000
Period for commencing monthly principal payments	36 months
Amortization period	24 months
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

Debt - Current and Non current

Debt - Current and Non current (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt
Principal amount of long-term debt	$ 50,000	$ 25,000
Long term debt net of current maturities	50,000	25,000
Accrued final interest payment	213	16
Debt discount	(617)	(669)
Long‑term debt, net of current portion	$ 49,596	$ 24,347

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Convertible Preferred Stock
Proceeds from issuance of convertible preferred stock						$ 100,986	$ 120,067
Payments of initial public offering costs						$ 3,548
Preferred stock outstanding					0	0
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%
Series B convertible preferred stock
Convertible Preferred Stock
Preferred stock issued				14,362,344
Convertible preferred stock issue price (dollars per share)				$ 8.39
Proceeds from issuance of convertible preferred stock				$ 120,500
Payments of initial public offering costs				$ 400
Series C convertible preferred stock
Convertible Preferred Stock
Preferred stock issued			7,912,432
Convertible preferred stock issue price (dollars per share)			$ 12.79
Proceeds from issuance of convertible preferred stock			$ 101,200
Payments of initial public offering costs			$ 200
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	Dec. 31, 2018	Dec. 31, 2017	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
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
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

Equity (Details)

Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 20, 2018	Jul. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Aug. 01, 2019	Jun. 30, 2018	Apr. 30, 2018	Feb. 28, 2018	Jan. 31, 2018
Equity
Common stock, authorized (in shares)	150,000,000		150,000,000	150,000,000		79,000,000	78,800,000	75,000,000	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	10,000,000
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
Shares issued (in shares)				12,055,450
Conversion of Stock, Shares Issued	51,845,438
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438	51,845,438		51,845,438
IPO
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
Equity
Shares issued (in shares)	1,572,450
At the market offering
Equity
Shares issued (in shares)			0
Percentage of commission					3.00%
Aggregate offering price					$ 100,000

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - shares	Jan. 01, 2020	Jul. 06, 2018	Dec. 31, 2019
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			3,334,533
Annual increase of shares reserved for awards (as a percent)		4.00%
Increase in number of authorized shares reserved for issuance	3,200,649
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			1,751,650
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

Stock-Based Compensation - Empl

Stock-Based Compensation - Employees, directors and non-employees (Details) - Market-Based Stock Options	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
Risk-free interest rate	3.15%
Expected volatility	69.00%
Derived service period (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 - $ / shares	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value of stock awards granted prior to the adoption of ASU 2018-07
Risk-free interest rate	2.30%	2.07%	2.71%	2.05%
Expected volatility	74.00%	76.90%	74.00%	75.60%
Expected term (in years)	6 years 1 month 6 days	6 years 22 days	6 years 2 months 16 days	6 years 4 months 21 days
ASU 2018-07 | Minimum
Fair value of stock awards granted prior to the adoption of ASU 2018-07
Risk-free interest rate				2.02%
Expected volatility				74.00%
Expected term (in years)				6 years
Fair value of common stock				$ 0.19
ASU 2018-07 | Maximum
Fair value of stock awards granted prior to the adoption of ASU 2018-07
Risk-free interest rate				2.31%
Expected volatility				85.00%
Expected term (in years)				10 years
Fair value of common stock				$ 6.28

Stock-Based Compensation - Opti

Stock-Based Compensation - Option activity (Details) - Stock options. - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of shares
Outstanding at beginning of period	14,784,770
Options granted	3,059,004
Exercised	(1,449,309)
Forfeited	(1,290,558)
Outstanding at end of period	15,103,907	14,784,770
Vested and expected to vest as of December 31, 2019	15,103,907
Options exercisable as of December 31, 2019	5,182,808
Weighted average exercise price
Outstanding at the beginning (in dollars per share)	$ 8.93
Granted (in dollars per share)	12.67
Exercised (in dollars per share)	1.68
Forfeited (in dollars per share)	8.58
Outstanding at the end (in dollars per share)	10.41	$ 8.93
Vested and expected to vest (in dollars per share)	10.41
Options exercisable (in dollars per share	$ 6.33
Weighted average contractual term and aggregate intrinsic value
Outstanding at the beginning (in years)	8 years 2 months 19 days	8 years 10 months 21 days
Outstanding at the end (in years)	8 years 2 months 19 days	8 years 10 months 21 days
Vested and expected to vest (in years)	8 years 2 months 19 days
Options exercisable (in years)	7 years 5 months 27 days
Outstanding at the beginning (in dollars)	$ 126,367
Outstanding at the end (in dollars)	36,006	$ 126,367
Vested and expected to vest (in dollars)	36,006
Options exercisable (in dollars)	$ 21,743

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-Based Compensation
Proceeds from Stock Options Exercised					$ 2,413	$ 64	$ 37
Receivables from promissory note recorded in balance sheet				$ 0
Stock-based compensation expense			$ 1,000		41,271	27,528	17,903
Executive officer
Stock-Based Compensation
Options exercise price				$ 0.18
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				1,400,000
Proceeds from Stock Options Exercised				$ 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.
Stock-Based Compensation
Options granted					3,059,004
Options exercise price					$ 1.68
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period					1,449,309
Aggregate intrinsic value of stock options exercised					$ 17,700	$ 3,700	$ 3,700
Weighted average grant-date fair value (per share)					$ 8.58	$ 8.71	$ 2.74
Aggregate Intrinsic value					$ 36,006	$ 126,367
Liability from early exercise of stock options				$ 100
Stock-based compensation expense						$ 2,200
Stock options. | Performance-based vesting
Stock-Based Compensation
Options granted						447,000
Market-Based Stock Options
Stock-Based Compensation
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period					0
Aggregate intrinsic value of stock options exercised					$ 0
Weighted average grant-date fair value (per share)						$ 11.88
Market-Based Stock Options | Vesting based on service period | Executive officer
Stock-Based Compensation
Options granted		164,400
Options exercise price		$ 16.43
Stock-based compensation expense					900	$ 200
Market-Based Stock Options | Performance-based vesting | Executive officer
Stock-Based Compensation
Options granted		193,400
Options exercise price		$ 16.43
Stock-based compensation expense					$ 0

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, 2017USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)shares
Executive officer
Weighted Average Grant Date Fair Value
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						1,559,401
Vested						(1,258,837)
Forfeited						(201,250)
Unvested restricted common stock end of period						99,314	1,559,401
Weighted Average Grant Date Fair Value
Outstanding at the ending (in dollars per share) | $ / shares						$ 0.502
Weighted Average Grant Date Fair Value- vested (in dollars per share) | $ / shares						0.519
Weighted Average Grant Date Fair Value- Forfeited (in dollars per share) | $ / shares						0.387
Ending at the beginning (in dollars per share) | $ / shares						$ 0.526	$ 0.502
Number of shares exercised								1,400,000
Repurchase of unvested restricted common stock (in shares)						466,667
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
Unvested restricted common stock							670,834
The aggregate intrinsic value of restricted stock awards vested | $						$ 16,300	$ 34,000	$ 900
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
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
Interest on promissory note forgiven | $	$ 100
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
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
Market-Based Stock Options
Weighted Average Grant Date Fair Value
Weighted average grant-date fair value (per share) | $ / shares							$ 11.88
Market-Based Stock Options | Vesting based on service period | Executive officer
Weighted Average Grant Date Fair Value
Restricted stock exercise price | $ / shares		$ 16.43
Restricted Stock Units
Number of shares
Issued						277,200
Vested						0
Forfeited						0
Unvested restricted common stock end of period						277,200
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value- issued (in dollars per share) | $ / shares						$ 10.660
Ending at the beginning (in dollars per share) | $ / shares						$ 10.660
Vesting period						3 years

Stock-Based Compensation - Comp

Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 30, 2017
Stock-based compensation expense
Stock-based compensation expense		$ 1,000	$ 41,271	$ 27,528	$ 17,903
Research and development expenses
Stock-based compensation expense
Stock-based compensation expense			9,011	3,787	1,756
General and administrative expenses
Stock-based compensation expense
Stock-based compensation expense			32,260	23,741	$ 16,147
Employees, directors and non-employees
Stock-based compensation expense
Unrecognized compensation cost			$ 70,600
Unrecognized compensation cost expected to be recognized over a weighted average period			2 years 2 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
Purchase of an aggregate			3,059,004
Shares issued			0
Shares cancelled			257,854
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			$ 7,600	$ 7,300
Shares issued			99,314	1,559,401
Shares cancelled			201,250
Restricted Common Stock | Executive officer
Stock-based compensation expense
Shares issued						460,000
Market-Based Stock Options | Executive officer | Vesting based on service period
Stock-based compensation expense
Stock-based compensation expense			$ 900	$ 200
Purchase of an aggregate	164,400
Market-Based Stock Options | Executive officer | Performance-based vesting
Stock-based compensation expense
Stock-based compensation expense			$ 0
Purchase of an aggregate	193,400

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate	21.00%	21.00%
Federal Operating loss carryforwards	$ 222,900
State Operating loss carryforwards	227,000
Federal Operating loss carryforwards subject to expiration	37,200
Federal Operating loss carryforwards subject to expiration indefinitely	185,700
Research and development tax credit carryforwards	13,525	$ 3,824
Increase in gross deferred tax assets, before valuation allowance	52,200
Accrued interest or penalties	0	0
Uncertain tax positions	0	$ 0
U.S. federal
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	9,500
State
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	$ 5,100

Income Taxes - U S federal stat

Income Taxes - U S federal statutory income tax rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Income tax benefits - operating losses incurred	$ 0	$ 0
Income tax benefits - research and development tax credits	$ 0	$ 0
Federal statutory income tax rate	(21.00%)	(21.00%)
State taxes, net of federal benefit	(6.20%)	(4.80%)
Federal and state research and development tax credits	(6.00%)	(2.90%)
Stock-based compensation expense	0.70%	4.50%
Other	0.50%	0.50%
Increase in deferred tax asset valuation allowance	32.00%	23.70%

Income Taxes - Net deferred tax

Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 61,160	$ 25,508
Research and development tax credit carryforwards	13,525	3,824
Accrued expenses	1,142	1,139
Capitalized intellectual property costs	1,073	764
Capitalized research and development expense	109	120
Operating lease liabilities	12,952
Stock-based compensation expense	9,156	2,652
Total deferred tax assets	99,117	34,007
Deferred tax liabilities:
Operating lease assets	(12,720)
Depreciation and other	(513)	(341)
Total deferred tax liabilities	(13,233)	(341)
Valuation allowance	$ (85,884)	$ (33,666)	$ (12,575)

Income Taxes - Valuation allowa

Income Taxes - Valuation allowance for deferred tax assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Valuation allowance as of beginning of year	$ 33,666	$ 12,575
Increases recorded to income tax provision	52,218	21,091
Valuation allowance as of end of year	$ 85,884	$ 33,666

Commitments and Contingencies -

Commitments and Contingencies - Collaborative Arrangements And Non-collaborative Arrangement (Details) - WIBR $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
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_2

Commitments and Contingencies - Defined Contribution Plan (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Other Commitments [Line Items]
Employer matching contribution (as a percent)	50.00%
Company matching contribution		$ 0.6	$ 0.2
Maximum
Other Commitments [Line Items]
Maximum employee contribution	6.00%

Leases (Details)

Leases (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Nov. 30, 2018USD ($)	Jan. 31, 2018USD ($)Options	Dec. 31, 2019USD ($)leasepropertyitem	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Jan. 01, 2019USD ($)
Leases
Number of leases | lease			2
Number of expiration period | item			2
Number of leased properties | property			2
Lease term	9 years	8 years
Extended lease term		5 years
Number of options to renew lease | Options		1
Lessee, Operating Lease, Existence of Option to Extend [true false]				true
Initial annual base rent	$ 2,500	$ 3,800
Base rent incremental (as a percentage)	3.00%	3.00%
Benefit of the landlord secured by cash deposit	$ 600	$ 900
Tenant improvement allowance	9,200	9,400
Tenant improvement repayable to landlord	$ 2,000	$ 500
Construction-in-progress within property and equipment				$ 45,100
Operating lease, right-of-use-asset			$ 46,559			$ 1,751
Rent expense				$ 2,400	$ 1,000
Embedded lease contracts
Leases
Lease term			22 months
Lessee, Operating Lease, Existence of Option to Extend [true false]			false

Leases - Minimum lease payments

Leases - Minimum lease payments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Minimum lease payments
2020	$ 10,540
2021	7,574
2022	7,186
2023	7,390
2024	7,601
Thereafter	22,058
Payments due	62,349
Less: imputed interest	(14,942)
Operating lease liability	$ 47,407
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019		$ 5,458
2020		7,141
2021		7,168
2022		6,820
2023		7,024
Thereafter		27,777
Total		$ 61,388

Leases - Sublease (Details)

Leases - Sublease (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Sublease
Option to extend	false
Future undiscounted cash inflows under the sublease
2020	$ 735
2021	541
Total	$ 1,276

Leases - Lease Portfolio (Detai

Leases - Lease Portfolio (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019
Lease Cost
Operating lease cost	$ 7,208
Short-term lease cost	127
Variable lease cost	1,976
Sublease income	(908)
Total lease cost	8,403
Operating Leases
Operating lease, right-of-use-asset	46,559	$ 1,751
Operating lease liabilities	10,540	616
Operating lease liabilities, net of current portion	$ 36,867	$ 1,226
Weighted average remaining lease term - operating leases	7 years 5 months 19 days
Weighted-average discount rate - operating leases	7.58%

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net Loss									$ (163,458)	$ (89,195)	$ (43,847)
Accretion of Series A redeemable convertible preferred stock to redemption value											(656)
Net loss attributable to common stockholders	$ (44,472)	$ (47,015)	$ (39,390)	$ (32,581)	$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (163,458)	$ (89,195)	$ (44,503)
Denominator:
Weighted average common shares outstanding, basic and diluted									78,688,878	39,285,468	8,023,785
Net loss per share, basic and diluted	$ (0.56)	$ (0.59)	$ (0.50)	$ (0.42)	$ (0.35)	$ (0.42)	$ (2.43)	$ (1.72)	$ (2.08)	$ (2.27)	$ (5.55)

Net Loss Per Share - Antidiluti

Net Loss Per Share - Antidilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	15,480,421	17,015,005	55,007,222
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
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
Restricted Common Stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	376,514	2,230,235	6,277,014
Stock options
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	15,103,907	14,784,770	4,661,635
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, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Related Parties
Cash payments					$ 500
Accounts Receivable					$ 100
Amount due to related party							$ 0
Amount due from related party							0
Option to extend					false
Sublease income					$ 908
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%
January 2017 promissory note
Related Parties
Amount due to related party							0
Amount due from related party							0
Interest rate (as a percent)				1.97%
May 2017 promissory note
Related Parties
Amount due to related party							0
Amount due from related party							0
Interest rate (as a percent)		2.04%
Flagship | Services Agreement
Related Parties
Payment of general and administrative expenses						$ 1,300	900
Cash payments						1,300	$ 900
Amount due to related party					$ 0	$ 0
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 due from related party	200
Amount loaned or remitted to related parties			$ 200
Number of promissory notes | item			2
Number of restricted stock agreements | item			2
Interest income form related parties	$ 100

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statements of Operations Data:
Total operating expenses	$ 45,428	$ 48,482	$ 41,285	$ 34,406	$ 29,122	$ 27,554	$ 20,384	$ 14,603	$ 169,601	$ 91,663	$ 43,264
Loss from operations	(45,428)	(48,482)	(41,285)	(34,406)	(29,122)	(27,554)	(20,384)	(14,603)	(169,601)	(91,663)	(43,264)
Net loss attributable to common stockholders	$ (44,472)	$ (47,015)	$ (39,390)	$ (32,581)	$ (27,183)	$ (26,362)	$ (21,239)	$ (14,411)	$ (163,458)	$ (89,195)	$ (44,503)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.56)	$ (0.59)	$ (0.50)	$ (0.42)	$ (0.35)	$ (0.42)	$ (2.43)	$ (1.72)	$ (2.08)	$ (2.27)	$ (5.55)