Q32 Bio (QTTB) 10-K12 Mar 202019 FY Annual reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Mar. 06, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2019
Entity Registrant Name	Homology Medicines, Inc.
Entity Central Index Key	0001661998
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Trading Symbol	FIXX
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		45,202,999
Entity Public Float			$ 514
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.0001 par value
Security Exchange Name	NASDAQ
Entity File Number	001-38433
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-3468154
Entity Address, Address Line One	One Patriots Park
Entity Address, City or Town	Bedford
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01730
City Area Code	(781)
Local Phone Number	301-7277
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference [Text Block]	None.

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 53,774	$ 38,220
Short-term investments	208,614	176,517
Prepaid expenses and other current assets	4,189	6,948
Total current assets	266,577	221,685
Property and equipment, net	42,716	35,637
Restricted cash	1,274	1,772
Total assets	310,567	259,094
Current liabilities:
Accounts payable	2,608	15,732
Accrued expenses and other liabilities	7,644	5,040
Deferred rent	1,313	977
Deferred revenue	809	770
Total current liabilities	12,374	22,519
Non-current liabilities:
Deferred rent, net of current portion	9,544	9,470
Deferred revenue, net of current portion	30,142	30,750
Total liabilities	52,060	62,739
Commitments and contingencies (Note 8)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2019 and 2018, respectively; no shares issued and outstanding at December 31, 2019 and 2018, respectively
Common stock, $0.0001 par value; 200,000,000 shares authorized as of December 31, 2019 and 2018, respectively; 45,138,408 and 37,509,815 shares issued as of December 31, 2019 and 2018, respectively; and 45,116,742 and 37,358,526 shares outstanding as of December 31, 2019 and 2018, respectively	4	3
Additional paid-in capital	457,994	292,187
Accumulated other comprehensive loss	183	(77)
Accumulated deficit	(199,674)	(95,758)
Total stockholders’ equity	258,507	196,355
Total liabilities and stockholders' equity	$ 310,567	$ 259,094

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	45,138,408	37,509,815
Common stock, shares outstanding	45,116,742	37,358,526

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 1,666	$ 5,322
Operating expenses:
Research and development	89,398	47,948
General and administrative	22,211	17,300
Total operating expenses	111,609	65,248
Loss from operations	(109,943)	(59,926)
Other income:
Interest income	6,027	4,349
Total other income	6,027	4,349
Net loss	$ (103,916)	$ (55,577)
Net loss per share-basic and diluted	$ (2.47)	$ (1.95)
Weighted average common shares outstanding-basic and diluted	42,117,690	28,551,807

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (103,916)	$ (55,577)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available for sale securities, net	260	(4)
Total other comprehensive gain (loss)	260	(4)
Comprehensive loss	$ (103,656)	$ (55,581)

CONSOLIDATED STATEMENTS OF CONV

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Initial Public Offering	Follow On Offering	ATM	Convertible Preferred Stock	Common Stock	Common StockInitial Public Offering	Common StockFollow On Offering	Common StockATM	Additional Paid-in Capital	Additional Paid-in CapitalInitial Public Offering	Additional Paid-in CapitalFollow On Offering	Additional Paid-in CapitalATM	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance balance at Dec. 31, 2017					$ 137,762
Beginning balance at Dec. 31, 2017	$ (39,454)									$ 800				$ (73)	$ (40,181)
Beginning balance, Shares at Dec. 31, 2017					127,199,705
Beginning balance, Shares at Dec. 31, 2017						2,637,011
Conversion of convertible preferred stock into common stock upon initial public offering	137,762				$ (137,762)	$ 2				137,760
Conversion of convertible preferred stock into common stock upon initial public offering, Shares					(127,199,705)
Conversion of convertible preferred stock into common stock upon initial public offering, Shares						24,168,656
Issuance of common stock, net of discounts and issuance costs		$ 150,843					$ 1				$ 150,842
Issuance of common stock, net of discounts and issuance costs, Shares							10,350,000
Vesting of common stock from option exercises	75									75
Vesting of common stock from option exercises, Shares						129,959
Issuance of common stock from option exercises	87									87
Issuance of common stock from option exercises, Shares						72,900
Stock-based compensation	2,623									2,623
Other comprehensive gain (loss)	(4)													(4)
Net loss	(55,577)														(55,577)
Ending balance at Dec. 31, 2018	196,355					$ 3				292,187				(77)	(95,758)
Ending balance, Shares at Dec. 31, 2018						37,358,526
Issuance of common stock, net of discounts and issuance costs			$ 134,524	$ 22,409				$ 1				$ 134,523	$ 22,409
Issuance of common stock, net of discounts and issuance costs, Shares				1,105,000				6,388,889	1,105,000
Issuance of common stock pursuant to employee stock purchase plan	829									829
Issuance of common stock pursuant to employee stock purchase plan, Shares						55,234
Vesting of common stock from option exercises	60									60
Vesting of common stock from option exercises, Shares						94,816
Issuance of common stock from option exercises	$ 351									351
Issuance of common stock from option exercises, Shares	114,277					114,277
Stock-based compensation	$ 7,635									7,635
Other comprehensive gain (loss)	260													260
Net loss	(103,916)														(103,916)
Ending balance at Dec. 31, 2019	$ 258,507					$ 4				$ 457,994				$ 183	$ (199,674)
Ending balance, Shares at Dec. 31, 2019						45,116,742

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (103,916)	$ (55,577)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	6,318	1,289
Stock-based compensation expense	7,635	2,623
Amortization of premium on short-term investments	(1,871)	(1,589)
Loss on disposal of property and equipment	13	127
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,759	(5,003)
Accounts payable	(4,452)	4,383
Accrued expenses and other liabilities	2,315	3,042
Deferred revenue	(569)	(1,890)
Deferred rent	410	10,033
Net cash used in operating activities	(91,358)	(42,562)
Cash flows from investing activities:
Purchases of short-term investments	(286,391)	(245,328)
Maturities of short-term investments	256,425	148,480
Purchases of property and equipment	(21,833)	(24,939)
Net cash used in investing activities	(51,799)	(121,787)
Cash flows from financing activities:
Proceeds from issuance of common stock in initial public offering, net of discounts and issuance costs		150,843
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	134,524
Proceeds from issuance of common stock from option exercises	351	87
Proceeds from issuance of common stock pursuant to employee stock purchase plan	829
Proceeds from issuance of restricted common stock		64
Net cash provided by financing activities	158,213	150,994
Net change in cash, cash equivalents and restricted cash	15,056	(13,355)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	55,048	39,992
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	60	75
Property and equipment additions included in accounts payable	455	9,127
Property and equipment additions included in accrued expenses and other liabilities	249
Unrealized gain on available for sale securities, net	260	(4)
Offering costs included in accrued expenses and other liabilities	100
ATM
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs	$ 22,509
Series A Convertible Preferred Stock \| Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		42,994
Series B Convertible Preferred Stock \| Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		$ 94,768

Nature of Business and Basis of

Nature of Business and Basis of Presentation	12 Months Ended
Nature of Business and Basis of Presentation	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
NATURE OF BUSINESS AND BASIS OF PRESENTATION	1. NATURE OF BUSINESS AND BASIS OF PRESENTATION Nature of Business —Homology Medicines, Inc. (the “Company”) is a clinical stage biopharmaceutical company dedicated to translating proprietary gene therapy and gene editing technology into novel treatments for patients with rare genetic diseases with significant unmet medical needs by curing the underlying cause of the disease. The Company was founded in March 2015 as a Delaware corporation. Its principal offices are in Bedford, Massachusetts. Since its inception, the Company has devoted substantially all of its resources to recruiting personnel, developing its technology platform and advancing its pipeline of product candidates, developing and implementing manufacturing processes, building out manufacturing and research and development space, and maintaining and building its intellectual property portfolio. The Company is subject to a number of risks similar to those of other companies conducting high-risk, early-stage research and development of product candidates. Principal among these risks are dependency on key individuals and intellectual property, competition from other products and companies, and the technical risks associated with the successful research, development and clinical manufacturing of its product candidates. The Company’s success is dependent upon its ability to continue to raise additional capital in order to fund ongoing research and development, obtain regulatory approval of its products, successfully commercialize its products, generate revenue, meet its obligations, and, ultimately, attain profitable operations. On April 2, 2018, the Company completed its initial public offering (“IPO”) with the sale of 10,350,000 shares of its common stock, including shares issued upon the exercise in full of the underwriters’ over-allotment option, at a public offering price of $16.00 per share, resulting in net proceeds of $150.8 million, after deducting underwriting discounts and commissions and offering expenses. Upon the closing of the IPO, all of the Company’s outstanding shares of convertible preferred stock automatically converted into 24,168,656 shares of common stock at the applicable conversion ratio then in effect. Subsequent to the closing of the IPO, there were no shares of preferred stock outstanding. On April 1, 2019, the Company filed a Registration Statement on Form S-3 (File No. 333-230664) (the “Shelf”) with the Securities and Exchange Commission (“SEC”) in relation to the registration of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof in the aggregate amount of up to $350.0 million for a period up to three years from the date of the filing. The Shelf became effective on April 9, 2019. The Company also simultaneously entered into a sales agreement with Cowen and Company, LLC, as sales agent, providing for the offering, issuance and sale by the Company of up to an aggregate $100.0 million of its common stock from time to time in “at-the-market” offerings under the Shelf (the “ATM”). During the year ended December 31, 2019, the Company sold 1,105,000 shares of common stock and received net proceeds of $22.5 million pursuant to the ATM. At December 31, 2019, there remained $76.8 million of common stock available for sale under the ATM. On April 12, 2019, pursuant to the Shelf, the Company completed a follow-on public offering of its common stock. The Company sold 5,555,556 shares of its common stock at a public offering price of $22.50 per share and received net proceeds of $116.9 million, after deducting underwriting discounts and commissions and offering expenses. In addition, on April 26, 2019 and May 7, 2019, in connection with the exercise in full of the underwriters’ option to purchase additional shares, the Company issued an aggregate of 833,333 shares of its common stock at a public offering price of $22.50 per share and received net proceeds of $17.6 million, after deducting underwriting discounts and commissions. To date, the Company has not generated any revenue from product sales and does not expect to generate any revenue from the sale of product in the foreseeable future. Through December 31, 2019, the Company has financed its operations primarily through public offerings of its common stock, the issuance of convertible preferred stock, and with proceeds from its collaboration and license agreement with Novartis (see Note 15). During the year ended December 31, 2019, the Company incurred a net loss of $103.9 million and as of December 31, 2019, had $199.7 million in accumulated deficit. The Company expects to incur additional operating losses and negative operating cash flows for the foreseeable future. Based on current projections, management believes that existing cash, cash equivalents and short-term investments will enable the Company to continue its operations into the fourth quarter of 2021. In the absence of a significant source of recurring revenue, the continued viability of the Company beyond that point is dependent on its ability to continue to raise additional capital to finance its operations. There can be no assurance that the Company will be able to obtain sufficient capital to cover its costs on acceptable terms, if at all. Basis of Presentation —The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business .

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2019
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Principles of Consolidation —The Company’s consolidated financial statements include the accounts of the Company and its subsidiary, Homology Medicines Securities Corporation, a wholly owned Massachusetts corporation, for the sole purpose of buying, selling, and holding securities on the Company’s behalf. All intercompany balances and transactions have been eliminated in the consolidated financial statements. Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue, and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, accrued research and development expenses and useful lives assigned to property and equipment. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates. Comprehensive Income (Loss) — Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments. Cash and Cash Equivalents and Restricted Cash —Cash and cash equivalents consist of standard checking accounts, money market accounts and certain investments. The Company considers all highly liquid investments with original or remaining maturities at the time of purchase of 90 days or less to be cash equivalents. Restricted cash consists of cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the consolidated statements of cash flows: December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 53,774 $ 38,220 Restricted cash 1,274 1,772 Total cash, cash equivalents and restricted cash $ 55,048 $ 39,992 Short-Term Investments —Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses, reported within accumulated other comprehensive income as a separate component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities, are included in interest income in the Company’s consolidated statements of operations. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other income. Concentrations of Credit Risk —Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents and short-term investments. Periodically, the Company may maintain deposits in financial institutions in excess of government insured limits. We believe that we are not exposed to significant credit risk as our deposits are held at financial institutions that management believes to be of high credit quality and the Company has not experienced any losses on these deposits. We regularly invest excess cash with major financial institutions in money market funds, U.S. government and corporate debt securities and commercial paper, all of which can be readily purchased and sold using established markets. As of December 31, 2019, the Company’s cash and cash equivalents were held with two financial institutions. We believe that the market risk arising from our holdings of these financial instruments is mitigated based on the fact that many of these securities are either government backed or of high credit rating. Deferred Offering Costs —The Company capitalizes incremental legal, professional accounting and other third-party fees that are directly associated with equity financings as other current assets until the transactions are completed. After equity financings are complete, these costs are recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering. Guarantees and Indemnifications —As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2019, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related liabilities have been established. Property and Equipment —Property and equipment are recorded at cost. Expenditures for repairs and maintenance are expensed as incurred. When assets are retired or disposed of, the assets and related accumulated depreciation are derecognized from the accounts, and any resulting gain or loss is included in the determination of net loss. Depreciation is provided using the straight-line method over the estimated useful lives of the related assets. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset. Computer equipment and software 3 years Laboratory equipment and office furniture 5 years Manufacturing equipment 5 - 7 years Leasehold improvements Shorter of the lease term or estimated useful life Impairment of Long-Lived Assets —The Company evaluates its long-lived assets, which consist primarily of property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to the future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the asset exceeds the fair value of the asset. To date, no impairments have been recognized for these assets. Research and Development Costs —Research and development costs are charged to expense as incurred. Research and development expense consists of expenses incurred in performing research and development activities, including salaries and benefits, materials and supplies, preclinical and clinical expenses, stock-based compensation expense, depreciation of equipment, contract services, and other outside expenses. Costs for certain development activities are recognized based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its vendors on their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense. Income Taxes —The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the Company’s consolidated financial statements and tax returns. Deferred tax assets and liabilities are determined based upon the differences between the financial statement carrying amounts and the tax bases of existing assets and liabilities and for loss and credit carryforwards, using enacted tax rates expected to be in effect in the year in which the differences are expected to reverse. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that these assets may not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. Segment Information —Operating segments are identified as components of an enterprise about which separate discrete financial information is made available for evaluation by the chief operating decision maker (“CODM”) in making decisions regarding resource allocation and assessing performance. The CODM is the Company’s Chief Executive Officer. The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is dedicated to translating proprietary gene editing and gene therapy technology into novel treatments for patients with rare genetic diseases. All of the Company’s tangible assets are held in the United States. Revenue Recognition— In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue (Topic 606): Revenue from Contracts with Customers (“ASU 2014-09”), which amends the guidance for accounting for revenue from contracts with customers. ASU 2014-09 supersedes the revenue recognition requirements in FASB ASC Topic 605, Revenue Recognition (“ASC 605”), and creates a new Topic 606, Revenue from Contracts with Customers (“ASC 606”). On January 1, 2019, the Company adopted ASC 606 using the full retrospective transition method. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangements would likely consist of a license, rights to the Company’s intellectual property or research, development and manufacturing services. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of consideration to which the Company expects to be entitled to. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts may include development and regulatory milestone payments that are assessed under the most likely amount method and constrained until it is probable that a significant revenue reversal would not occur. Milestone payments that are not within the Company’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development and regulatory milestones and any related constraint, and if necessary, adjust its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from the Company’s collaboration arrangement. The Company allocates the transaction price based on the estimated standalone selling price of each performance obligation. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress for its over-time arrangements at each reporting period and, if necessary, updates the measure of progress and revenue recognized. The Company adopted ASC 606 effective January 1, 2019, using the full retrospective transition method. Under this method, the Company revised its consolidated financial statements for prior period amounts as if ASC 606 had been effective for such periods. The references "as revised" used herein refer to revisions of data for the year ended December 31, 2018 as a result of the adoption of ASC 606. The adoption of ASC 606 did not have an impact on periods prior to January 1, 2018. In addition to revisions to financial statement amounts and related tables, Note 15 has been revised to reflect the adoption of ASC 606. As part of the adoption, the Company reviewed its collaboration and license agreement with Novartis to determine the cumulative effect impact related to the adoption of ASC 606. (For a complete discussion of the accounting for the Company’s agreement with Novartis, see Note 15.) The adoption of ASC 606 resulted in a change to the pattern of revenue recognition whereby the Company expects to recognize revenue from its collaboration agreement with Novartis as costs are incurred, which likely will not occur evenly over the performance period as a result of applying the cost-to-cost method, in contrast to recognizing revenue on a straight-line basis over the estimated performance period under ASC 605. The impact on previously reported amounts as a result of the adoption of ASC 606 is as follows: Consolidated Balance Sheets December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Current portion of deferred revenue $ 770 $ 3,684 $ (2,914 ) Long-term portion of deferred revenue $ 30,750 $ 29,474 $ 1,276 Accumulated deficit $ (95,758 ) $ (97,396 ) $ 1,638 Consolidated Statements of Operations and Comprehensive Loss For the Year Ended December 31, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 5,322 $ 3,684 $ 1,638 Loss from operations $ (59,926 ) $ (61,564 ) $ 1,638 Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Net loss per share attributable to common stockholders-basic and diluted $ (1.95 ) $ (2.00 ) $ 0.05 Consolidated Statement of Cash Flows For the Year Ended December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (1,890 ) $ (252 ) $ (1,638 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 39,992 $ 39,992 $ — Stock-based Compensation —The Company recognizes compensation expense for awards to employees based on the grant date fair value of stock-based awards on a straight-line basis over the period during which an award holder provides service in exchange for the award. The fair value of options on the date of grant is calculated using the Black-Scholes option pricing model based on key assumptions such as stock price, expected volatility and expected term. The Company’s estimates of these assumptions are primarily based on the trading price of the Company’s stock, historical data, peer company data and judgment regarding future trends and factors. Stock-based awards granted to nonemployees are initially recorded at fair value and are re-measured at each reporting period as the awards vest and expense is recognized over the period the services are provided. The purchase price of common stock under the Company’s employee stock purchase plan (“ESPP”) is equal to 85% of the lesser of (i) the fair market value per share of the common stock on the first business day of an offering period and (ii) the fair market value per share of the common stock on the purchase date. The fair value of the look-back provision under the ESPP is calculated using the Black-Scholes option pricing model. The fair value of the look-back provision plus the 15% discount is recognized as compensation expense over the 180-day purchase period. Fair Value Measurements —Certain assets and liabilities are reported on a recurring basis at fair value. 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 (unadjusted) 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. Net Loss per Share —Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period. Diluted net loss per share is computed using the weighted-average number of common shares outstanding during the period and, if dilutive, the weighted-average number of potential shares of common stock. The weighted-average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options and unvested shares of common stock. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is generally the same as basic net loss per share attributable to common stockholders since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Recent Accounting Pronouncements —The Jumpstart Our Business Startups Act of 2012 permits an emerging growth company to take advantage of an extended transition period to comply with new or revised accounting standards applicable to public companies until those standards would otherwise apply to private companies. As an emerging growth company, the Company has elected to take advantage of this extended transition period. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) , The Company adopted this standard early on January 1, 2020. The Company expects to elect the package of practical expedients permitted under the transition guidance within the new standard, which, among other things, allows the Company to carry forward historical lease classification. The Company has substantially completed its evaluation of the impact of the adoption of ASU 2016-02 on its consolidated financial statements and upon adoption, expects to recognize a lease liability and related right-of-use asset on its consolidated balance sheet of approximately $17.0 million to $18.0 million and approximately $6.0 million to $7.0 million, respectively, and a reduction to deferred rent of approximately $10.0 million to $11.0 million to eliminate the deferred rent balance. The Company does not expect the standard to have a material impact on its results of operations or cash flows. In addition, the Company is currently implementing changes to processes and controls to support lease accounting and related disclosures under the new standard. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In December 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force) , In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ,

Cash and Cash Equivalents

Cash and Cash Equivalents	12 Months Ended
Cash and Cash Equivalents	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Cash and Cash Equivalents	3. CASH AND CASH EQUIVALENTS From time to time, the Company may have cash balances in financial institutions in excess of federal deposit insurance limits. The Company has never experienced any losses related to these balances. The Company considers only those investments that are highly liquid, readily convertible to cash, and that mature within three months from date of purchase to be cash equivalents. The following table summarizes the Company’s cash and cash equivalents: December 31, 2019 2018 (in thousands) Cash $ 250 $ 471 U.S. Treasury securities — 35 Commercial paper — 5,974 Repurchase agreements 14,000 — Money market funds 39,524 31,740 Total cash and cash equivalents $ 53,774 $ 38,220

Short-Term Investments

Short-Term Investments	12 Months Ended
Short-Term Investments	Dec. 31, 2019
Investments Disclosure [Abstract]
SHORT-TERM INVESTMENTS	4. SHORT-TERM INVESTMENTS The Company invests its excess cash in fixed income instruments denominated and payable in U.S. dollars including U.S. treasury securities, commercial paper, corporate debt securities and asset-backed securities in accordance with the Company’s investment policy that primarily seeks to maintain adequate liquidity and preserve capital. The Company has designated all investments as available-for-sale and therefore such investments are reported at fair value. Unrealized gains or losses on investments are recorded in accumulated other comprehensive income or loss, a component of stockholders’ equity, on the Company’s consolidated balance sheets. The following table summarizes the Company’s investments, which are included in cash equivalents and short-term investments: As of December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Repurchase agreements $ 14,000 $ — $ — 14,000 Asset-backed securities 14,866 10 — 14,876 Money market mutual funds 39,524 — — 39,524 Commercial paper 41,259 — — 41,259 U.S. Treasury securities 59,926 45 — 59,971 Corporate debt securities 92,380 128 — 92,508 Total $ 261,955 $ 183 $ — $ 262,138 As of December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Asset-backed securities $ 19,541 $ — $ (21 ) $ 19,520 U.S. Treasury securities 29,980 — (6 ) 29,974 Money market mutual funds 31,740 — — 31,740 Corporate debt securities 54,511 — (50 ) 54,461 Commercial paper 78,571 — — 78,571 Total $ 214,343 $ — $ (77 ) $ 214,266 As of December 31, 2019, the Company had no securities that are in an unrealized loss position. The Company utilizes the specific identification method in computing realized gains and losses. The Company had no realized gains and losses on its available-for-sale securities for the years ended December 31, 2019 and 2018. The contractual maturity dates of all of the Company’s investments are less than one year.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
Fair Value Measurements	Dec. 31, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	5. FAIR VALUE MEASUREMENTS The Company’s financial instruments consist of cash and cash equivalents, short-term investments, restricted cash and accounts payable. The carrying amount of cash, restricted cash and accounts payable are each considered a reasonable estimate of fair value due to the short-term maturity. Assets measured at fair value on a recurring basis were as follows: Description December 31, 2019 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 39,524 $ 39,524 $ — $ — Repurchase agreements 14,000 — 14,000 — Total cash equivalents $ 53,524 $ 39,524 $ 14,000 $ — Short-term investments: Asset-backed securities $ 14,876 $ — $ 14,876 $ — Commercial paper 41,259 — 41,259 — U.S. Treasury securities 59,971 — 59,971 — Corporate debt securities 92,508 — 92,508 — Total short-term investments $ 208,614 $ — $ 208,614 $ — Total financial assets $ 262,138 $ 39,524 $ 222,614 $ — Description December 31, 2018 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 31,740 $ 31,740 $ — $ — Commercial paper 5,974 — 5,974 — U.S. Treasury securities 35 35 — — Total cash equivalents $ 37,749 $ 31,775 $ 5,974 $ — Short-term investments: Asset-backed securities $ 19,520 $ — $ 19,520 $ — U.S. Treasury securities 29,939 — 29,939 — Corporate debt securities 54,461 — 54,461 — Commercial paper 72,597 — 72,597 — Total short-term investments $ 176,517 $ — $ 176,517 $ — Total financial assets $ 214,266 $ 31,775 $ 182,491 $ — Short-term securities are valued using models or other valuation methodologies that use Level 2 inputs. These models are primarily industry-standard models that consider various assumptions, including time value, yield curve, volatility factors, default rates, current market and contractual prices for the underlying financial instruments, as well as other relevant economic measures. Substantially all of these assumptions are observable in the marketplace, can be derived from observable data or are supported by observable levels at which transactions are executed in the marketplace. There were no transfers between fair value measure levels during the years ended December 31, 2019 and 2018.

Property and Equipment

Property and Equipment	12 Months Ended
Property and Equipment	Dec. 31, 2019
Property Plant And Equipment [Abstract]
PROPERTY AND EQUIPMENT	6. PROPERTY AND EQUIPMENT Property and equipment, net consists of the following: December 31, 2019 2018 (in thousands) Laboratory equipment $ 10,837 $ 9,309 Manufacturing equipment 4,911 — Computers and purchased software 640 248 Furniture and fixtures 1,363 1,066 Leasehold improvements 30,862 24,520 Assets not yet in service 2,513 2,602 Property and equipment, at cost 51,126 37,745 Less accumulated depreciation and amortization (8,410 ) (2,108 ) Property and equipment, net $ 42,716 $ 35,637 Depreciation expense for the years ended December 31, 2019 and 2018 was approximately $6.3 million and $1.3 million, respectively. The Company disposed of $13,000 and approximately $0.1 million of property and equipment, net during the years ended December 31, 2019 and 2018, respectively. Leasehold improvements consist primarily of costs associated with the buildout of the Company’s research and development, manufacturing and general office space in Bedford, Massachusetts, which the Company occupied as of December 31, 2018.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
Accrued Expenses and Other Liabilities	Dec. 31, 2019
Payables And Accruals [Abstract]
ACCRUED EXPENSES AND OTHER LIABILITIES	7. ACCRUED EXPENSES AND OTHER LIABILITIES Accrued expenses and other liabilities consist of the following: December 31, 2019 2018 (in thousands) Accrued compensation and benefits $ 4,551 $ 3,370 Accrued research and development expenses 2,258 1,059 Accrued professional fees 542 195 Accrued unvested common stock subject to repurchase 37 112 Accrued other 256 304 Total accrued expenses and other liabilities $ 7,644 $ 5,040

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	8. COMMITMENTS AND CONTINGENCIES Operating Leases —In September 2016, the Company entered into a noncancelable operating lease beginning in November 2016 for office, laboratory and manufacturing space in Bedford, Massachusetts, that expires in October 2021, with an option for an additional three-year term. In 2018, the Company entered into a sublease agreement for the entire leased premises. The rent commencement date of the sublease was December 2018, and the sublease will terminate on the scheduled termination date of the original lease. Under the terms of the sublease, the subtenant is obligated to pay the Company aggregate base rent of approximately $2.7 million over the term of the sublease in addition to a passthrough of operating expenses and real estate taxes charged by the landlord. The Company did not record a loss on the sublease as future payments to its landlord were not materially different from future rent payments expected from the subtenant over the term of the sublease. In December 2017, the Company entered into a noncancelable operating lease for approximately 67,000 square feet of research and development, manufacturing and general office space in Bedford, Massachusetts. The lease expires in February 2027 with an option for an additional five-year term. The initial annual base rent was $39.50 per square foot and increases by three percent annually. The Company is obligated to pay, on a pro-rata basis, real estate taxes and operating costs related to the premises. Future minimum lease payments, net of anticipated sublease payments, as of December 31, 2019, are as follows: Amount For the Years Ending December 31, (in thousands) 2020 $ 2,810 2021 2,815 2022 2,987 2023 3,077 2024 3,169 Thereafter 7,197 Total future minimum lease payments $ 22,055 The lease agreement entered into in December 2017 allowed for a tenant improvement allowance not to exceed $10.9 million to be applied to the total cost of tenant improvements to the leased premises. The total tenant improvement allowance of $10.9 million was recorded as deferred rent incentives on the Company’s consolidated balance sheets and is being recognized as a reduction to rent expense over the term of the lease. As of December 31, 2019, deferred rent incentives totaled $8.3 million. Rent expense, net of amortization of the deferred rent incentive, for the years ended December 31, 2019 and 2018 was $1.4 million and $2.7 million, respectively. The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $1.3 million and $1.8 million as of December 31, 2019 and 2018, respectively, in conjunction with its current leases.

License Agreements

License Agreements	12 Months Ended
License Agreements	Dec. 31, 2019
License Agreement [Abstract]
LICENSE AGREEMENTS	9. LICENSE AGREEMENTS City of Hope In April 2016, the Company entered into a license agreement with City of Hope (“COH”), an academic research and medical center. The license term extends until the last to expire patent, unless terminated earlier by either party under certain provisions. The Company is required to pay an annual license fee of $25,000, reimburse COH for patent costs incurred, pay amounts up to $3.2 million upon the achievement of certain development and commercialization milestones for each product under the license, pay royalties on future sales in the low single- digits and royalties on sublicense revenue in the low double-digits, if any. During the year ended December 31, 2019, the Company paid $50,000 to COH upon dosing the first patient in the pheNIX Phase 1/2 clinical trial. In May 2015, the Company entered into a sponsored research agreement with COH with a goal to identify potential treatments for diseases in humans. Under this agreement, the Company recorded $0.1 million in research and development expense for each of the years ended December 31, 2019 and 2018. The agreement terminated in September 2019 in accordance with its terms. California Institute of Technology In September 2016, the Company entered into a co-exclusive license agreement with the California Institute of Technology (“Caltech”), an academic research institute. The license term extends until the expiration, revocation, invalidation or unenforceability of the licensed patent rights. The Company is required to pay an annual minimal royalty fee of $20,000, reimburse for patent costs incurred, pay an amount up to $7.2 million upon the achievement of certain development and regulatory milestones and pay royalties on future sales in the low single-digits and royalties on sublicense revenue in the mid to high single-digits, if any.

Income Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2019
Income Tax Disclosure [Abstract]
INCOME TAXES	10. INCOME TAXES A reconciliation between the U.S. federal statutory tax rate and the Company’s effective tax rate is summarized as follows: For the Years Ended December 31, 2019 2018 Federal statutory rate 21.0 % 21.0 % Tax credits 13.4 % 17.7 % State taxes, net of federal tax benefit 6.1 % 5.5 % Non-deductible expenses (1.0 %) (0.3 %) Change in valuation allowance (39.5 %) (43.9 %) Effective income tax rate — % — % The principal components of the Company’s deferred tax assets and liabilities consist of the following: December 31, 2019 2018 (in thousands) Deferred tax assets: Net operating losses $ 41,589 $ 14,848 R&D credits 25,328 12,506 Deferred revenue 8,456 8,159 Deferred rent 2,966 2,854 Capitalized R&D costs 1,207 1,375 Equity compensation 883 443 Accrued expense and other 108 115 Total deferred tax assets 80,537 40,300 Deferred tax liabilities: Depreciation (2,744 ) (2,680 ) Total deferred tax liabilities (2,744 ) (2,680 ) Valuation allowance (77,793 ) (37,620 ) Net deferred taxes $ — $ — The Company has no income tax expense due to the operating loss incurred for the years ended December 31, 2019 and 2018. The Company has provided a valuation allowance for the full amount of the net deferred tax assets as the realization of the net deferred tax assets is not determined to be more likely than not. At December 31, 2019, the Company had $150.7 million and $157.3 million of federal and state net operating loss carryforwards, respectively, that expire at various dates through 2039. Included in the federal net operating loss carryforwards of $150.7 million is $119.6 million that can be carried forward indefinitely. At December 31, 2019, the Company had $21.2 million and $5.2 million of federal and state research and development credit carryforwards, respectively, that expire at various dates through 2039. Included in the $21.2 million of federal research and development credit carryforwards is $17.3 million of orphan drug credit carryforwards. The valuation allowance increased in 2019 and 2018 by $40.2 million and $25.1 million, respectively, due to the increase in the deferred tax assets by the same amounts, primarily due to the net operating loss carryforwards and research and development tax credits not utilized. For all years through December 31, 2019, the Company generated research credits but has not conducted a study to document the qualified activities. This study may result in an adjustment to the Company’s research and development credit carryforwards. Since a full valuation allowance has been provided against the Company’s research and development credits, any reduction in the gross deferred tax asset established for the research and development credit carryforwards would not result in any net impact to the Company’s consolidated financial statements. Realization of the future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. Under the provisions of the Internal Revenue Code, certain substantial changes in the Company’s ownership, including a sale of the Company or significant changes in ownership due to sales of equity, may have limited, or may limit in the future, the amount of net operating loss carryforwards that could be used annually to offset future taxable income. The Company has not completed a study to assess whether a change of control has occurred or whether there have been multiple changes of control since the Company’s formation due to the significant complexity and cost associated with such study and because there could be additional changes in control in the future. As a result, the Company is not able to estimate the effect a change in control would have, if any, on the Company’s ability to utilize its net operating loss and research and development credit carryforwards in the future. The Company files tax returns in the United States and Massachusetts. All tax years since inception remain open to examination by the major taxing jurisdictions to which the Company is subject, as carryforward attributes generated in years past may still be adjusted upon examination by the Internal Revenue Service (“IRS”) or other authorities if they have or will be used in a future period. The Company is not currently under examination by the IRS or any other jurisdictions for any tax years. As of December 31, 2019, the Company had no uncertain tax positions. The Company has elected to recognize interest and penalties related to income tax matters as a component of income tax expense, of which no interest or penalties were recorded for the years ended December 31, 2019 and 2018.

Stockholder's Equity

Stockholder's Equity	12 Months Ended
Stockholder's Equity	Dec. 31, 2019
Equity [Abstract]
STOCKHOLDERS' EQUITY	11. STOCKHOLDERS’ EQUITY Common Stock —Voting, dividend and liquidation rights of the holders of the common stock are subject to and qualified by the rights, powers and preferences of the holders of the preferred stock. Voting —Each holder of outstanding shares of common stock are entitled to one vote in respect of each share. The holders of outstanding shares of common stock, voting together as a single class, shall be entitled to elect one director. The number of authorized shares of common stock may be increased or decreased by the affirmative vote of a majority of the outstanding shares of common stock and preferred stock voting together as a single class. Dividends —Subject to the payment in full of any preferential dividends to which the holders of preferred stock are entitled, the holders of common stock shall be entitled to receive dividends out of funds legally available therefore at such times and in such amounts as the Board of Directors may determine in its sole discretion. Liquidation Rights —In the event of any voluntary or involuntary liquidation, dissolution or winding-up of the Company, after the payment or provision for payment of all debts and liabilities of the Company and any preferential amounts to which the holders of preferred stock are entitled with respect to the distribution of assets in liquidation, the holders of common stock shall be entitled to share ratably in the remaining assets of the Company available for distribution. There were 45,116,742 and 37,358,526 shares of common stock outstanding at December 31, 2019 and 2018, respectively. Preferred Stock —As of December 31, 2019 and 2018, there were no shares of preferred stock issued and outstanding.

Stock Incentive Plans

Stock Incentive Plans	12 Months Ended
Stock Incentive Plans	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK INCENTIVE PLANS	12. STOCK INCENTIVE PLANS 2015 Stock Incentive Plan In December 2015, the Company’s Board of Directors adopted the 2015 Stock Incentive Plan (the “2015 Plan”), which provided for the grant of qualified incentive and nonqualified stock options or restricted stock awards to the Company’s employees, officers, directors, advisors, and outside consultants. Stock options generally vest over a four-year period and expire ten years from the date of grant. Certain options provide for accelerated vesting if there is a change in control, as defined in the 2015 Plan. At December 31, 2019, there were no additional shares available for future grant under the 2015 Plan. 2018 Incentive Award Plan In March 2018, the Company’s Board of Directors adopted and the Company’s stockholders approved the Homology Medicines, Inc. 2018 Incentive Award Plan (the “2018 Plan” and, together with the 2015 Plan, the “Plans”), which became effective on the day prior to the first public trading date of the Company’s common stock. Upon effectiveness of the 2018 Plan, the Company ceased granting new awards under the 2015 Plan. The 2018 Plan provides for the grant of incentive stock options, nonqualified stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock or cash-based awards to employees and consultants of the Company and certain affiliates and directors of the Company. The number of shares of common stock initially available for issuance under the 2018 Plan was 3,186,205 shares of common stock plus the number of shares subject to awards outstanding under the 2015 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2018 Plan. In addition, the number of shares of common stock available for issuance under the 2018 Plan is subject to an annual increase on the first day of each calendar year beginning on January 1, 2019 and ending on and including January 1, 2028 equal to the lesser of (i) 4% of the Company’s outstanding shares of common stock on the final day of the immediately preceding calendar year and (ii) such smaller number of shares of common stock as determined by the Company’s Board of Directors, provided that not more than 20,887,347 shares of common stock may be issued under the 2018 Plan upon the exercise of incentive stock options. Therefore, on January 1, 2020, an additional 1,804,670 shares were added to the 2018 Plan, representing 4% of total common shares outstanding at December 31, 2019. As of December 31, 2019, there were 1,637,094 shares available for future grant under the 2018 Plan. 2018 Employee Stock Purchase Plan In March 2018, the Company’s Board of Directors adopted and the Company’s stockholders approved the Homology Medicines, Inc. 2018 Employee Stock Purchase Plan (the “2018 ESPP”). The 2018 ESPP allows employees to buy Company stock through after-tax payroll deductions at a discount from market value. The number of shares of common stock initially available for issuance under the 2018 ESPP was 353,980 shares of common stock plus an annual increase on the first day of each calendar year beginning on January 1, 2019 and ending on and including January 1, 2028 equal to the lesser of (i) 1% of the Company’s outstanding shares of common stock on the final day of the immediately preceding calendar year and (ii) such smaller number of shares of common stock as determined by the Company’s Board of Directors, provided that not more than 4,778,738 shares of common stock may be issued under the 2018 ESPP. Therefore, on January 1, 2020, an additional 451,167 shares were added to the 2018 ESPP, representing 1% of total common shares outstanding at December 31, 2019. At December 31, 2019, there were 672,331 shares available for future issuance under the 2018 ESPP. The Company commenced offerings under the 2018 ESPP on September 1, 2018. Under the 2018 ESPP, employees may purchase common stock through after-tax payroll deductions at a price equal to 85% of the lower of the fair market value on the first trading day of an offering period or the last trading day of an offering period. The 2018 ESPP generally provides for offering periods of six months in duration that end on the final trading day of each February and August. In accordance with the Internal Revenue Code, no employee will be permitted to accrue the right to purchase stock under the ESPP at a rate in excess of $25,000 worth of shares during any calendar year during which such a purchase right is outstanding (based on the fair market value per share of our common stock as of the first day of the offering period). During the year ended December 31, 2019, 55,234 shares were issued under the 2018 ESPP for aggregate proceeds to the Company of $0.8 million. There were no shares issued pursuant to the 2018 ESPP during the year ended December 31, 2018. The Company recorded $0.2 million and $0.1 million of stock-based compensation pursuant to the 2018 ESPP for the years ended December 31, 2019 and 2018, respectively. Stock-based compensation expense The Company recognizes compensation expense for awards to employees based on the grant date fair value of stock-based awards on a straight-line basis over the period during which an award holder provides service in exchange for the award, which is generally the vesting period. The fair value of each option award is estimated on the date of grant using the Black-Scholes option- pricing model, with the assumptions noted in the table below. Expected volatility for the Company’s common stock was determined based on an average of the historical volatility of a peer group of publicly traded companies that are similar to the Company. The expected term of options granted to employees was calculated using the simplified method, which represents the average of the contractual term of the option and the weighted-average vesting period of the option. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate expected term. The contractual life of the options was used for the expected term of options granted to non-employees. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for periods commensurate with the expected term of the award. The Company recognizes forfeitures as they occur. The Company recorded stock-based compensation expense related to stock options and shares purchased under the 2018 ESPP as follows: For the Years Ended December 31, 2019 2018 (in thousands) Research and development $ 4,164 $ 1,117 General and administrative 3,471 1,506 $ 7,635 $ 2,623 As of December 31, 2019, there was $31.5 million of unrecognized compensation expense related to unvested employee and non-employee share-based compensation arrangements granted under the Plans. The unrecognized compensation expense is estimated to be recognized over a period of 2.9 years at December 31, 2019. In determining the exercise prices for options granted, the Company’s Board of Directors considered the fair value of the common stock as of the measurement date. For awards granted prior the Company’s IPO, the Board of Directors determined the fair value of the common stock at each award grant date based upon a variety of factors, including the results obtained from an independent third-party valuation, the Company’s financial position and historical financial performance, the status of technological developments within the Company’s proposed products, an evaluation or benchmark of the Company’s competition, the current business climate in the marketplace, the illiquid nature of the common stock, arm’s length sales of the Company’s capital stock, including convertible preferred stock, the effect of the rights and preferences of the preferred shareholders, and the prospects of a liquidity event, among others. For awards granted after the Company’s IPO, the exercise price is equal to the closing price as quoted by Nasdaq on the grant date of the awards. The assumptions used in the Black-Scholes option pricing model are as follows: For the Years Ended December 31, 2019 2018 Expected volatility 62.4% — 64.7% 52.80% — 60.12% Weighted-average risk-free interest rate 1.36% — 2.60% 2.33% — 3.08% Expected dividend yield — % — % Expected term (in years) 6.25 5.5 — 7.6 Underlying common stock fair value $12.73 — $30.34 $6.63 — $24.28 A summary of option activity under the Plans during the year ended December 31, 2019 is as follows: Number of Options Weighted- Average Exercise Price per Share Weighted- Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value (in thousands) Outstanding as of January 1, 2019 3,203,122 $ 10.47 8.9 $ 39,018 Granted 1,898,215 $ 21.60 Exercised (114,277 ) $ 3.07 Cancelled/Forfeited (144,042 ) $ 18.06 Outstanding at December 31, 2019 4,843,018 $ 14.78 8.5 $ 33,809 Vested and expected to vest at December 31, 2019 4,843,018 $ 14.78 8.5 $ 33,809 Exercisable at December 31, 2019 1,593,532 $ 8.72 7.6 $ 20,021 The total intrinsic value of options exercised during the year ended December 31, 2019 and 2018 was $1.9 million and $1.8 million, respectively. The weighted-average grant date fair value of options granted during the years ended December 31, 2019 and 2018 was $12.90 and $10.41, respectively. Stock options granted pursuant to the 2015 Plan permit option holders to elect to exercise unvested options in exchange for unvested common stock. Options granted under the 2015 Plan that are exercised prior to vesting will continue to vest according to the respective option agreement, and such unvested shares are subject to repurchase by the Company at the optionee’s original exercise price in the event the optionee’s service with the Company voluntarily or involuntarily terminates. A summary of the Company’s unvested common stock from early exercises that is subject to repurchase by the Company is as follows: Shares Unvested shares—January 1, 2019 151,289 Vested (94,816 ) Issued — Repurchased (34,807 ) Unvested shares—December 31, 2019 21,666 As of December 31, 2019 and 2018, 21,666 shares and 151,289 shares, respectively, remained subject to a repurchase right by the Company, with a related liability included in accrued expenses and other liabilities in the consolidated balance sheets of approximately $0.1 million as of each date.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
Net Loss Per Share	Dec. 31, 2019
Earnings Per Share [Abstract]
NET LOSS PER SHARE	13. NET LOSS PER SHARE The Company’s potential dilutive securities, which include unvested common stock from the early-exercise of stock options and outstanding common stock options, 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, presented based on amounts outstanding at December 31, 2019 and 2018, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of December 31, 2019 2018 Unvested common stock from early exercise of options 21,666 151,289 Stock options to purchase common stock 4,843,018 3,203,122 Total 4,864,684 3,354,411

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
Defined Contribution Plan	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
DEFINED CONTRIBUTION PLAN	14. DEFINED CONTRIBUTION PLAN The Company has a 401(k) defined contribution plan (the “401(k) Plan”) for all of its employees. Eligible employees may make pretax contributions to the 401(k) Plan up to statutory limits, while the Company contributes to the plan at the discretion of the Board of Directors. The Company’s discretionary match made under the 401(k) Plan for the years ended December 31, 2019 and 2018 was $0.5 million and $0.3 million, respectively.

Collaboration and License Agree

Collaboration and License Agreement	12 Months Ended
Collaboration and License Agreement	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AND LICENSE AGREEMENT	Error extracting Word content

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation —The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business .
Principles of Consolidation	Principles of Consolidation —The Company’s consolidated financial statements include the accounts of the Company and its subsidiary, Homology Medicines Securities Corporation, a wholly owned Massachusetts corporation, for the sole purpose of buying, selling, and holding securities on the Company’s behalf. All intercompany balances and transactions have been eliminated in the consolidated financial statements.
Use of Estimates	Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue, and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, accrued research and development expenses and useful lives assigned to property and equipment. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Comprehensive Income (Loss)	Comprehensive Income (Loss) — Comprehensive income (loss) is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. The Company’s only element of other comprehensive income (loss) is unrealized gains and losses on available-for-sale investments.
Cash and Cash Equivalents and Restricted Cash	Cash and Cash Equivalents and Restricted Cash —Cash and cash equivalents consist of standard checking accounts, money market accounts and certain investments. The Company considers all highly liquid investments with original or remaining maturities at the time of purchase of 90 days or less to be cash equivalents. Restricted cash consists of cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the consolidated statements of cash flows: December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 53,774 $ 38,220 Restricted cash 1,274 1,772 Total cash, cash equivalents and restricted cash $ 55,048 $ 39,992
Short-Term Investments	Short-Term Investments —Short-term investments represent holdings of available-for-sale marketable securities in accordance with the Company’s investment policy and cash management strategy. Short-term investments mature within one-year from the balance sheet date. Investments in marketable securities are recorded at fair value, with any unrealized gains and losses, reported within accumulated other comprehensive income as a separate component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion, together with interest on securities, are included in interest income in the Company’s consolidated statements of operations. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of other income.
Concentration of Credit Risk	Concentrations of Credit Risk —Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents and short-term investments. Periodically, the Company may maintain deposits in financial institutions in excess of government insured limits. We believe that we are not exposed to significant credit risk as our deposits are held at financial institutions that management believes to be of high credit quality and the Company has not experienced any losses on these deposits. We regularly invest excess cash with major financial institutions in money market funds, U.S. government and corporate debt securities and commercial paper, all of which can be readily purchased and sold using established markets. As of December 31, 2019, the Company’s cash and cash equivalents were held with two financial institutions. We believe that the market risk arising from our holdings of these financial instruments is mitigated based on the fact that many of these securities are either government backed or of high credit rating.
Deferred Offering Costs	Deferred Offering Costs —The Company capitalizes incremental legal, professional accounting and other third-party fees that are directly associated with equity financings as other current assets until the transactions are completed. After equity financings are complete, these costs are recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering.
Guarantees and Indemnifications	Guarantees and Indemnifications —As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2019, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related liabilities have been established.
Property and Equipment	Property and Equipment —Property and equipment are recorded at cost. Expenditures for repairs and maintenance are expensed as incurred. When assets are retired or disposed of, the assets and related accumulated depreciation are derecognized from the accounts, and any resulting gain or loss is included in the determination of net loss. Depreciation is provided using the straight-line method over the estimated useful lives of the related assets. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset. Computer equipment and software 3 years Laboratory equipment and office furniture 5 years Manufacturing equipment 5 - 7 years Leasehold improvements Shorter of the lease term or estimated useful life
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets —The Company evaluates its long-lived assets, which consist primarily of property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to the future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the asset exceeds the fair value of the asset. To date, no impairments have been recognized for these assets.
Research and Development Costs	Research and Development Costs —Research and development costs are charged to expense as incurred. Research and development expense consists of expenses incurred in performing research and development activities, including salaries and benefits, materials and supplies, preclinical and clinical expenses, stock-based compensation expense, depreciation of equipment, contract services, and other outside expenses. Costs for certain development activities are recognized based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its vendors on their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense.
Income Taxes	Income Taxes —The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the Company’s consolidated financial statements and tax returns. Deferred tax assets and liabilities are determined based upon the differences between the financial statement carrying amounts and the tax bases of existing assets and liabilities and for loss and credit carryforwards, using enacted tax rates expected to be in effect in the year in which the differences are expected to reverse. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that these assets may not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes.
Segment Information	Segment Information —Operating segments are identified as components of an enterprise about which separate discrete financial information is made available for evaluation by the chief operating decision maker (“CODM”) in making decisions regarding resource allocation and assessing performance. The CODM is the Company’s Chief Executive Officer. The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is dedicated to translating proprietary gene editing and gene therapy technology into novel treatments for patients with rare genetic diseases. All of the Company’s tangible assets are held in the United States.
Revenue Recognition	Revenue Recognition— In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue (Topic 606): Revenue from Contracts with Customers (“ASU 2014-09”), which amends the guidance for accounting for revenue from contracts with customers. ASU 2014-09 supersedes the revenue recognition requirements in FASB ASC Topic 605, Revenue Recognition (“ASC 605”), and creates a new Topic 606, Revenue from Contracts with Customers (“ASC 606”). On January 1, 2019, the Company adopted ASC 606 using the full retrospective transition method. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangements would likely consist of a license, rights to the Company’s intellectual property or research, development and manufacturing services. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of consideration to which the Company expects to be entitled to. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts may include development and regulatory milestone payments that are assessed under the most likely amount method and constrained until it is probable that a significant revenue reversal would not occur. Milestone payments that are not within the Company’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development and regulatory milestones and any related constraint, and if necessary, adjust its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenue in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from the Company’s collaboration arrangement. The Company allocates the transaction price based on the estimated standalone selling price of each performance obligation. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress for its over-time arrangements at each reporting period and, if necessary, updates the measure of progress and revenue recognized. The Company adopted ASC 606 effective January 1, 2019, using the full retrospective transition method. Under this method, the Company revised its consolidated financial statements for prior period amounts as if ASC 606 had been effective for such periods. The references "as revised" used herein refer to revisions of data for the year ended December 31, 2018 as a result of the adoption of ASC 606. The adoption of ASC 606 did not have an impact on periods prior to January 1, 2018. In addition to revisions to financial statement amounts and related tables, Note 15 has been revised to reflect the adoption of ASC 606. As part of the adoption, the Company reviewed its collaboration and license agreement with Novartis to determine the cumulative effect impact related to the adoption of ASC 606. (For a complete discussion of the accounting for the Company’s agreement with Novartis, see Note 15.) The adoption of ASC 606 resulted in a change to the pattern of revenue recognition whereby the Company expects to recognize revenue from its collaboration agreement with Novartis as costs are incurred, which likely will not occur evenly over the performance period as a result of applying the cost-to-cost method, in contrast to recognizing revenue on a straight-line basis over the estimated performance period under ASC 605. The impact on previously reported amounts as a result of the adoption of ASC 606 is as follows: Consolidated Balance Sheets December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Current portion of deferred revenue $ 770 $ 3,684 $ (2,914 ) Long-term portion of deferred revenue $ 30,750 $ 29,474 $ 1,276 Accumulated deficit $ (95,758 ) $ (97,396 ) $ 1,638 Consolidated Statements of Operations and Comprehensive Loss For the Year Ended December 31, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 5,322 $ 3,684 $ 1,638 Loss from operations $ (59,926 ) $ (61,564 ) $ 1,638 Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Net loss per share attributable to common stockholders-basic and diluted $ (1.95 ) $ (2.00 ) $ 0.05 Consolidated Statement of Cash Flows For the Year Ended December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (1,890 ) $ (252 ) $ (1,638 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 39,992 $ 39,992 $ —
Stock Based Compensation	Stock-based Compensation —The Company recognizes compensation expense for awards to employees based on the grant date fair value of stock-based awards on a straight-line basis over the period during which an award holder provides service in exchange for the award. The fair value of options on the date of grant is calculated using the Black-Scholes option pricing model based on key assumptions such as stock price, expected volatility and expected term. The Company’s estimates of these assumptions are primarily based on the trading price of the Company’s stock, historical data, peer company data and judgment regarding future trends and factors. Stock-based awards granted to nonemployees are initially recorded at fair value and are re-measured at each reporting period as the awards vest and expense is recognized over the period the services are provided. The purchase price of common stock under the Company’s employee stock purchase plan (“ESPP”) is equal to 85% of the lesser of (i) the fair market value per share of the common stock on the first business day of an offering period and (ii) the fair market value per share of the common stock on the purchase date. The fair value of the look-back provision under the ESPP is calculated using the Black-Scholes option pricing model. The fair value of the look-back provision plus the 15% discount is recognized as compensation expense over the 180-day purchase period.
Fair Value Measurements	Fair Value Measurements —Certain assets and liabilities are reported on a recurring basis at fair value. 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 (unadjusted) 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.
Net Loss per Share	Net Loss per Share —Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period. Diluted net loss per share is computed using the weighted-average number of common shares outstanding during the period and, if dilutive, the weighted-average number of potential shares of common stock. The weighted-average number of common shares included in the computation of diluted net loss gives effect to all potentially dilutive common equivalent shares, including outstanding stock options and unvested shares of common stock. Common stock equivalent shares are excluded from the computation of diluted net loss per share if their effect is antidilutive. In periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is generally the same as basic net loss per share attributable to common stockholders since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements —The Jumpstart Our Business Startups Act of 2012 permits an emerging growth company to take advantage of an extended transition period to comply with new or revised accounting standards applicable to public companies until those standards would otherwise apply to private companies. As an emerging growth company, the Company has elected to take advantage of this extended transition period. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) , The Company adopted this standard early on January 1, 2020. The Company expects to elect the package of practical expedients permitted under the transition guidance within the new standard, which, among other things, allows the Company to carry forward historical lease classification. The Company has substantially completed its evaluation of the impact of the adoption of ASU 2016-02 on its consolidated financial statements and upon adoption, expects to recognize a lease liability and related right-of-use asset on its consolidated balance sheet of approximately $17.0 million to $18.0 million and approximately $6.0 million to $7.0 million, respectively, and a reduction to deferred rent of approximately $10.0 million to $11.0 million to eliminate the deferred rent balance. The Company does not expect the standard to have a material impact on its results of operations or cash flows. In addition, the Company is currently implementing changes to processes and controls to support lease accounting and related disclosures under the new standard. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In December 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force) , In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting ,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
Summary of Significant Accounting Policies (Tables)	Dec. 31, 2019
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash to amounts shown in the consolidated statements of cash flows: December 31, 2019 2018 (in thousands) Cash and cash equivalents $ 53,774 $ 38,220 Restricted cash 1,274 1,772 Total cash, cash equivalents and restricted cash $ 55,048 $ 39,992
Schedule of Estimated Useful Life of Asset	Computer equipment and software 3 years Laboratory equipment and office furniture 5 years Manufacturing equipment 5 - 7 years Leasehold improvements Shorter of the lease term or estimated useful life
ASC 606
Impact of Adoption of ASC 606 on Previously Reported Amounts of Financial Statement	As part of the adoption, the Company reviewed its collaboration and license agreement with Novartis to determine the cumulative effect impact related to the adoption of ASC 606. (For a complete discussion of the accounting for the Company’s agreement with Novartis, see Note 15.) The adoption of ASC 606 resulted in a change to the pattern of revenue recognition whereby the Company expects to recognize revenue from its collaboration agreement with Novartis as costs are incurred, which likely will not occur evenly over the performance period as a result of applying the cost-to-cost method, in contrast to recognizing revenue on a straight-line basis over the estimated performance period under ASC 605. The impact on previously reported amounts as a result of the adoption of ASC 606 is as follows: Consolidated Balance Sheets December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Current portion of deferred revenue $ 770 $ 3,684 $ (2,914 ) Long-term portion of deferred revenue $ 30,750 $ 29,474 $ 1,276 Accumulated deficit $ (95,758 ) $ (97,396 ) $ 1,638 Consolidated Statements of Operations and Comprehensive Loss For the Year Ended December 31, 2018 (in thousands, except per share amounts) As revised under ASC 606 As originally reported under ASC 605 Effect of change Collaboration revenue $ 5,322 $ 3,684 $ 1,638 Loss from operations $ (59,926 ) $ (61,564 ) $ 1,638 Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Net loss per share attributable to common stockholders-basic and diluted $ (1.95 ) $ (2.00 ) $ 0.05 Consolidated Statement of Cash Flows For the Year Ended December 31, 2018 (in thousands) As revised under ASC 606 As originally reported under ASC 605 Effect of change Net loss $ (55,577 ) $ (57,215 ) $ 1,638 Adjustments to reconcile net loss to net cash used in operating activities: Deferred revenue $ (1,890 ) $ (252 ) $ (1,638 ) Cash, cash equivalents and restricted cash, beginning of period $ 53,347 $ 53,347 $ — Cash, cash equivalents and restricted cash, end of period $ 39,992 $ 39,992 $ —

Cash and Cash Equivalents (Tabl

Cash and Cash Equivalents (Tables)	12 Months Ended
Cash and Cash Equivalents (Tables)	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Summary of Cash and Cash Equivalents	The following table summarizes the Company’s cash and cash equivalents: December 31, 2019 2018 (in thousands) Cash $ 250 $ 471 U.S. Treasury securities — 35 Commercial paper — 5,974 Repurchase agreements 14,000 — Money market funds 39,524 31,740 Total cash and cash equivalents $ 53,774 $ 38,220

Short-Term Investments (Tables)

Short-Term Investments (Tables)	12 Months Ended
Short-Term Investments (Tables)	Dec. 31, 2019
Investments Disclosure [Abstract]
Summary of Investments Included in Cash Equivalents and Short Term Investments	The following table summarizes the Company’s investments, which are included in cash equivalents and short-term investments: As of December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Repurchase agreements $ 14,000 $ — $ — 14,000 Asset-backed securities 14,866 10 — 14,876 Money market mutual funds 39,524 — — 39,524 Commercial paper 41,259 — — 41,259 U.S. Treasury securities 59,926 45 — 59,971 Corporate debt securities 92,380 128 — 92,508 Total $ 261,955 $ 183 $ — $ 262,138 As of December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Asset-backed securities $ 19,541 $ — $ (21 ) $ 19,520 U.S. Treasury securities 29,980 — (6 ) 29,974 Money market mutual funds 31,740 — — 31,740 Corporate debt securities 54,511 — (50 ) 54,461 Commercial paper 78,571 — — 78,571 Total $ 214,343 $ — $ (77 ) $ 214,266

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
Fair Value Measurements (Tables)	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	Assets measured at fair value on a recurring basis were as follows: Description December 31, 2019 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 39,524 $ 39,524 $ — $ — Repurchase agreements 14,000 — 14,000 — Total cash equivalents $ 53,524 $ 39,524 $ 14,000 $ — Short-term investments: Asset-backed securities $ 14,876 $ — $ 14,876 $ — Commercial paper 41,259 — 41,259 — U.S. Treasury securities 59,971 — 59,971 — Corporate debt securities 92,508 — 92,508 — Total short-term investments $ 208,614 $ — $ 208,614 $ — Total financial assets $ 262,138 $ 39,524 $ 222,614 $ — Description December 31, 2018 Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Money market mutual funds $ 31,740 $ 31,740 $ — $ — Commercial paper 5,974 — 5,974 — U.S. Treasury securities 35 35 — — Total cash equivalents $ 37,749 $ 31,775 $ 5,974 $ — Short-term investments: Asset-backed securities $ 19,520 $ — $ 19,520 $ — U.S. Treasury securities 29,939 — 29,939 — Corporate debt securities 54,461 — 54,461 — Commercial paper 72,597 — 72,597 — Total short-term investments $ 176,517 $ — $ 176,517 $ — Total financial assets $ 214,266 $ 31,775 $ 182,491 $ —

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
Property and Equipment (Tables)	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2019 2018 (in thousands) Laboratory equipment $ 10,837 $ 9,309 Manufacturing equipment 4,911 — Computers and purchased software 640 248 Furniture and fixtures 1,363 1,066 Leasehold improvements 30,862 24,520 Assets not yet in service 2,513 2,602 Property and equipment, at cost 51,126 37,745 Less accumulated depreciation and amortization (8,410 ) (2,108 ) Property and equipment, net $ 42,716 $ 35,637

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
Accrued Expenses and Other Liabilities (Tables)	Dec. 31, 2019
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consist of the following: December 31, 2019 2018 (in thousands) Accrued compensation and benefits $ 4,551 $ 3,370 Accrued research and development expenses 2,258 1,059 Accrued professional fees 542 195 Accrued unvested common stock subject to repurchase 37 112 Accrued other 256 304 Total accrued expenses and other liabilities $ 7,644 $ 5,040

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
Commitments and Contingencies (Tables)	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments Net of Anticipated Sublease Payments	Future minimum lease payments, net of anticipated sublease payments, as of December 31, 2019, are as follows: Amount For the Years Ending December 31, (in thousands) 2020 $ 2,810 2021 2,815 2022 2,987 2023 3,077 2024 3,169 Thereafter 7,197 Total future minimum lease payments $ 22,055

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Reconciliation between U.S. Federal Statutory Tax Rate and Effective Tax Rate	A reconciliation between the U.S. federal statutory tax rate and the Company’s effective tax rate is summarized as follows: For the Years Ended December 31, 2019 2018 Federal statutory rate 21.0 % 21.0 % Tax credits 13.4 % 17.7 % State taxes, net of federal tax benefit 6.1 % 5.5 % Non-deductible expenses (1.0 %) (0.3 %) Change in valuation allowance (39.5 %) (43.9 %) Effective income tax rate — % — %
Schedule of Deferred Tax Assets and Liabilities	The principal components of the Company’s deferred tax assets and liabilities consist of the following: December 31, 2019 2018 (in thousands) Deferred tax assets: Net operating losses $ 41,589 $ 14,848 R&D credits 25,328 12,506 Deferred revenue 8,456 8,159 Deferred rent 2,966 2,854 Capitalized R&D costs 1,207 1,375 Equity compensation 883 443 Accrued expense and other 108 115 Total deferred tax assets 80,537 40,300 Deferred tax liabilities: Depreciation (2,744 ) (2,680 ) Total deferred tax liabilities (2,744 ) (2,680 ) Valuation allowance (77,793 ) (37,620 ) Net deferred taxes $ — $ —

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	12 Months Ended
Stock Incentive Plans (Tables)	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options and shares purchased under the 2018 ESPP as follows: For the Years Ended December 31, 2019 2018 (in thousands) Research and development $ 4,164 $ 1,117 General and administrative 3,471 1,506 $ 7,635 $ 2,623
Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model	The assumptions used in the Black-Scholes option pricing model are as follows: For the Years Ended December 31, 2019 2018 Expected volatility 62.4% — 64.7% 52.80% — 60.12% Weighted-average risk-free interest rate 1.36% — 2.60% 2.33% — 3.08% Expected dividend yield — % — % Expected term (in years) 6.25 5.5 — 7.6 Underlying common stock fair value $12.73 — $30.34 $6.63 — $24.28
Summary of Option Activity under Plans	A summary of option activity under the Plans during the year ended December 31, 2019 is as follows: Number of Options Weighted- Average Exercise Price per Share Weighted- Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value (in thousands) Outstanding as of January 1, 2019 3,203,122 $ 10.47 8.9 $ 39,018 Granted 1,898,215 $ 21.60 Exercised (114,277 ) $ 3.07 Cancelled/Forfeited (144,042 ) $ 18.06 Outstanding at December 31, 2019 4,843,018 $ 14.78 8.5 $ 33,809 Vested and expected to vest at December 31, 2019 4,843,018 $ 14.78 8.5 $ 33,809 Exercisable at December 31, 2019 1,593,532 $ 8.72 7.6 $ 20,021
Summary of Unvested Common Stock from Early Exercises Subject to Repurchase	A summary of the Company’s unvested common stock from early exercises that is subject to repurchase by the Company is as follows: Shares Unvested shares—January 1, 2019 151,289 Vested (94,816 ) Issued — Repurchased (34,807 ) Unvested shares—December 31, 2019 21,666

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
Net Loss Per Share (Tables)	Dec. 31, 2019
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Effect Excluded from Computation of Diluted Net Loss Per Share Attributable to Common Stockholders	The Company excluded the following potential common shares, presented based on amounts outstanding at December 31, 2019 and 2018, from the computation of diluted net loss per share attributable to common stockholders because including them would have had an anti-dilutive effect: As of December 31, 2019 2018 Unvested common stock from early exercise of options 21,666 151,289 Stock options to purchase common stock 4,843,018 3,203,122 Total 4,864,684 3,354,411

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 26, 2019	Apr. 12, 2019	Apr. 01, 2019	Apr. 02, 2018	Dec. 31, 2019	Dec. 31, 2018	Apr. 04, 2018
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses						$ 150,843
Preferred stock, shares outstanding					0	0
Conversion of convertible preferred stock into common stock upon initial public offering, Shares				24,168,656
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs					$ 134,524
Net loss					103,916	$ 55,577
Accumulated deficit					$ 199,674	$ 95,758
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Conversion of convertible preferred stock into common stock upon initial public offering, Shares						24,168,656
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold				10,350,000
Common stock at a public offering price							$ 16
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses				$ 150,800
Preferred stock, shares outstanding				0
Initial Public Offering \| Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold						10,350,000
Common Stock, Preferred Stock, Debt Securities, Warrants and Units \| Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 350,000
ATM
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold					1,105,000
Common stock available for sale					76,800,000
ATM \| Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold					1,105,000
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs					$ 22,500
ATM \| Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 100,000
Follow On Offering \| Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold		5,555,556			6,388,889
Shares issued price per share		$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses		$ 116,900
Underwriters Option \| Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold	833,333
Shares issued price per share	$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses	$ 17,600

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 53,774	$ 38,220
Restricted cash	1,274	1,772
Total cash, cash equivalents and restricted cash	$ 55,048	$ 39,992

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Jan. 01, 2020	Sep. 01, 2018	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Outstanding claim			$ 0
Impairment of long-lived assets			$ 0
Tax positions, description			The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency.
Reduction to deferred rent			$ (410)	$ (10,033)
ASU 2016-02 \| Subsequent Event \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Lease liability	$ 17,000
Right-of-use asset	6,000
Reduction to deferred rent	(10,000)
ASU 2016-02 \| Subsequent Event \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Lease liability	18,000
Right-of-use asset	7,000
Reduction to deferred rent	$ (11,000)
Employee Stock Purchase Plan
Summary Of Significant Accounting Policies [Line Items]
Purchase of common stock through payroll deductions expressed in percentage of fair market value		85.00%	85.00%
Share based compensation, discount from market price			15.00%
Share based compensation arrangement by share based payment purchase period			180 days

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Estimated Useful Life of Asset (Details)	12 Months Ended
	Dec. 31, 2019
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Estimated useful life of asset	3 years
Laboratory Equipment and Office Furniture
Property Plant And Equipment [Line Items]
Estimated useful life of asset	5 years
Manufacturing Equipment \| Minimum
Property Plant And Equipment [Line Items]
Estimated useful life of asset	5 years
Manufacturing Equipment \| Maximum
Property Plant And Equipment [Line Items]
Estimated useful life of asset	7 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Estimated useful life of asset, term	Shorter of the lease term or estimated useful life

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current portion of deferred revenue	$ 809	$ 770
Long-term portion of deferred revenue	30,142	30,750
Accumulated deficit	$ (199,674)	(95,758)
ASC 606 \| As Originally Reported Under ASC 605
Current portion of deferred revenue		3,684
Long-term portion of deferred revenue		29,474
Accumulated deficit		(97,396)
ASC 606 \| Effect of Change
Current portion of deferred revenue		(2,914)
Long-term portion of deferred revenue		1,276
Accumulated deficit		$ 1,638

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Collaboration revenue	$ 1,666	$ 5,322
Loss from operations	(109,943)	(59,926)
Net loss	$ (103,916)	$ (55,577)
Net loss per share attributable to common stockholders-basic and diluted	$ (2.47)	$ (1.95)
ASC 606 \| As Originally Reported Under ASC 605
Collaboration revenue		$ 3,684
Loss from operations		(61,564)
Net loss		$ (57,215)
Net loss per share attributable to common stockholders-basic and diluted		$ (2)
ASC 606 \| Effect of Change
Collaboration revenue		$ 1,638
Loss from operations		1,638
Net loss		$ 1,638
Net loss per share attributable to common stockholders-basic and diluted		$ 0.05

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Impact of Adoption of ASC 606 on Previously Reported Amounts of Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Net loss	$ (103,916)	$ (55,577)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue	(569)	(1,890)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	55,048	39,992
ASC 606 \| As Originally Reported Under ASC 605
Net loss		(57,215)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		(252)
Cash, cash equivalents and restricted cash, beginning of period	$ 39,992	53,347
Cash, cash equivalents and restricted cash, end of period		39,992
ASC 606 \| Effect of Change
Net loss		1,638
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		$ (1,638)

Summary of Cash and Cash Equiva

Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 53,774	$ 38,220
Cash
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	250	471
U.S. Treasury securities
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents		35
Commercial Paper
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents		5,974
Money Market Funds
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	39,524	$ 31,740
Repurchase Agreements
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 14,000

Short-Term Investments - Summar

Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 261,955	$ 214,343
Cash equivalents and short-term investments, Unrealized Gains	183
Cash equivalents and short-term investments, Unrealized Losses	0	(77)
Cash equivalents and short-term investments, Fair Value	262,138	214,266
Money Market Mutual Funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	39,524	31,740
Cash equivalents and short-term investments, Fair Value	39,524	31,740
Repurchase Agreements
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	14,000
Cash equivalents and short-term investments, Fair Value	14,000
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	14,866	19,541
Cash equivalents and short-term investments, Unrealized Gains	10
Cash equivalents and short-term investments, Unrealized Losses		(21)
Cash equivalents and short-term investments, Fair Value	14,876	19,520
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	41,259	78,571
Cash equivalents and short-term investments, Fair Value	41,259	78,571
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	92,380	54,511
Cash equivalents and short-term investments, Unrealized Gains	128
Cash equivalents and short-term investments, Unrealized Losses		(50)
Cash equivalents and short-term investments, Fair Value	92,508	54,461
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	59,926	29,980
Cash equivalents and short-term investments, Unrealized Gains	45
Cash equivalents and short-term investments, Unrealized Losses		(6)
Cash equivalents and short-term investments, Fair Value	$ 59,971	$ 29,974

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash Cash Equivalents And Available For Sale Securities [Abstract]
Unrealized losss position on securities	$ 0	$ 77,000
Realized gains and losses on available-for-sale securities	$ 0	$ 0
Contractual maturity date of investments	Less than one year

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 208,614	$ 176,517
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	53,524	37,749
Short-term investments, fair value	208,614	176,517
Financial assets, fair value	262,138	214,266
Fair Value, Measurements, Recurring \| Money Market Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	39,524	31,740
Fair Value, Measurements, Recurring \| U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		35
Short-term investments, fair value	59,971	29,939
Fair Value, Measurements, Recurring \| Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	14,000
Fair Value, Measurements, Recurring \| Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	14,876	19,520
Fair Value, Measurements, Recurring \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		5,974
Short-term investments, fair value	41,259	72,597
Fair Value, Measurements, Recurring \| Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	92,508	54,461
Fair Value, Measurements, Recurring \| Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	39,524	31,775
Financial assets, fair value	39,524	31,775
Fair Value, Measurements, Recurring \| Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) \| Money Market Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	39,524	31,740
Fair Value, Measurements, Recurring \| Quoted Prices (Unadjusted) in Active Markets for Identical Assets (Level 1) \| U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		35
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	14,000	5,974
Short-term investments, fair value	208,614	176,517
Financial assets, fair value	222,614	182,491
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	59,971	29,939
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	14,000
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	14,876	19,520
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value		5,974
Short-term investments, fair value	41,259	72,597
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 92,508	$ 54,461

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Transfers between fair value measure levels	$ 0	$ 0

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 51,126	$ 37,745
Less accumulated depreciation and amortization	(8,410)	(2,108)
Property and equipment, net	42,716	35,637
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	10,837	9,309
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	4,911
Computers and Purchased Software
Property Plant And Equipment [Line Items]
Property and equipment, at cost	640	248
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, at cost	1,363	1,066
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, at cost	30,862	24,520
Assets Not Yet in Service
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 2,513	$ 2,602

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation	$ 6,318	$ 1,289
Disposal of property and equipment	$ 13,000	$ 100

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued compensation and benefits	$ 4,551	$ 3,370
Accrued research and development expenses	2,258	1,059
Accrued professional fees	542	195
Accrued unvested common stock subject to repurchase	37	112
Accrued other	256	304
Total accrued expenses and other liabilities	$ 7,644	$ 5,040

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended		12 Months Ended
	Dec. 31, 2017USD ($)ft²	Sep. 30, 2016	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Commitment And Contingencies [Line Items]
Deferred rent incentives			$ 8,300,000
Rent expense, net			1,400,000	$ 2,700,000
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit			1,300,000	1,800,000
Maximum
Commitment And Contingencies [Line Items]
Tenant improvement allowance			$ 10,900,000
Bedford, Massachusetts
Commitment And Contingencies [Line Items]
Operating lease beginning year and month		2016-11
Operating lease expiration year and month	2027-02	2021-10
Operating lease agreements additional term		3 years
Sublease aggregate base rent obligation				$ 2,700,000
Office space leased \| ft²	67,000
Lessee, operating lease, lease not yet commenced, renewal term	5 years
Initial annual base rent per square foot	$ 39.50
Percentage increase in initial annual base rent per square foot.	3.00%

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 2,810
2021	2,815
2022	2,987
2023	3,077
2024	3,169
Thereafter	7,197
Total future minimum lease payments	$ 22,055

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Sep. 30, 2016	Apr. 30, 2016	Dec. 31, 2019	Dec. 31, 2018
License Agreement [Line Items]
Research and development			$ 89,398,000	$ 47,948,000
City Of Hope
License Agreement [Line Items]
Annual license fee		$ 25,000
Payment for execution of agreements			50,000,000
City Of Hope \| Sponsored Research Agreement
License Agreement [Line Items]
Research and development			$ 100,000	$ 100,000
City Of Hope \| Maximum
License Agreement [Line Items]
Development and commercialization milestone payment		$ 3,200,000
California Institute Of Technology \| Maximum \| Co Exclusive License Agreement
License Agreement [Line Items]
Development and regulatory milestone payments	$ 7,200,000
California Institute Of Technology \| Minimum \| Co Exclusive License Agreement
License Agreement [Line Items]
Annual minimal royalty fee	$ 20,000

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation between U.S. Federal Statutory Tax Rate and Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	21.00%
Tax credits	13.40%	17.70%
State taxes, net of federal tax benefit	6.10%	5.50%
Non-deductible expenses	(1.00%)	(0.30%)
Change in valuation allowance	(39.50%)	(43.90%)

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating losses	$ 41,589	$ 14,848
R&D credits	25,328	12,506
Deferred revenue	8,456	8,159
Deferred rent	2,966	2,854
Capitalized R&D costs	1,207	1,375
Equity compensation	883	443
Accrued expense and other	108	115
Total deferred tax assets	80,537	40,300
Deferred tax liabilities:
Depreciation	(2,744)	(2,680)
Total deferred tax liabilities	(2,744)	(2,680)
Valuation allowance	$ (77,793)	$ (37,620)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
Income Taxes - Additional Information (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Income tax expense	$ 0	$ 0
Increase in valuation allowance due to deferred tax assets	40,200,000	25,100,000
Uncertain tax positions	0
Income tax expense, interest or penalties	0	$ 0
Orphan Drug
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 17,300,000
Tax credit carryforwards expiration year	2039
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 150,700,000
Net operating loss carryforwards indefinitely	119,600,000
Federal \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards	21,200,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	157,300,000
State \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 5,200,000
Federal and State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards expiration year	2039
Federal and State \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards expiration year	2039

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Common stock, voting rights	Each holder of outstanding shares of common stock are entitled to one vote in respect of each share.
Common stock, shares outstanding	45,116,742	37,358,526
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2020	Jan. 01, 2019	Sep. 01, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate proceeds from shares issued under the plan					$ 829
Stock-based compensation					7,635	$ 2,623
Total intrinsic value of options exercised					$ 1,900	$ 1,800
Weighted-average grant date fair value per share for options granted					$ 12.90	$ 10.41
Remaining shares subject to repurchase					21,666	151,289
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities					$ 37	$ 112
Accrued Expenses and Other Liabilities
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities					$ 100	100
2015 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting period					4 years
Stock options expiration period					10 years
Number of additional shares available for future grant					0
2018 Incentive Award Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance				3,186,205
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock				4.00%
Maximum shares of common stock may be issued				20,887,347
Number of shares outstanding available for future grant					1,637,094
2018 Incentive Award Plan \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	4.00%
Number of additional shares added to the plan	1,804,670
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance				353,980
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock		1.00%		1.00%
Number of shares outstanding available for future grant					672,331
Maximum shares allowed to be issued under ESPP				4,778,738
Purchase of common stock through payroll deductions expressed in percentage of fair market value			85.00%		85.00%
Common stock offering period			6 months
Number of shares issued to the plan					55,234
Aggregate proceeds from shares issued under the plan					$ 800	0
Stock-based compensation					200	$ 100
2018 Employee Stock Purchase Plan \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of additional shares added to the plan	451,167
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense					$ 31,500
Unrecognized compensation expense estimated to be recognized over period					2 years 10 months 24 days

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 7,635	$ 2,623
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	4,164	1,117
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 3,471	$ 1,506

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	62.40%	52.80%
Expected volatility, maximum	64.70%	60.12%
Weighted-average risk-free interest rate, minimum	1.36%	2.33%
Weighted-average risk-free interest rate, maximum	2.60%	3.08%
Expected term (in years)	6 years 3 months
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		5 years 6 months
Underlying common stock fair value	$ 12.73	$ 6.63
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		7 years 7 months 6 days
Underlying common stock fair value	$ 30.34	$ 24.28

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Option Activity under Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Options, Outstanding at Beginning Balance	3,203,122
Number of Options, Granted	1,898,215
Number of Options, Exercised	(114,277)
Number of Options, Cancelled/Forfeited	(144,042)
Number of Options, Outstanding at Ending Balance	4,843,018	3,203,122
Number of Options, Vested and Expected to vest at December 31, 2019	4,843,018
Number of Options, Exercisable at December 31, 2019	1,593,532
Weighted-Average Exercise Price per Share, Outstanding at Beginning Balance	$ 10.47
Weighted-Average Exercise Price per Share, Granted	21.60
Weighted-Average Exercise Price per Share, Exercised	3.07
Weighted-Average Exercise Price per Share, Cancelled/Forfeited	18.06
Weighted-Average Exercise Price per Share, Outstanding at Ending Balance	14.78	$ 10.47
Weighted-Average Exercise Price per Share, Vested and Expected to vest at December 31, 2019	14.78
Weighted-Average Exercise Price Per Share, Exercisable at December 31, 2019	$ 8.72
Weighted Average Remaining Contractual Term, Outstanding	8 years 6 months	8 years 10 months 24 days
Weighted Average Remaining Contractual Term, Vested and Expected to vest at December 31, 2019	8 years 6 months
Weighted Average Remaining Contractual Term, Exercisable at December 31, 2019	7 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding Beginning Balance	$ 33,809	$ 39,018
Aggregate Intrinsic Value, Vested and Expected to vest at December 31, 2019	33,809
Aggregate Intrinsic Value, Exercisable at December 31, 2019	$ 20,021

Stock Incentive Plans - Summa_2

Stock Incentive Plans - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	12 Months Ended
	Dec. 31, 2019shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares—Beginning Balance	151,289
Vested	(94,816)
Repurchased	(34,807)
Unvested shares—Ending Balance	21,666

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Anti-dilutive Effect Excluded from Computation of Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	4,864,684	3,354,411
Unvested Common Stock from Early Exercise of Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	21,666	151,289
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	4,843,018	3,203,122

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Discretionary match made under the 401(k) Plan by employer	$ 0.5	$ 0.3

Collaboration and License Agr_2

Collaboration and License Agreement - Additional Information (Details)	1 Months Ended	12 Months Ended
	Nov. 30, 2017USD ($)TargetCandidateorProduct	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Jan. 01, 2019USD ($)	Jan. 01, 2018USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		$ 1,666,000	$ 5,322,000
Novartis
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received	$ 35,000,000
Allocated to collaboration agreement	33,300,000
Collaborative agreement target fee	$ 5,000,000
Collaboration agreement, research term	5 years
Novartis \| Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement revenue recognized		1,700,000	5,300,000
Deferred revenue		31,000,000	31,500,000	$ 600,000	$ 1,800,000
Novartis \| Collaborative Arrangement \| Reimbursable Research and Development Costs
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable		400,000	$ 800,000
Novartis \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price		$ 33,300,000
Novartis \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected \| Target	2
Number of targets during research term \| Target	2
Collaboration agreement, reimbursement of manufacturing activities during research term	$ 17,000,000
Collaboration agreement number of candidates or products for develop and commercialize \| CandidateorProduct	2
Collaboration agreement eligible milestone payments eligible to receive	$ 530,000,000
Collaboration agreement eligible development milestone payments eligible to receive	180,000,000
Collaboration agreement eligible regulatory milestone payments eligible to receive	170,000,000
Collaboration agreement eligible commercial milestone payments eligible to receive	180,000,000
Novartis \| Series B Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received	1,700,000
Proceeds from issuance of shares of preferred stock	10,000,000
Preferred stock estimated fair value	$ 11,700,000

Collaboration and License Agr_3

Collaboration and License Agreement - Additional Information (Details1) - Novartis $ in Millions	Dec. 31, 2019USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2017-11-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation, unsatisfied portion	$ 31
Remaining performance obligation, expected timing of satisfaction, period	10 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2027-04-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining performance obligation, unsatisfied portion	$ 31
Remaining performance obligation, expected timing of satisfaction, period	10 years