Q32 Bio (QTTB) 10-K 12 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 07, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2018
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	Non-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		37,538,555
Entity Public Float			$ 365

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 38,220	$ 51,575
Short-term investments	176,517	78,084
Prepaid expenses and other current assets	6,948	1,945
Total current assets	221,685	131,604
Property and equipment, net	35,637	3,154
Deferred offering costs		1,000
Restricted cash	1,772	1,772
Total assets	259,094	137,530
Current liabilities:
Accounts payable	15,732	2,538
Accrued expenses and other liabilities	5,040	2,860
Deferred rent	977	123
Deferred revenue	3,684	3,341
Total current liabilities	25,433	8,862
Non-current liabilities:
Deferred rent, net of current portion	9,470	291
Deferred revenue, net of current portion	29,474	30,069
Total liabilities	64,377	39,222
Commitments and contingencies (Note 8)
Convertible preferred stock:
Total convertible preferred stock		137,762
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of December 31, 2018 and no shares authorized as of December 31, 2017; no shares issued and outstanding at December 31, 2018 and December 31, 2017
Common stock, $0.0001 par value; 200,000,000 and 170,000,000 shares authorized as of December 31, 2018 and 2017, respectively; 37,509,815 and 2,902,109 shares issued as of December 31, 2018 and 2017, respectively; and 37,358,526 and 2,637,011 shares outstanding as of December 31, 2018 and 2017, respectively	3
Additional paid-in capital	292,187	800
Accumulated other comprehensive loss	(77)	(73)
Accumulated deficit	(97,396)	(40,181)
Total stockholders’ equity (deficit)	194,717	(39,454)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 259,094	137,530
Series A Convertible Preferred Stock
Convertible preferred stock:
Total convertible preferred stock		42,994
Series B Convertible Preferred Stock
Convertible preferred stock:
Total convertible preferred stock		$ 94,768

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	170,000,000
Common stock, shares issued	37,509,815	2,902,109
Common stock, shares outstanding	37,358,526	2,637,011
Series A Convertible Preferred Stock
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, shares authorized		62,304,354
Convertible preferred stock, shares issued		62,269,144
Convertible preferred stock, shares outstanding		62,269,144
Convertible preferred stock, liquidation preference		$ 44,211
Series B Convertible Preferred Stock
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, shares authorized		64,930,561
Convertible preferred stock, shares issued		64,930,561
Convertible preferred stock, shares outstanding		64,930,561
Convertible preferred stock, liquidation preference		$ 93,500

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 3,684
Operating expenses:
Research and development	47,948	$ 21,378
General and administrative	17,300	8,279
Total operating expenses	65,248	29,657
Loss from operations	(61,564)	(29,657)
Other income (expense):
Changes in fair value of convertible preferred stock tranche liability		(876)
Interest income	4,349	542
Total other income (expense)	4,349	(334)
Net loss and net loss attributable to common stockholders-basic and diluted	$ (57,215)	$ (29,991)
Net loss per share attributable to common stockholders-basic and diluted	$ (2)	$ (12.10)
Weighted average common shares outstanding-basic and diluted	28,551,807	2,479,432

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (57,215)	$ (29,991)
Other comprehensive loss:
Change in unrealized gain (loss) on available for sale securities, net	(4)	(73)
Total other comprehensive loss	(4)	(73)
Comprehensive loss	$ (57,219)	$ (30,064)

CONSOLIDATED STATEMENTS OF CONV

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance balance at Dec. 31, 2016		$ 17,392
Beginning balance at Dec. 31, 2016	$ (9,892)				$ 298		$ (10,190)
Beginning balance, Shares at Dec. 31, 2016		33,395,907
Beginning balance, Shares at Dec. 31, 2016				2,121,156
Issuance of convertible preferred stock, net of issuance costs		$ 20,479	$ 93,101
Issuance of convertible preferred stock, net of issuance costs, Shares		28,873,237	64,930,561
Reclassification of tranche liability upon issuance of convertible preferred stock		$ 5,123
Allocation of collaboration proceeds to carrying value of Series B convertible preferred stock			$ 1,667
Vesting of common stock from option exercise	225				225
Vesting of common stock from option exercise, Shares				483,200
Vesting of founders restricted common stock	$ 21				21
Vesting of founders restricted common stock, Shares				32,655
Issuance of common stock from option exercises, Shares	0
Stock-based compensation	$ 256				256
Other comprehensive loss	(73)					$ (73)
Net loss	(29,991)						(29,991)
Ending balance at Dec. 31, 2017	(39,454)				800	(73)	(40,181)
Ending balance, Shares at Dec. 31, 2017		62,269,144	64,930,561
Ending balance at Dec. 31, 2017	137,762	$ 42,994	$ 94,768
Ending balance, Shares at Dec. 31, 2017				2,637,011
Conversion of convertible preferred stock into common stock upon initial public offering		$ (42,994)	$ (94,768)
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		(62,269,144)	(64,930,561)
Conversion of convertible preferred stock into common stock upon initial public offering	137,762			$ 2	137,760
Conversion of convertible preferred stock into common stock upon initial public offering, Shares				24,168,656
Issuance of common stock in initial public offering, net of discounts and issuance costs	150,843			$ 1	150,842
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares				10,350,000
Vesting of common stock from option exercise	75				75
Vesting of common stock from option exercise, Shares				129,959
Issuance of common stock from option exercises	$ 87				87
Issuance of common stock from option exercises, Shares	89,050			72,900
Stock-based compensation	$ 2,623				2,623
Other comprehensive loss	(4)					(4)
Net loss	(57,215)						(57,215)
Ending balance at Dec. 31, 2018	$ 194,717			$ 3	$ 292,187	$ (77)	$ (97,396)
Ending balance, Shares at Dec. 31, 2018				37,358,526

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (57,215)	$ (29,991)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation	1,289	684
Stock-based compensation expense	2,623	277
Accretion on short-term investments	(1,589)	(85)
Loss on disposal of property and equipment	127
Change in fair value associated with convertible preferred stock tranche liability		876
Changes in operating assets and liabilities:
Prepaid expense and other current assets	(5,003)	(1,463)
Accounts payable	4,383	1,720
Accrued expenses and other liabilities	3,042	864
Deferred revenue	(252)	33,411
Deferred rent	10,033	186
Net cash (used in) provided by operating activities	(42,562)	6,479
Cash flows from investing activities:
Purchases of short-term investments	(245,328)	(78,071)
Maturities of short-term investments	148,480
Purchases of property and equipment	(24,939)	(1,958)
Changes in restricted cash		(1,497)
Net cash used in investing activities	(121,787)	(81,526)
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 from option exercises	87
Proceeds from issuance of restricted common stock	64
Repurchase of unvested common stock		(17)
Net cash provided by financing activities	150,994	115,230
Net change in cash and cash equivalents	(13,355)	40,183
Cash and cash equivalents, beginning of period	51,575	11,392
Cash and cash equivalents, end of period	38,220	51,575
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	75	225
Property and equipment additions included in accounts payable	9,127	167
Deferred offering costs included in accrued expenses		1,000
Reclassification of tranche liability upon issuance of convertible preferred stock		5,123
Series A Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		20,479
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
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 94,768
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
	Dec. 31, 2018
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 pre-clinical stage biopharmaceutical company dedicated to translating proprietary gene editing and gene therapy 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 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. 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, 2018, the Company has financed its operations primarily through the On April 2, 2018, the Company completed an initial public offering (“IPO”) in which the Company issued and sold 10,350,000 shares of its common stock at a public offering price of $16.00 per share, for aggregate gross proceeds of $165.6 million before fees and expenses. In connection with the IPO, the Company effected a one-for-5.263 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the conversion ratios for the Company’s Series A and Series B preferred stock. Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. The reverse stock split became effective on March 16, 2018. Upon the closing of the IPO, all of the 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. Management believes that existing cash, cash equivalents and short-term investments will allow the Company to continue its operations through the end of 2020. 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
	Dec. 31, 2018
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 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, useful lives assigned to property and equipment, as well as the fair values of common stock, convertible preferred stock and convertible preferred stock tranche liability. 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 —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. 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 (deficit) 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 on 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 (expense), net. Restricted Cash —The Company had restricted cash of $1.8 million as of December 31, 2018 and 2017, which represents cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts. Concentrations of Credit Risk —Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents, short-term investments and restricted cash. 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, 2018, 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 capitalized incremental legal, professional accounting and other third-party fees that were directly associated with its planned IPO as other non-current assets until the IPO was consummated. After consummation of the IPO, these costs were recorded in stockholders’ equity (deficit) 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, 2018, 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 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. Derivative Instruments— The Company determined that its obligation to issue, and the Company’s investors’ obligation to purchase, additional shares of Series A convertible preferred stock in the second of two tranches represented a freestanding financial instrument. The freestanding tranche liability was initially recorded at fair value, with gains and losses arising from changes in fair value recognized in other income (expense) in the statements of operations at each period end while such instruments were outstanding. The liability was valued using an income approach, specifically the discounted cash flow method. On February 10, 2017, the Company issued 28,873,237 shares of Series A Preferred Stock at $0.71 per share upon the achievement of certain development milestones, resulting in net proceeds of $20.5 million. Accordingly, the convertible preferred stock tranche liability was re-measured at fair value on February 10, 2017 using an income approach and then derecognized with a corresponding amount recorded to Series A Preferred Stock. The Company had no liability related to the convertible preferred stock as of December 31, 2017 or December 31, 2018. 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 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— The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the Company’s price to the buyer is fixed or determinable; and collectability is reasonably assured. The Company records as deferred revenue any amounts received or billed prior to satisfying the revenue recognition criteria. Deferred revenue not expected to be recognized within the next twelve months is reported as non-current deferred revenue. In November 2017, the Company entered into a collaboration and license agreement for research, development, manufacturing and commercialization of products using the Company’s gene editing technology for the treatment of certain diseases (see Note 16). Consideration the Company may receive under the collaboration and license agreement include upfront nonrefundable payments, payments for research and manufacturing activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. Multiple Element Arrangements The terms of the collaboration and license agreement contain multiple deliverables, including licenses, research and development activities, participation on steering committees and manufacturing activities. The Company evaluates the activities in its collaboration agreements to determine if the activities are consistent with a typical vendor-customer relationship, and if so, accounts for them in accordance with Accounting Standards Codification (“ASC”) Topic 605-25, Revenue Recognition – Multiple Element Arrangements The Company evaluates multiple element arrangements to determine the deliverables included in the arrangement and whether the individual deliverables represent separate units of accounting, or whether they must be accounted for as a combined unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit. This evaluation requires the Company to make judgments about the individual deliverables and whether such deliverables (1) have value to the customer on a standalone basis and (2) if the arrangement includes a general right of return with respect to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use any other deliverable for its intended purpose without the receipt of the remaining deliverables, whether the value of the deliverable is dependent on any undelivered item, and whether there are other vendors that can provide the undelivered items. The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (“VSOE”), if available; third-party evidence (“TPE”) of selling price if VSOE is not available; or best estimate of selling price (“BESP”), if neither VSOE nor TPE is available. The Company typically uses BESP to estimate the selling price as it generally does not have VSOE or TPE of selling price for its units of accounting. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria are satisfied for that particular unit of accounting. The Company recognizes revenue from a combined unit of accounting over the contractual or estimated performance period for the undelivered items. If there is no discernible pattern of performance or objectively measurable performance measures do not exist for a unit of accounting, then the Company recognizes revenue on a straight-line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance over which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Amounts received prior to satisfying the associated revenue recognition criteria are recorded as deferred revenue on the consolidated balance sheets. Amounts not expected to be recognized within one year following the balance sheet date are classified as non-current deferred revenue. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations. Consideration for development and sales milestones are generally not considered fixed or determinable until the milestone is achieved. Consideration due to or received by the Company for the achievement of milestones are allocated to the units of accounting, if applicable, and recognized as revenue for the portion of the performance obligation that is complete at the time the milestone is achieved. The Company will defer the remaining portion of the milestone payment and recognize it as revenue over the remaining term of the performance obligation. If no such performance obligation exists, milestone payments are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met. Royalties earned on product sales, if any, are recognized based on contractual terms of the agreement when reported sales are reliably measurable and collectibility is reasonably assured, provided that there are no performance obligations then remaining. To date, none of the Company’s product candidates have been approved and, therefore, the Company has not earned any royalty revenue from product sales. In the event that the agreement was to be terminated and the Company had no further performance obligations at that time, the Company would recognize as revenue any portion of the upfront payment and other payments that had not previously been recorded as revenue and were classified as deferred revenue at the date of such termination. 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. Net loss per share was calculated using the two-class method, which is an earnings allocation formula that determines net loss per share for the holders of the Company’s common shares and participating securities. The Company’s convertible preferred stock contained participation rights in any dividend paid by the Company and was deemed to be a participating security. Net loss attributable to common stockholders and participating preferred shares was allocated to each share on an as-converted basis as if all of the earnings for the period had been distributed. The participating securities do not include a contractual obligation to share in losses of the Company and are not included in the calculation of net loss per share in the periods in which a net loss is recorded. Diluted net loss per share is computed using the more dilutive of (a) the two-class method or (b) the if converted method. The Company allocates earnings first to preferred stockholders based on dividend rights and then to common and preferred stockholders based on ownership interests. 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, unvested shares of common stock and convertible preferred 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 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”) The Company has substantially completed its evaluation of the impact of the adoption of ASU 2014-09 on the consolidated financial statements. Under ASU 2014-09, with respect to the Company’s collaboration with Novartis, the Company expects to account for the (1) research, development and commercialization and manufacturing licenses, (2) the research activities performed by us, (3) service on the joint committees and (4) manufacturing during the research and development terms as a single performance obligation, which is consistent with the accounting under the existing standard. The Company expects to recognize revenue from this collaboration using the cost-to-cost method, whereby the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected. Significant judgment is required in making estimates of total expected costs to complete the single performance obligation over the performance period. Revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimated transaction price will include variable consideration for milestone payments, however such milestone payments will remain fully constrained as of the adoption date as it is not probable at this time that the Company will achieve those milestones. The estimate of total costs expected and the estimate of variable consideration to be included in the transaction price will be updated at each reporting date. The cumulative effect of revisions to these estimates will be recorded in the period in which changes are identified. Under the new standard, the Company expects to recognize revenue from its collaboration agreement with Novartis earlier during 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 the existing standard. The Company plans to adopt the new standard using the full retrospective method, and thus will apply the new standard retrospectively to each prior reporting period. As a result, the Company expects an increase to collaboration revenue for the year ended December 31, 2018 of approximately $1.5 to $2.0 million, with a corresponding decrease to deferred revenue. The impact to the year ended December 31, 2017 is insignificant. 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 February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), 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
	Dec. 31, 2018
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, 2018 2017 (in thousands) U.S. Treasury securities $ 35 $ — Cash 471 7,393 Commercial paper 5,974 — Money market funds 31,740 44,182 Total cash and cash equivalents $ 38,220 $ 51,575

Short-Term Investments

Short-Term Investments	12 Months Ended
	Dec. 31, 2018
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 assets-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 (deficit), 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: December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 31,740 $ — $ — $ 31,740 Asset-backed securities 19,541 — (21 ) 19,520 Commercial paper 78,571 — — 78,571 Corporate debt securities 54,511 — (50 ) 54,461 U.S. Treasury securities 29,980 — (6 ) 29,974 Total $ 214,343 $ — $ (77 ) $ 214,266 December 31, 2017 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 44,182 $ — $ — $ 44,182 Asset-backed securities 7,428 — (6 ) 7,422 Commercial paper 34,882 — — 34,882 Corporate debt securities 26,906 — (50 ) 26,856 U.S. Treasury securities 8,941 — (17 ) 8,924 Total $ 122,339 $ — $ (73 ) $ 122,266 As of December 31, 2018, the Company does not consider those securities that are in an unrealized loss position to be other-than-temporarily impaired, as it has the ability to hold such investments until recovery of the fair value. 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, 2018 and 2017. The contractual maturity dates of all of the Company’s investments are less than one year.

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
PROPERTY AND EQUIPMENT	5. PROPERTY AND EQUIPMENT Property and equipment, net consists of the following: December 31, 2018 2017 (in thousands) Laboratory equipment $ 9,309 $ 3,714 Computers and purchased software 248 253 Furniture and fixtures 1,066 51 Leasehold improvements 24,520 34 Assets not yet in service 2,602 — Property and equipment, at cost 37,745 4,052 Less accumulated depreciation and amortization (2,108 ) (898 ) Property and equipment—net $ 35,637 $ 3,154 Depreciation expense for the years ended December 31, 2018 and 2017 was approximately $1.3 million and $0.7 million, respectively. The Company disposed of approximately $0.1 million of property and equipment, net during the year ended December 31, 2018, primarily related to laboratory equipment no longer in use. There were no disposals during the year ended December 31, 2017. Leasehold improvements as of December 31, 2018 consist primarily of costs associated with the buildout of the Company’s new research and development, manufacturing and general office space in Bedford, Massachusetts, which the Company occupied as of December 31, 2018. Assets not yet in service consists of manufacturing equipment and furniture purchased for the new space that was not in service as of December 31, 2018.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	6. 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, 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 $ — Commercial paper 72,597 — 72,597 — Corporate debt securities 54,461 — 54,461 — U.S. Treasury securities 29,939 — 29,939 — Total short-term investments $ 176,517 $ — $ 176,517 $ — Total financial assets $ 214,266 $ 31,775 $ 182,491 $ — Description December 31, 2017 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 $ 44,182 $ 44,182 $ — $ — Total cash equivalents $ 44,182 $ 44,182 $ — $ — Short-term investments: Asset-backed securities $ 7,422 $ — $ 7,422 $ — Commercial paper 34,882 — 34,882 — Corporate debt securities 26,856 — 26,856 — U.S. Treasury securities 8,924 — 8,924 — Total short-term investments $ 78,084 $ — $ 78,084 $ — Total financial assets $ 122,266 $ 44,182 $ 78,084 $ — 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. The convertible preferred stock tranche liability, which was derecognized in February 2017 (see Note 11), was stated at fair value and was measured using a Level 3 input because the fair value measurement was based, in part, on significant inputs not observed in the market. The reconciliations of changes in the fair value of financial instruments based on Level 3 inputs for the year ended December 31, 2017 consisted of: (in thousands) Fair value as of January 1, 2017 $ 4,247 Change in fair value of convertible preferred stock tranche liability 876 Reduction in tranche liability due to preferred stock issuance (5,123 ) Fair value as of December 31, 2017 $ — The Company had no liability related to the convertible preferred stock as of December 31, 2018. There were no transfers between fair value measure levels during the years ended December 31, 2018 and 2017.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 (in thousands) Accrued compensation and benefits $ 3,370 $ 1,435 Accrued research and development expenses 1,059 182 Accrued professional fees 195 1,120 Accrued unvested common stock subject to repurchase 112 123 Accrued other 304 — Total accrued expenses and other liabilities $ 5,040 $ 2,860

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
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 addition to the leased space, the Company has certain rights to expand the lease to include certain adjacent property. As of December 31, 2018, no expansion rights had been exercised. On August 13, 2018, the Company entered into a sublease agreement for the entire leased premises, with a related party. 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. Rent is due under the lease in two phases; rent on the first 46,000 square feet started in September 2018 and rent on the remaining 21,000 square feet will start in March 2019. The initial annual base rent is $39.50 per square foot and will increase 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, 2018 are as follows: Amount For the Years Ending December 31, (in thousands) 2019 $ 2,537 2020 2,755 2021 2,758 2022 2,928 2023 3,016 Thereafter 10,161 Total future minimum lease payments $ 24,155 The lease agreement entered into in December 2017 allows 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 tenant improvement allowance must be used on or before August 31, 2019 or it will be deemed forfeited with no further obligation by the landlord. Tenant improvement allowances due or received are recorded as deferred rent incentives on the Company’s consolidated balance sheets and are recognized as a reduction to rent expense over the term of the lease. As of December 31, 2018, deferred rent incentives totaled $8.0 million. Rent expense, net of amortization of the deferred rent incentive, for the years ended December 31, 2018 and 2017 was $2.7 million and $0.9 million, respectively. The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $1.8 million as of December 31, 2018 and 2017, in conjunction with its current leases.

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2018
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. As a result of the execution of the Collaboration Agreement with Novartis (see Note 16), the Company paid $4.5 million to COH in December 2017, under the terms of the license agreement. In May 2015, the Company entered into a sponsored research agreement with COH with a goal to identify potential treatments for diseases in humans. The agreed upon commitment for research and development services is for up to $1.1 million which continues through 2019. Under this agreement, the Company has recorded $0.1 million in research and development expense for each of the years ended December 31, 2018 and 2017. The Company’s future contractual obligation under the sponsored research agreement is $0.6 million as of December 31, 2018. 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. As a result of the execution of the Collaboration Agreement with Novartis (see Note 16), the Company paid $0.1 million to Caltech in December 2017, under the terms of the license agreement.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Federal statutory rate 21.0 % 34.0 % Tax credits 17.7 % 4.8 % State taxes, net of federal tax benefit 5.5 % 5.0 % Change in tranche liability — % (1.0 %) Non-deductible expenses (0.3 %) (1.8 %) Impact of federal rate change — % (15.2 %) Change in valuation allowance (43.9 %) (25.8 %) Effective income tax rate — % — % The principal components of the Company’s deferred tax assets and liabilities consist of the following: December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating losses $ 14,848 $ 9,180 R&D credits 12,506 1,776 Deferred revenue 8,159 — Deferred rent 2,854 113 Capitalized R&D costs 1,375 1,362 Equity compensation 443 27 Accrued expense and other 115 196 Total deferred tax assets 40,300 12,654 Deferred tax liabilities: Depreciation (2,680 ) (166 ) Total deferred tax liabilities (2,680 ) (166 ) Valuation allowance (37,620 ) (12,488 ) Net deferred taxes $ — $ — On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (“TCJA”) tax reform legislation. This legislation makes significant change in U.S. tax law including a reduction in the corporate tax rates, changes to net operating loss carryforwards and carrybacks, and a repeal of the corporate alternative minimum tax. The legislation reduced the U.S. corporate tax rate from the current rate of 34% down to 21% starting on January 1, 2018. As a result of the enacted law, the Company was required to revalue its deferred tax assets and liabilities at the 21% rate as of December 22, 2017. The Company has no income tax expense due to the operating loss incurred for the years ended December 31, 2018 and 2017. 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, 2018, the Company has $52.4 million and $46.6 million of federal and state net operating loss carryforwards, respectively, that expire at various dates through 2038. Included in the federal net operating loss of $52.4 million is $21.3 million that can be carried forward indefinitely. At December 31, 2018, the Company has $2.5 million and $3.5 million of federal and state research and development credit carryforwards, respectively, that expire at various dates through 2038. At December 31, 2018, the Company has $7.3 million of orphan drug credit carryforwards that expire at various dates through 2038. The valuation allowance increased in 2018 and 2017 by $25.1 million and $7.7 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, 2018, 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, 2018, 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, 2018 and 2017.

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
CONVERTIBLE PREFERRED STOCK	11. CONVERTIBLE PREFERRED STOCK On April 2, 2018, the Company closed its initial public offering with the sale of 10,350,000 shares of 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. Prior to the IPO, the Company had financed its operations primarily through the sales of preferred stock and through funding from a collaboration partner. In December 2015, the Company authorized the sale and issuance of up to 62,269,145 shares of Series A preferred stock. The Series A preferred financing was structured to close in two tranches. The first tranche closed on December 22, 2015 with the issuance of 28,873,237 shares at $0.71 per share resulting in gross cash proceeds of $20.5 million. Issuance costs totaled $0.1 million. As part of the closing on December 22, 2015, the Company also issued 4,522,670 shares of Series A in connection with the conversion of notes payable to investors that were originally issued in 2015. The investors in the first tranche were granted the right to purchase an additional 28,873,237 shares of Series A preferred stock to be offered in the second tranche at $0.71 per share. The Company determined that the right of the investors to purchase Series A preferred stock in the second tranche met the definition of a freestanding financial instrument and therefore was recognized as a liability at fair value until the tranche right was exercised in February 2017. Upon the first tranche closing, the Company recognized a liability of $6.2 million for the fair value of the convertible preferred stock tranche liability representing the future obligation. The convertible preferred stock tranche liability was re-measured with a fair value of $5.1 million as of February 9, 2017. The fair value of the convertible preferred stock tranche liability was determined using an option pricing model with the following assumptions: February 9, 2017 Probability of milestone closing 99.9 % Expected years closing 0.0 Discount rate 1.0 % Risk-free interest rate 0.84 % Expected dividend yield 0.0 % The Company adjusted the carrying value of the convertible preferred stock tranche liability to its estimated fair value at each reporting date and upon issuance of the second tranche of Series A preferred stock on February 10, 2017, recognizing the changes in fair value in other income (expense) in the consolidated statement of operations. During the year ended December 31, 2017, the Company recognized total other income (expense) of $(0.9) million related to changes in the fair value of the convertible preferred stock tranche liability. On February 10, 2017, the Company issued 28,873,237 shares of Series A preferred stock at $0.71 per share for gross proceeds of $20.5 million. Issuance costs were less than $0.1 million. Accordingly, the convertible preferred stock tranche liability was re-measured at fair value and then derecognized with a corresponding amount of $5.1 million reclassified to Series A preferred stock. On July 28, 2017, the Company authorized the sale of 64,930,561 shares of Series B convertible preferred stock and issued 57,986,116 shares of Series B convertible preferred stock at $1.44 per share, for gross proceeds of $83.5 million upon closing. Total issuance costs were $0.4 million. All holders of Series A convertible preferred stock participated in the Series B issuance along with new investors. On November 6, 2017, the Company entered into a Collaboration and License Agreement with Novartis for the development and commercialization of products using the Company’s gene editing technology for the treatment of certain ophthalmic targets and sickle cell disease (see Note 16). Under the terms of the Collaboration Agreement, Novartis invested $10.0 million to purchase 6,944,445 shares of Series B convertible preferred stock. The difference between the cash proceeds received from Novartis for the purchase of Series B preferred stock and the $11.7 million estimated fair value of the Series B at the time of purchase was allocated from the collaboration proceeds to Series B preferred stock.

Stockholder's Equity

Stockholder's Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
STOCKHOLDERS’ EQUITY	12. 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.

Stock Option Plan

Stock Option Plan	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK OPTION PLAN	13. STOCK OPTION PLAN 2015 Stock Incentive Plan In December 2015, the 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. In February 2017 and July 2017, the Board of Directors amended the 2015 Plan to increase the number of shares available for issue under the 2015 Plan to 2,446,323 and 3,225,346, respectively. 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, 2018, there were no 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, 2019, an additional 1,494,341 shares were added to the 2018 Plan, representing 4% of total common shares outstanding at December 31, 2018. As of December 31, 2018, there were 1,896,926 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, 2019, an additional 373,585 shares were added to the 2018 ESPP, representing 1% of total common shares outstanding at December 31, 2018. The 2018 ESPP is intended to qualify as an “employee stock purchase plan” under Section 423 of the Internal Revenue Code. 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 with purchase periods ending 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). The Company recorded $0.1 million of stock-based compensation pursuant to the 2018 ESPP during the year ended December 31, 2018. 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, 2018 2017 (in thousands) Research and development $ 1,117 $ 115 General and administrative 1,506 141 $ 2,623 $ 256 As of December 31, 2018, there was $15.7 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, 2018. 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. 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, 2018 2017 Expected volatility 52.80% - 60.12% 52.73% - 59.40% Weighted-average risk-free interest rate 2.33% - 3.08% 2.04% - 2.37% Expected dividend yield — % — % Expected term (in years) 5.5 - 7.6 5.9 - 8.6 Underlying common stock fair value $6.63 - $24.28 $0.63 - $6.63 A summary of option activity under the Plans during the year ended December 31, 2018 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, 2018 1,971,711 $ 3.61 9.3 $ 5,977 Granted 1,392,259 $ 19.05 Exercised (89,050 ) $ 1.70 Cancelled/Forfeited (71,798 ) $ 2.72 Outstanding at December 31, 2018 3,203,122 $ 10.47 8.9 $ 39,018 Vested and expected to vest at December 31, 2018 3,203,122 $ 10.47 8.9 $ 39,018 Exercisable at December 31, 2018 716,531 $ 3.82 8.1 $ 13,287 The total intrinsic value of options exercised during the year ended December 31, 2018 was $1.8 million. There were no option exercises in 2017. The weighted-average grant date fair value of options granted during the years ended December 31, 2018 and 2017 was $10.41 and $2.78, 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, 2018 265,098 Vested (129,959 ) Issued 16,150 Repurchased — Unvested shares—December 31, 2018 151,289 As of December 31, 2018 and 2017, 151,289 shares and 265,098 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 $0.1 million as of each date.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
NET LOSS PER SHARE	14. 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, 2018 and 2017, 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, 2018 2017 Convertible preferred shares (as converted to common stock) — 24,168,656 Unvested common stock from early exercise of options 151,289 265,098 Stock options to purchase common stock 3,203,122 1,971,711 Total 3,354,411 26,405,465

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
DEFINED CONTRIBUTION PLAN	15. 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 year ended December 31, 2018 was $0.3 million. There was no discretionary match made under the 401(k) Plan for the year ended December 31, 2017.

Collaboration and License Agree

Collaboration and License Agreement	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AND LICENSE AGREEMENT	16. COLLABORATION AND LICENSE AGREEMENT In November 2017, the Company entered into a collaboration and license agreement (the “Collaboration Agreement”) with Novartis Institutes of BioMedical Research, Inc. (“Novartis”) for the research, development, manufacturing and commercialization of products using the Company’s gene-editing technology for the treatment of certain ophthalmic targets and sickle cell disease. In February 2019, Novartis elected to discontinue the sickle cell disease program under the Collaboration Agreement effective in August 2019. The Company continues to work with Novartis to identify new targets for the partnership based on the existing exploratory research component of the agreement, and the collaboration on two ophthalmic programs also continues. The Company has assumed all worldwide rights to the sickle cell disease program, and its efforts to develop an in vivo Under the terms of the Collaboration Agreement, taking into account Novartis’ election to discontinue the sickle cell disease program, the Company granted Novartis a research license, a development and commercialization license, and a manufacturing license, under certain of its intellectual property rights to research, develop, manufacture and commercialize the ophthalmic targets. Upon entering into the Collaboration Agreement, the Company received an upfront, nonrefundable payment of $35.0 million and issued additional shares of its Series B preferred stock to Novartis for consideration of $10.0 million. The Collaboration Agreement consists of a research term, where the Company and Novartis are collaborating to perform research and conduct preclinical development to identify candidates that modulate the ophthalmic targets. The Collaboration Agreement also includes exploratory work on the applicability of the gene-editing technology with respect to other gene targets. The Company’s obligation to perform such exploratory research concludes in November 2020. Novartis may select up to four targets, with limited substitution rights. The Company is responsible for the manufacturing of proprietary research grade human hematopoietic stem cell derived adeno-associated virus vectors (“AAVHSCs”) during the research term. Research activities performed by the Company are being reimbursed at a full-time equivalent rate (“FTE”) and manufacturing activities will be reimbursed at cost, as specified and defined in the Collaboration Agreement. Novartis is required to pay the Company a target fee of $5.0 million for each target that meets certain success criteria during the research term (the “target fee trigger date”), up to a maximum of four targets. The research term will continue for five years from the effective date of the Collaboration Agreement. Pursuant to the Collaboration Agreement, the Company will also participate on a joint steering committee and a joint manufacturing subcommittee, with equal representation from both the Company and Novartis. Novartis has the exclusive right to develop and commercialize up to four candidates or products arising from the research activities. Subject to certain limitations pursuant to the terms of the Collaboration Agreement, Novartis will fund all development and commercialization costs. The Company will be responsible for manufacturing candidates and products for Novartis during the development and commercialization terms. The Company’s manufacturing activities will be reimbursed at cost during the development term and at cost plus a margin during the commercialization term, as defined in the Collaboration Agreement. If the Company is not able to manufacture candidates or products that meet the quality or quantity requirements of Novartis, then Novartis shall have the right to designate a third-party contract manufacturer or manufacture such candidates or products itself. In accordance with the Collaboration Agreement, taking into consideration Novartis’ election to discontinue the sickle cell disease program, the Company is also eligible to receive up to a total of $530.0 million in milestone payments, including up to $180.0 million in development milestone payments, up to $170.0 million in regulatory milestone payments and up to $180.0 million in commercial milestone payments, with respect to the licensed products. The Company is also eligible to earn tiered royalties on net sales of licensed products by Novartis, its affiliates or sublicensees, ranging from mid single-digit, to sub-teen double-digit percentages, and such royalties are potentially subject to various reductions and offsets. If any of the exploratory research efforts are advanced into formal research and development programs, the parties will negotiate the economics including potential milestone payments for such programs. Unless earlier terminated, the Collaboration Agreement will continue on a target-by-target basis until the expiration of all applicable royalty terms with respect to all products that modulate such target on a country-by-country-basis. Either party may terminate the agreement on a target-by-target basis for the other party’s material breach with respect to such target, or in the event of the other party’s bankruptcy. Novartis may terminate the agreement for convenience on a target-by-target basis. The Company may terminate the agreement if Novartis files, or joins a third party in filing or maintaining, a patent challenge against certain of the patent rights licensed to Novartis under the terms of the agreement. There are no performance, cancellation, termination or refund provisions in the arrangement that contain material financial consequences to the Company. Revenue Recognition The Company has identified the following deliverables in the Collaboration Agreement in accordance with the provisions of ASC Topic 605-25, Revenue Recognition—Multiple Element Arrangements Upon entering into the Collaboration Agreement, the Company received a nonrefundable upfront payment of $35.0 million and a $10.0 million investment in its Series B preferred stock by Novartis. The Company recorded the Series B preferred stock at its estimated fair value of $11.7 million, At the inception of the Collaboration Agreement, the Company could not objectively measure the level of effort required to fulfill its obligations for the combined unit of accounting. Therefore, revenue is being recognized on a straight-line basis over the estimated period of performance for the combined unit of accounting, which the Company estimates to be approximately ten years from the inception of the Collaboration Agreement. The Company commenced revenue recognition upon delivery of the final deliverable included in the combined unit of accounting which occurred in January 2018. Accordingly, no amounts of revenue were recognized for the year ended December 31, 2017. All payments due or received from Novartis as of December 31, 2017, including amounts due for research activities performed, were recorded as deferred revenue as of December 31, 2017. The Company recognized revenue of $3.7 million for the year ended December 31, 2018, based on a straight-line basis over the estimated period of performance taking into consideration all upfront payments and research funding payments together as a single unit. The Company had a $0.8 million receivable from Novartis as of December 31, 2018, which was included in prepaid expenses and other current assets on the Company’s consolidated balance sheet. The amount recorded as deferred revenue under this agreement totaled $33.2 million as of December 31, 2018.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 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, useful lives assigned to property and equipment, as well as the fair values of common stock, convertible preferred stock and convertible preferred stock tranche liability. 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	Cash and Cash Equivalents —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.
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 (deficit) 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 on 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 (expense), net.
Restricted Cash	Restricted Cash —The Company had restricted cash of $1.8 million as of December 31, 2018 and 2017, which represents cash serving as collateral for letters of credit issued for security deposits for the Company’s facility leases in Bedford, Massachusetts.
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, short-term investments and restricted cash. 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, 2018, 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 capitalized incremental legal, professional accounting and other third-party fees that were directly associated with its planned IPO as other non-current assets until the IPO was consummated. After consummation of the IPO, these costs were recorded in stockholders’ equity (deficit) 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, 2018, 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 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.
Derivative Instruments	Derivative Instruments— The Company determined that its obligation to issue, and the Company’s investors’ obligation to purchase, additional shares of Series A convertible preferred stock in the second of two tranches represented a freestanding financial instrument. The freestanding tranche liability was initially recorded at fair value, with gains and losses arising from changes in fair value recognized in other income (expense) in the statements of operations at each period end while such instruments were outstanding. The liability was valued using an income approach, specifically the discounted cash flow method. On February 10, 2017, the Company issued 28,873,237 shares of Series A Preferred Stock at $0.71 per share upon the achievement of certain development milestones, resulting in net proceeds of $20.5 million. Accordingly, the convertible preferred stock tranche liability was re-measured at fair value on February 10, 2017 using an income approach and then derecognized with a corresponding amount recorded to Series A Preferred Stock. The Company had no liability related to the convertible preferred stock as of December 31, 2017 or December 31, 2018.
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 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— The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the Company’s price to the buyer is fixed or determinable; and collectability is reasonably assured. The Company records as deferred revenue any amounts received or billed prior to satisfying the revenue recognition criteria. Deferred revenue not expected to be recognized within the next twelve months is reported as non-current deferred revenue. In November 2017, the Company entered into a collaboration and license agreement for research, development, manufacturing and commercialization of products using the Company’s gene editing technology for the treatment of certain diseases (see Note 16). Consideration the Company may receive under the collaboration and license agreement include upfront nonrefundable payments, payments for research and manufacturing activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales. Multiple Element Arrangements The terms of the collaboration and license agreement contain multiple deliverables, including licenses, research and development activities, participation on steering committees and manufacturing activities. The Company evaluates the activities in its collaboration agreements to determine if the activities are consistent with a typical vendor-customer relationship, and if so, accounts for them in accordance with Accounting Standards Codification (“ASC”) Topic 605-25, Revenue Recognition – Multiple Element Arrangements The Company evaluates multiple element arrangements to determine the deliverables included in the arrangement and whether the individual deliverables represent separate units of accounting, or whether they must be accounted for as a combined unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit. This evaluation requires the Company to make judgments about the individual deliverables and whether such deliverables (1) have value to the customer on a standalone basis and (2) if the arrangement includes a general right of return with respect to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use any other deliverable for its intended purpose without the receipt of the remaining deliverables, whether the value of the deliverable is dependent on any undelivered item, and whether there are other vendors that can provide the undelivered items. The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices of the separate units of accounting. The Company determines the estimated selling price for units of accounting within each arrangement using vendor-specific objective evidence (“VSOE”), if available; third-party evidence (“TPE”) of selling price if VSOE is not available; or best estimate of selling price (“BESP”), if neither VSOE nor TPE is available. The Company typically uses BESP to estimate the selling price as it generally does not have VSOE or TPE of selling price for its units of accounting. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria are satisfied for that particular unit of accounting. The Company recognizes revenue from a combined unit of accounting over the contractual or estimated performance period for the undelivered items. If there is no discernible pattern of performance or objectively measurable performance measures do not exist for a unit of accounting, then the Company recognizes revenue on a straight-line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance over which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Amounts received prior to satisfying the associated revenue recognition criteria are recorded as deferred revenue on the consolidated balance sheets. Amounts not expected to be recognized within one year following the balance sheet date are classified as non-current deferred revenue. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company expects to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations. Consideration for development and sales milestones are generally not considered fixed or determinable until the milestone is achieved. Consideration due to or received by the Company for the achievement of milestones are allocated to the units of accounting, if applicable, and recognized as revenue for the portion of the performance obligation that is complete at the time the milestone is achieved. The Company will defer the remaining portion of the milestone payment and recognize it as revenue over the remaining term of the performance obligation. If no such performance obligation exists, milestone payments are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met. Royalties earned on product sales, if any, are recognized based on contractual terms of the agreement when reported sales are reliably measurable and collectibility is reasonably assured, provided that there are no performance obligations then remaining. To date, none of the Company’s product candidates have been approved and, therefore, the Company has not earned any royalty revenue from product sales. In the event that the agreement was to be terminated and the Company had no further performance obligations at that time, the Company would recognize as revenue any portion of the upfront payment and other payments that had not previously been recorded as revenue and were classified as deferred revenue at the date of such termination.
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 and Basic 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. Net loss per share was calculated using the two-class method, which is an earnings allocation formula that determines net loss per share for the holders of the Company’s common shares and participating securities. The Company’s convertible preferred stock contained participation rights in any dividend paid by the Company and was deemed to be a participating security. Net loss attributable to common stockholders and participating preferred shares was allocated to each share on an as-converted basis as if all of the earnings for the period had been distributed. The participating securities do not include a contractual obligation to share in losses of the Company and are not included in the calculation of net loss per share in the periods in which a net loss is recorded. Diluted net loss per share is computed using the more dilutive of (a) the two-class method or (b) the if converted method. The Company allocates earnings first to preferred stockholders based on dividend rights and then to common and preferred stockholders based on ownership interests. 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, unvested shares of common stock and convertible preferred 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 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”) The Company has substantially completed its evaluation of the impact of the adoption of ASU 2014-09 on the consolidated financial statements. Under ASU 2014-09, with respect to the Company’s collaboration with Novartis, the Company expects to account for the (1) research, development and commercialization and manufacturing licenses, (2) the research activities performed by us, (3) service on the joint committees and (4) manufacturing during the research and development terms as a single performance obligation, which is consistent with the accounting under the existing standard. The Company expects to recognize revenue from this collaboration using the cost-to-cost method, whereby the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected. Significant judgment is required in making estimates of total expected costs to complete the single performance obligation over the performance period. Revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimated transaction price will include variable consideration for milestone payments, however such milestone payments will remain fully constrained as of the adoption date as it is not probable at this time that the Company will achieve those milestones. The estimate of total costs expected and the estimate of variable consideration to be included in the transaction price will be updated at each reporting date. The cumulative effect of revisions to these estimates will be recorded in the period in which changes are identified. Under the new standard, the Company expects to recognize revenue from its collaboration agreement with Novartis earlier during 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 the existing standard. The Company plans to adopt the new standard using the full retrospective method, and thus will apply the new standard retrospectively to each prior reporting period. As a result, the Company expects an increase to collaboration revenue for the year ended December 31, 2018 of approximately $1.5 to $2.0 million, with a corresponding decrease to deferred revenue. The impact to the year ended December 31, 2017 is insignificant. 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 February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), 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
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Estimated Useful Life of Asset	Computer equipment and software 3 years Laboratory equipment and office furniture 5 years Leasehold improvements Shorter of the lease term or estimated useful life

Cash and Cash Equivalents (Tabl

Cash and Cash Equivalents (Tables)	12 Months Ended
	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Summary of Cash and Cash Equivalents	The following table summarizes the Company’s cash and cash equivalents: December 31, 2018 2017 (in thousands) U.S. Treasury securities $ 35 $ — Cash 471 7,393 Commercial paper 5,974 — Money market funds 31,740 44,182 Total cash and cash equivalents $ 38,220 $ 51,575

Short-Term Investments (Tables)

Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2018
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: December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 31,740 $ — $ — $ 31,740 Asset-backed securities 19,541 — (21 ) 19,520 Commercial paper 78,571 — — 78,571 Corporate debt securities 54,511 — (50 ) 54,461 U.S. Treasury securities 29,980 — (6 ) 29,974 Total $ 214,343 $ — $ (77 ) $ 214,266 December 31, 2017 Amortized Cost Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market mutual funds $ 44,182 $ — $ — $ 44,182 Asset-backed securities 7,428 — (6 ) 7,422 Commercial paper 34,882 — — 34,882 Corporate debt securities 26,906 — (50 ) 26,856 U.S. Treasury securities 8,941 — (17 ) 8,924 Total $ 122,339 $ — $ (73 ) $ 122,266

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2018 2017 (in thousands) Laboratory equipment $ 9,309 $ 3,714 Computers and purchased software 248 253 Furniture and fixtures 1,066 51 Leasehold improvements 24,520 34 Assets not yet in service 2,602 — Property and equipment, at cost 37,745 4,052 Less accumulated depreciation and amortization (2,108 ) (898 ) Property and equipment—net $ 35,637 $ 3,154

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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, 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 $ — Commercial paper 72,597 — 72,597 — Corporate debt securities 54,461 — 54,461 — U.S. Treasury securities 29,939 — 29,939 — Total short-term investments $ 176,517 $ — $ 176,517 $ — Total financial assets $ 214,266 $ 31,775 $ 182,491 $ — Description December 31, 2017 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 $ 44,182 $ 44,182 $ — $ — Total cash equivalents $ 44,182 $ 44,182 $ — $ — Short-term investments: Asset-backed securities $ 7,422 $ — $ 7,422 $ — Commercial paper 34,882 — 34,882 — Corporate debt securities 26,856 — 26,856 — U.S. Treasury securities 8,924 — 8,924 — Total short-term investments $ 78,084 $ — $ 78,084 $ — Total financial assets $ 122,266 $ 44,182 $ 78,084 $ —
Schedule of Reconciliation of Changes in the Fair Value of Financial Instruments	The reconciliations of changes in the fair value of financial instruments based on Level 3 inputs for the year ended December 31, 2017 consisted of: (in thousands) Fair value as of January 1, 2017 $ 4,247 Change in fair value of convertible preferred stock tranche liability 876 Reduction in tranche liability due to preferred stock issuance (5,123 ) Fair value as of December 31, 2017 $ —

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consist of the following: December 31, 2018 2017 (in thousands) Accrued compensation and benefits $ 3,370 $ 1,435 Accrued research and development expenses 1,059 182 Accrued professional fees 195 1,120 Accrued unvested common stock subject to repurchase 112 123 Accrued other 304 — Total accrued expenses and other liabilities $ 5,040 $ 2,860

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	Future minimum lease payments, net of anticipated sublease payments, as of December 31, 2018 are as follows: Amount For the Years Ending December 31, (in thousands) 2019 $ 2,537 2020 2,755 2021 2,758 2022 2,928 2023 3,016 Thereafter 10,161 Total future minimum lease payments $ 24,155

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 Federal statutory rate 21.0 % 34.0 % Tax credits 17.7 % 4.8 % State taxes, net of federal tax benefit 5.5 % 5.0 % Change in tranche liability — % (1.0 %) Non-deductible expenses (0.3 %) (1.8 %) Impact of federal rate change — % (15.2 %) Change in valuation allowance (43.9 %) (25.8 %) 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, 2018 2017 (in thousands) Deferred tax assets: Net operating losses $ 14,848 $ 9,180 R&D credits 12,506 1,776 Deferred revenue 8,159 — Deferred rent 2,854 113 Capitalized R&D costs 1,375 1,362 Equity compensation 443 27 Accrued expense and other 115 196 Total deferred tax assets 40,300 12,654 Deferred tax liabilities: Depreciation (2,680 ) (166 ) Total deferred tax liabilities (2,680 ) (166 ) Valuation allowance (37,620 ) (12,488 ) Net deferred taxes $ — $ —

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Temporary Equity Disclosure [Abstract]
Summary of Fair Value of Convertible Preferred Stock Tranche Liability Determined Using Option Pricing Model	The fair value of the convertible preferred stock tranche liability was determined using an option pricing model with the following assumptions: February 9, 2017 Probability of milestone closing 99.9 % Expected years closing 0.0 Discount rate 1.0 % Risk-free interest rate 0.84 % Expected dividend yield 0.0 %

Stock Option Plan (Tables)

Stock Option Plan (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 (in thousands) Research and development $ 1,117 $ 115 General and administrative 1,506 141 $ 2,623 $ 256
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, 2018 2017 Expected volatility 52.80% - 60.12% 52.73% - 59.40% Weighted-average risk-free interest rate 2.33% - 3.08% 2.04% - 2.37% Expected dividend yield — % — % Expected term (in years) 5.5 - 7.6 5.9 - 8.6 Underlying common stock fair value $6.63 - $24.28 $0.63 - $6.63
Summary of Option Activity under Plans	A summary of option activity under the Plans during the year ended December 31, 2018 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, 2018 1,971,711 $ 3.61 9.3 $ 5,977 Granted 1,392,259 $ 19.05 Exercised (89,050 ) $ 1.70 Cancelled/Forfeited (71,798 ) $ 2.72 Outstanding at December 31, 2018 3,203,122 $ 10.47 8.9 $ 39,018 Vested and expected to vest at December 31, 2018 3,203,122 $ 10.47 8.9 $ 39,018 Exercisable at December 31, 2018 716,531 $ 3.82 8.1 $ 13,287
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, 2018 265,098 Vested (129,959 ) Issued 16,150 Repurchased — Unvested shares—December 31, 2018 151,289

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017, 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, 2018 2017 Convertible preferred shares (as converted to common stock) — 24,168,656 Unvested common stock from early exercise of options 151,289 265,098 Stock options to purchase common stock 3,203,122 1,971,711 Total 3,354,411 26,405,465

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands	Apr. 02, 2018USD ($)$ / sharesshares	Mar. 16, 2018	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	May 14, 2018shares
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net loss | $			$ 57,215	$ 29,991
Accumulated deficit | $			$ 97,396	$ 40,181
Reverse stock split ratio of issued and outstanding shares of common stock			one-for-5.263
Reverse stock split ratio of issued and outstanding shares of common stock, conversion		5.263
Reverse stock split effective date			Mar. 16, 2018
Convertible preferred shares converted to common stock	24,168,656
Preferred stock outstanding			0	0
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 | $ / shares	$ 16
Proceeds from issuance of common stock | $	$ 165,600
Preferred stock outstanding					0

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	Sep. 01, 2018	Jul. 28, 2017	Feb. 10, 2017	Dec. 22, 2015	Dec. 31, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Restricted cash					$ 1,800,000	$ 1,800,000
Outstanding claim					0
Impairment of long-lived assets					$ 0
Net proceeds form issuance of preferred stock		$ 83,500,000
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.
Collaboration revenue					$ 3,684,000
ASU 2014-09 | Minimum | Deferred Revenue
Summary Of Significant Accounting Policies [Line Items]
Effect of adoption of new accounting standard					(1,500,000)
ASU 2014-09 | Minimum | Collaboration Revenue
Summary Of Significant Accounting Policies [Line Items]
Effect of adoption of new accounting standard					1,500,000
ASU 2014-09 | Maximum | Deferred Revenue
Summary Of Significant Accounting Policies [Line Items]
Effect of adoption of new accounting standard					(2,000,000)
ASU 2014-09 | Maximum | Collaboration Revenue
Summary Of Significant Accounting Policies [Line Items]
Effect of adoption of new accounting standard					$ 2,000,000
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
Royalty
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue					$ 0
Series A Convertible Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Preferred stock shares issued			28,873,237	28,873,237		28,873,237
Preferred stock per share			$ 0.71
Net proceeds form issuance of preferred stock			$ 20,500,000	$ 20,500,000		$ 20,479,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Estimated Useful Life of Asset (Details)	12 Months Ended
	Dec. 31, 2018
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
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 Cash and Cash Equiva

Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	$ 38,220	$ 51,575	$ 11,392
U.S. Treasury securities
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	35
Cash
Cash And Cash Equivalents [Line Items]
Total cash and cash equivalents	471	7,393
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	$ 31,740	$ 44,182

Short-Term Investments - Summar

Short-Term Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	$ 214,343	$ 122,339
Cash equivalents and short-term investments, Unrealized Losses	(77)	(73)
Cash equivalents and short-term investments, Fair Value	214,266	122,266
Money Market Mutual Funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	31,740	44,182
Cash equivalents and short-term investments, Fair Value	31,740	44,182
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	19,541	7,428
Cash equivalents and short-term investments, Unrealized Losses	(21)	(6)
Cash equivalents and short-term investments, Fair Value	19,520	7,422
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	78,571	34,882
Cash equivalents and short-term investments, Fair Value	78,571	34,882
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	54,511	26,906
Cash equivalents and short-term investments, Unrealized Losses	(50)	(50)
Cash equivalents and short-term investments, Fair Value	54,461	26,856
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	29,980	8,941
Cash equivalents and short-term investments, Unrealized Losses	(6)	(17)
Cash equivalents and short-term investments, Fair Value	$ 29,974	$ 8,924

Short-Term Investments - Additi

Short-Term Investments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents And Available For Sale Securities [Abstract]
Realized gains and losses on available-for-sale securities	$ 0	$ 0
Contractual maturity date of investments	Less than one year

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 37,745	$ 4,052
Less accumulated depreciation and amortization	(2,108)	(898)
Property and equipment—net	35,637	3,154
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, at cost	9,309	3,714
Computers and Purchased Software
Property Plant And Equipment [Line Items]
Property and equipment, at cost	248	253
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, at cost	1,066	51
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, at cost	24,520	$ 34
Assets Not Yet in Service
Property Plant And Equipment [Line Items]
Property and equipment, at cost	$ 2,602

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation	$ 1,289,000	$ 684,000
Disposal of property and equipment, net	$ 100,000	$ 0

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments, fair value	$ 176,517	$ 78,084
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents, fair value	37,749	44,182
Short-term investments, fair value	176,517	78,084
Financial assets, fair value	214,266	122,266
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	31,740	44,182
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	19,520	7,422
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	72,597	34,882
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	29,939	8,924
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	54,461	26,856
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	31,775	44,182
Financial assets, fair value	31,775	44,182
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	31,740	44,182
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	5,974
Short-term investments, fair value	176,517	78,084
Financial assets, fair value	182,491	78,084
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	19,520	7,422
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	72,597	34,882
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	29,939	8,924
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	$ 54,461	$ 26,856

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Reconciliation of Changes in the Fair Value of Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Change in fair value associated with convertible preferred stock tranche liability	$ 876
Reduction in tranche liability due to preferred stock issuance	(5,123)
Significant Unobservable Inputs (Level 3)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value as of January 1, 2017	4,247
Change in fair value associated with convertible preferred stock tranche liability	876
Reduction in tranche liability due to preferred stock issuance	$ (5,123)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurement [Line Items]
Transfers between fair value measure levels	$ 0	$ 0
Convertible Preferred Stock
Fair Value Measurement [Line Items]
Financial liabilities fair value disclosure	$ 0

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued compensation and benefits	$ 3,370	$ 1,435
Accrued research and development expenses	1,059	182
Accrued professional fees	195	1,120
Accrued unvested common stock subject to repurchase	112	123
Accrued other	304
Total accrued expenses and other liabilities	$ 5,040	$ 2,860

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended		12 Months Ended
	Dec. 31, 2017USD ($)ft²Phase	Sep. 30, 2016	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)ft²
Commitment And Contingencies [Line Items]
Deferred rent incentives			$ 8,000,000
Rent expense, net			2,700,000	$ 900,000
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit	$ 1,800,000		1,800,000	$ 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
Expansion Rights Exercised			no
Sublease aggregate base rent obligation			$ 2,700,000
Office space leased | ft²	67,000			67,000
Lessee, operating lease, lease not yet commenced, renewal term	5 years			5 years
Number of phases | Phase	2
Initial annual base rent per square foot	$ 39.50
Percentage increase in initial annual base rent per square foot.	3.00%
Bedford, Massachusetts | Phase One
Commitment And Contingencies [Line Items]
Office space leased | ft²	46,000			46,000
Rent start date	2018-09
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000			21,000
Rent start date	2019-03

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 2,537
2020	2,755
2021	2,758
2022	2,928
2023	3,016
Thereafter	10,161
Total future minimum lease payments	$ 24,155

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($)	1 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2016	Apr. 30, 2016	May 31, 2015	Dec. 31, 2018	Dec. 31, 2017
License Agreement [Line Items]
Research and development					$ 47,948,000	$ 21,378,000
City Of Hope
License Agreement [Line Items]
Annual license fee			$ 25,000
City Of Hope | Collaboration Agreement
License Agreement [Line Items]
Payment for execution of agreements	$ 4,500,000
City Of Hope | Sponsored Research Agreement
License Agreement [Line Items]
Research and development					100,000	$ 100,000
Contractual obligation					$ 600,000
City Of Hope | Maximum
License Agreement [Line Items]
Development and commercialization milestone payment			$ 3,200,000
City Of Hope | Maximum | Sponsored Research Agreement
License Agreement [Line Items]
Collaborative arrangement research and development services				$ 1,100,000
California Institute Of Technology | Collaboration Agreement
License Agreement [Line Items]
Payment for execution of agreements	$ 100,000
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)	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	21.00%	34.00%
Tax credits		17.70%	4.80%
State taxes, net of federal tax benefit		5.50%	5.00%
Change in tranche liability			(1.00%)
Non-deductible expenses		(0.30%)	(1.80%)
Impact of federal rate change			(15.20%)
Change in valuation allowance		(43.90%)	(25.80%)

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating losses	$ 14,848	$ 9,180
R&D credits	12,506	1,776
Deferred revenue	8,159
Deferred rent	2,854	113
Capitalized R&D costs	1,375	1,362
Equity compensation	443	27
Accrued expense and other	115	196
Total deferred tax assets	40,300	12,654
Deferred tax liabilities:
Depreciation	(2,680)	(166)
Total deferred tax liabilities	(2,680)	(166)
Valuation allowance	$ (37,620)	$ (12,488)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
U.S. corporate tax rate	21.00%	21.00%	34.00%
Income tax expense		$ 0	$ 0
Increase in valuation allowance due to deferred tax assets		25,100,000	7,700,000
Uncertain tax positions		0
Income tax expense, interest or penalties		0	$ 0
Orphan Drug
Income Tax Disclosure [Line Items]
Tax credit carryforwards		$ 7,300,000
Tax credit carryforwards expiration year		2038
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards		$ 52,400,000
Net operating loss carryforwards indefinitely		21,300,000
Federal | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards		2,500,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards		46,600,000
State | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards		$ 3,500,000
Federal and State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards expiration year		2038
Federal and State | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards expiration year		2038

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Details) $ / shares in Units, $ in Thousands	Apr. 02, 2018USD ($)$ / sharesshares	Nov. 06, 2017USD ($)shares	Jul. 28, 2017USD ($)$ / sharesshares	Feb. 10, 2017USD ($)$ / sharesshares	Dec. 22, 2015USD ($)$ / sharesshares	Dec. 31, 2015Trancheshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Feb. 09, 2017USD ($)
Temporary Equity [Line Items]
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses | $							$ 150,843
Conversion of convertible preferred stock into common stock upon initial public offering, Shares	24,168,656
Proceeds from issuance of convertible preferred stock, net of issuance costs | $			$ 83,500
Changes in fair value of convertible preferred stock tranche liability | $								$ (876)
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized						62,269,145		62,304,354
Number of tranches | Tranche						2
Preferred stock shares issued				28,873,237	28,873,237			28,873,237
Issue price, per share | $ / shares				$ 0.71	$ 0.71
Proceeds from issuance of convertible preferred stock, net of issuance costs | $				$ 20,500	$ 20,500			$ 20,479
Issuance costs | $					$ 100
Convertible preferred stock, shares issued				28,873,237	4,522,670			62,269,144
Additional shares granted to purchase					28,873,237
Fair value of convertible preferred stock tranche liability | $					$ 6,200
Fair value of convertible preferred stock tranche liability remeasured | $				$ 5,100					$ 5,100
Series A Convertible Preferred Stock | Maximum
Temporary Equity [Line Items]
Issuance costs | $				$ 100
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			64,930,561					64,930,561
Preferred stock shares issued								64,930,561
Issue price, per share | $ / shares			$ 1.44
Proceeds from issuance of convertible preferred stock, net of issuance costs | $		$ 10,000						$ 94,768
Issuance costs | $			$ 400
Convertible preferred stock, shares issued		6,944,445	57,986,116					64,930,561
Preferred stock estimated fair value | $		$ 11,700
Common Stock
Temporary Equity [Line Items]
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares							10,350,000
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses | $	$ 150,800
Conversion of convertible preferred stock into common stock upon initial public offering, Shares							24,168,656
Initial Public Offering
Temporary Equity [Line Items]
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares	10,350,000
Initial Public Offering | Common Stock
Temporary Equity [Line Items]
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares	10,350,000
Offering price | $ / shares	$ 16
Conversion of convertible preferred stock into common stock upon initial public offering, Shares	24,168,656

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Fair Value of Convertible Preferred Stock Tranche Liability Determined Using Option Pricing Model (Details)	Feb. 09, 2017yr
Probability of Milestone Closing
Class of Stock [Line Items]
Convertible preferred stock tranche liability, measurement input	0.999
Expected Years Closing
Class of Stock [Line Items]
Convertible preferred stock tranche liability, measurement input	0
Discount Rate
Class of Stock [Line Items]
Convertible preferred stock tranche liability, measurement input	0.010
Risk-free Interest Rate
Class of Stock [Line Items]
Convertible preferred stock tranche liability, measurement input	0.0084
Expected Dividend Yield
Class of Stock [Line Items]
Convertible preferred stock tranche liability, measurement input	0

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details)	12 Months Ended
	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.

Stock Option Plan - Additional

Stock Option Plan - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2019	Sep. 01, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2017	Feb. 28, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation				$ 2,623	$ 256
Total intrinsic value of options exercised				$ 1,800
Weighted-average grant date fair value per share for options granted				$ 10.41	$ 2.78
Stock option exercised				89,050	0
Remaining shares subject to repurchase				151,289	265,098
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities				$ 112	$ 123
2015 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issue						3,225,346	2,446,323
Stock option vesting period				4 years
Stock options expiration period				10 years
Number of 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 issue			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				1,896,926
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	1,494,341
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issue			353,980
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock			1.00%
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
Stock-based compensation				$ 100
Employee Stock Purchase 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	1.00%
Number of additional shares added to the plan	373,585
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense				$ 15,700
Unrecognized compensation expense estimated to be recognized over period				2 years 10 months 24 days

Stock Option Plan - Schedule of

Stock Option Plan - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 2,623	$ 256
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	1,117	115
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 1,506	$ 141

Stock Option Plan - Schedule _2

Stock Option Plan - Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	52.80%	52.73%
Expected volatility, maximum	60.12%	59.40%
Weighted-average risk-free interest rate, minimum	2.33%	2.04%
Weighted-average risk-free interest rate, maximum	3.08%	2.37%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 10 months 24 days
Underlying common stock fair value	$ 6.63	$ 0.63
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	7 years 7 months 6 days	8 years 7 months 6 days
Underlying common stock fair value	$ 24.28	$ 6.63

Stock Option Plan - Summary of

Stock Option Plan - Summary of Option Activity under Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Options, Outstanding at Beginning Balance	1,971,711
Number of Options, Granted	1,392,259
Number of Options, Exercised	(89,050)	0
Number of Options, Cancelled/Forfeited	(71,798)
Number of Options, Outstanding at Ending Balance	3,203,122	1,971,711
Number of Options, Vested and Expected to vest at December 31, 2018	3,203,122
Number of Options, Exercisable at December 31, 2018	716,531
Weighted-Average Exercise Price per Share, Outstanding at Beginning Balance	$ 3.61
Weighted-Average Exercise Price per Share, Granted	19.05
Weighted-Average Exercise Price per Share, Exercised	1.70
Weighted-Average Exercise Price per Share, Cancelled/Forfeited	2.72
Weighted-Average Exercise Price per Share, Outstanding at Ending Balance	10.47	$ 3.61
Weighted-Average Exercise Price per Share, Vested and Expected to vest at December 31, 2018	10.47
Weighted-Average Exercise Price Per Share, Exercisable at December 31, 2018	$ 3.82
Weighted Average Remaining Contractual Term, Outstanding	8 years 10 months 24 days	9 years 3 months 18 days
Weighted Average Remaining Contractual Term, Vested and Expected to vest at December 31, 2018	8 years 10 months 24 days
Weighted Average Remaining Contractual Term, Exercisable at December 31, 2018	8 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding Ending Balance	$ 39,018	$ 5,977
Aggregate Intrinsic Value, Vested and Expected to vest at December 31, 2018	39,018
Aggregate Intrinsic Value, Exercisable at December 31, 2018	$ 13,287

Stock Option Plan - Summary o_2

Stock Option Plan - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	12 Months Ended
	Dec. 31, 2018shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares—Beginning Balance	265,098
Vested	(129,959)
Issued	16,150
Unvested shares—Ending Balance	151,289

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, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	3,354,411	26,405,465
Convertible Preferred Shares (as Converted to Common Stock)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share		24,168,656
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	151,289	265,098
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	3,203,122	1,971,711

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Discretionary match made under the 401(k) Plan by employer	$ 300,000	$ 0

Collaboration and License Agr_2

Collaboration and License Agreement - Additional Information (Details)	Jul. 28, 2017USD ($)	Nov. 30, 2017USD ($)TargetCandidateorProduct	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from issuance of shares of preferred stock	$ 83,500,000
Collaboration agreement revenue recognized			$ 3,684,000
Novartis
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront, nonrefundable payment received		$ 35,000,000
Collaborative agreement target fee		$ 5,000,000
Collaboration agreement, research term		5 years
Allocated to collaboration agreement		$ 33,300,000
Upfront nonrefundable consideration allocated to combined unit of accounting		$ 33,300,000
Novartis | Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognition on straight-line basis over estimated period of performance for combined unit of accounting		10 years
Collaboration agreement revenue recognized			3,700,000	$ 0
Deferred revenue			33,200,000
Novartis | Collaboration Agreement | Prepaid Expenses and Other Current Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivable			$ 800,000
Novartis | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected | Target		4
Number of targets during research term | Target		4
Collaboration agreement number of candidates or products for develop and commercialize | CandidateorProduct		4
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