Q32 Bio (QTTB) 10-Q 14 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	May 11, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	2,018
Entity Registrant Name	Homology Medicines, Inc.
Entity Central Index Key	1,661,998
Trading Symbol	FIXX
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		37,446,969

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
CURRENT ASSETS:
Cash and cash equivalents	$ 46,213,413	$ 51,574,932
Short-term investments	70,519,061	78,083,604
Prepaid expenses and other current assets	3,672,428	1,944,751
Total current assets	120,404,902	131,603,287
Property and equipment, net	3,828,104	3,154,205
Deferred offering costs	3,072,558	1,000,262
Restricted cash	1,772,587	1,772,587
TOTAL ASSETS	129,078,151	137,530,341
CURRENT LIABILITIES:
Accounts payable	3,776,474	2,538,057
Accrued expenses and other liabilities	2,258,037	2,860,025
Deferred rent		122,601
Deferred revenue	3,407,555	3,341,063
Total current liabilities	9,442,066	8,861,746
NON-CURRENT LIABILITIES:
Deferred rent, net of current portion	1,552,358	290,923
Deferred revenue, net of current portion	29,846,448	30,069,563
Total liabilities	40,840,872	39,222,232
COMMITMENTS (NOTE 7)
CONVERTIBLE PREFERRED STOCK:
Total convertible preferred stock	137,762,160	137,762,160
STOCKHOLDERS’ DEFICIT:
Common stock, $0.0001 par value; 170,000,000 shares authorized as of March 31, 2018 and December 31, 2017; 2,928,313 and 2,902,109 shares issued as of March 31, 2018 and December 31, 2017, respectively; and 2,697,352 and 2,637,011 shares outstanding as of March 31, 2018 and December 31, 2017, respectively	270	264
Additional paid-in capital	1,264,652	799,859
Accumulated other comprehensive loss	(86,165)	(73,308)
Accumulated deficit	(50,703,638)	(40,180,866)
Total stockholders’ deficit	(49,524,881)	(39,454,051)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ DEFICIT	129,078,151	137,530,341
Series A Convertible Preferred Stock
CONVERTIBLE PREFERRED STOCK:
Total convertible preferred stock	42,994,550	42,994,550
Series B Convertible Preferred Stock
CONVERTIBLE PREFERRED STOCK:
Total convertible preferred stock	$ 94,767,610	$ 94,767,610

CONDENSED CONSOLIDATED BALANCE3

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	170,000,000	170,000,000
Common stock, shares issued	2,928,313	2,902,109
Common stock, shares outstanding	2,697,352	2,637,011
Series A Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	62,304,354	62,304,354
Convertible preferred stock, shares issued	62,269,144	62,269,144
Convertible preferred stock, shares outstanding	62,269,144	62,269,144
Convertible preferred stock, liquidation preference	$ 44,211,092	$ 44,211,092
Series B Convertible Preferred Stock
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	64,930,561	64,930,561
Convertible preferred stock, shares issued	64,930,561	64,930,561
Convertible preferred stock, shares outstanding	64,930,561	64,930,561
Convertible preferred stock, liquidation preference	$ 93,500,008	$ 93,500,008

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue	$ 823,372
Operating expenses:
Research and development	7,997,605	$ 2,799,136
General and administrative	3,828,022	1,892,382
Total operating expenses	11,825,627	4,691,518
Loss from operations	(11,002,255)	(4,691,518)
Other income:
Change in fair value of convertible preferred stock tranche liability		(876,000)
Interest income	479,483	12,725
Total other income (expense)	479,483	(863,275)
Net loss and net loss attributable to common stockholders-basic and diluted	$ (10,522,772)	$ (5,554,793)
Net loss per share attributable to common stockholders-basic and diluted	$ (4.21)	$ (2.79)
Weighted average common shares outstanding-basic and diluted	2,500,178	1,992,014

CONDENSED CONSOLIDATED STATEME5

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (UNAUDITED) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (10,522,772)	$ (5,554,793)
Other comprehensive loss:
Unrealized losses on available for sale securities, net	(12,857)
Total other comprehensive loss	(12,857)
Comprehensive loss	$ (10,535,629)	$ (5,554,793)

CONDENSED CONSOLIDATED STATEME6

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (10,522,772)	$ (5,554,793)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	220,596	130,156
Stock-based compensation expense	380,120	27,680
Accretion on short-term investments	(123,258)
Change in fair value associated with convertible preferred stock tranche liability		876,000
Changes in operating assets and liabilities:
Prepaid expense and other current assets	(1,727,680)	(390,919)
Accounts payable	510,889	350,402
Accrued expenses and other liabilities	(763,948)	(69,114)
Deferred revenue	(156,622)
Deferred rent	1,138,834	105,753
Net cash used in operating activities	(11,043,841)	(4,524,835)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of short-term investments	(18,595,056)
Maturities of short-term investments	26,270,000
Purchases of property and equipment	(1,035,109)	(848,433)
Net cash provided by (used in) investing activities	6,639,835	(848,433)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	69,263
Payment of deferred offering costs	(1,026,776)
Proceeds from issuance of Series A convertible preferred stock, net of issuance costs		20,479,488
Net cash (used in) provided by financing activities	(957,513)	20,479,488
(DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(5,361,519)	15,106,220
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	51,574,932	11,392,207
CASH AND CASH EQUIVALENTS, END OF PERIOD	46,213,413	26,498,427
SUPPLEMENTAL DISCLOSURES OF NONCASH FINANCING ACTIVITIES:
Reclassification of liability for common stock vested	15,416	20,605
Property and equipment additions included in accounts payable	26,980	$ 214,734
Deferred offering costs included in accounts payable and accrued expenses	$ 2,045,782

Nature of Business and Basis of

Nature of Business and Basis of Presentation	3 Months Ended
	Mar. 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. 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 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 dependent 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 March 31, 2018, the Company has financed its operations primarily through the issuance of convertible preferred stock and with proceeds from its collaboration and license agreement with Novartis (see Note 10). 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. The Company expects to incur additional operating losses and negative operating cash flows for the foreseeable future. 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 existing 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. The financial statements as of March 31, 2018, including share and per share amounts, do not give effect to the IPO, as it closed subsequent to March 31, 2018. Management believes that existing cash, cash equivalents and short-term investments will allow the Company to continue its operations for at least two years from the issuance date of these condensed consolidated financial statements. 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 unaudited condensed consolidated financial statements as of March 31, 2018 and for the three months ended March 31, 2018 and 2017 have been prepared by the Company in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2017 included in the Company's final prospectus that forms a part of the Company’s Registration Statement on Form S-1 (Reg. No. 333-223409), filed with the SEC pursuant to Rule 424(b)(4) on March 29, 2018. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2018 and consolidated results of operations and cash flows for the three months ended March 31, 2018 and 2017. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Principles of Consolidation —The Company’s condensed 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 condensed 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 condensed consolidated financial statements include, but are not limited to, revenue recognition, 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. 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 maturities of 90 days or less at the time of purchase 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’ 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 condensed 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. 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 10). 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 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. For arrangements identified with multiple units of accounting, an allocation of the consideration is performed. 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. Net Loss per Share— Net loss per share and 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 attributable to common stockholders is 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 contains participation rights in any dividend paid by the Company and is deemed to be a participating security. Net loss attributable to common stockholders and participating preferred shares are 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. The Company reported a net loss attributable to common stockholders for the three months ended March 31, 2018 and 2017. 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”) In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting ASU No. 2016-09 was adopted by the Company on January 1, 2018 and the adoption did not have a material impact on its condensed consolidated financial statements. 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),

Cash, Cash Equivalents and Shor

Cash, Cash Equivalents and Short-Term Investments	3 Months Ended
	Mar. 31, 2018
Cash Cash Equivalents And Available For Sale Securities [Abstract]
INVESTMENTS	3. 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’ deficit, on the Company’s condensed consolidated balance sheets. The following table summarizes the Company’s investments, which are included in cash equivalents and short-term investments, as of March 31, 2018 and December 31, 2017: March 31, 2018 Amortized Cost Unrealized Gains Unrealized Loss Fair Value Cash equivalents and short-term investments Money market funds $ 28,831,110 $ — $ — $ 28,831,110 Asset-backed securities 8,099,245 — (8,721 ) 8,090,524 Commercial paper 32,277,246 — — 32,277,246 Corporate debt securities 21,433,498 — (53,474 ) 21,380,024 U.S. Treasury securities 24,916,290 — (23,970 ) 24,892,320 Total $ 115,557,389 $ — $ (86,165 ) $ 115,471,224 December 31, 2017 Amortized Cost Unrealized Gains Unrealized Loss Fair Value Cash equivalents and short-term investments Money market funds $ 44,181,756 $ — $ — $ 44,181,756 Asset-backed securities 7,428,021 — (6,318 ) 7,421,703 Commercial paper 34,882,298 — — 34,882,298 Corporate debt securities 26,905,815 — (49,532 ) 26,856,283 U.S. Treasury securities 8,940,778 — (17,458 ) 8,923,320 Total $ 122,338,668 $ — $ (73,308 ) $ 122,265,360 As of March 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 three months ended March 31, 2018 and year ended December 31, 2017. The contractual maturity dates of all of the Company’s investments are less than one year.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	4. 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 At March 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Money market funds $ 28,831,110 $ 28,831,110 $ — $ — Asset-backed securities 2,697,840 — 2,697,840 — Commercial paper 13,423,213 — 13,423,213 — Short-term investments: Asset-backed securities 5,392,684 — 5,392,684 — Commercial paper 18,854,033 — 18,854,033 — Corporate debt securities 21,380,024 — 21,380,024 — U.S. treasury securities 24,892,320 — 24,892,320 — Total financial assets $ 115,471,224 $ 28,831,110 $ 86,640,114 $ — Description At December 31, 2017 Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Money market funds $ 44,181,756 $ 44,181,756 $ — $ — Short-term investments: Asset-backed securities 7,421,703 — 7,421,703 — Commercial paper 34,882,298 — 34,882,298 — Corporate debt securities 26,856,283 — 26,856,283 — U.S. treasury securities 8,923,320 — 8,923,320 — Total financial assets $ 122,265,360 $ 44,181,756 $ 78,083,604 $ — 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 was stated at fair value and was measured using a Level 3 input because the fair value measurement is 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 period ended March 31, 2017 consisted of: Fair value as of January 1, 2017 $ 4,247,000 Change in fair value of convertible preferred stock tranche liability 876,000 Reduction in tranche liability due to preferred stock issuance (5,123,000 ) Fair value as of March 31, 2017 $ — There were no transfers between fair value measure levels during the three months ended March 31, 2018 and 2017.

Accrued Expenses And Other Liab

Accrued Expenses And Other Liabilities	3 Months Ended
	Mar. 31, 2018
Payables And Accruals [Abstract]
ACCRUED EXPENSES AND OTHER LIABILITIES	5. ACCRUED EXPENSES AND OTHER LIABILITIES Accrued expenses and other liabilities at March 31, 2018 and December 31, 2017 consist of the following: As of March 31, 2018 December 31, 2017 Accrued professional fees $ 1,156,451 $ 1,119,959 Accrued compensation and benefits 844,451 1,435,015 Accrued unvested common stock subject to repurchase 107,135 122,551 Accrued research and development expenses 150,000 182,500 $ 2,258,037 $ 2,860,025

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	6. 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 March 31, 2018, no expansion rights had been exercised. 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 will be due under the lease in two phases with rent on the first 46,000 square feet starting in September 2018 and with rent on the remaining 21,000 square feet starting 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 as of March 31, 2018 are as follows: Years Ending December 31 Amount 2018 $ 1,312,813 2019 3,505,225 2020 3,750,149 2021 3,696,598 2022 2,928,014 Thereafter 13,177,338 Total future minimum lease payments $ 28,370,137 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 condensed consolidated balance sheets, which are amortized to rent expense over the term of the lease. As of March 31, 2018, deferred rent incentives totaled $470,000. Rent expense for three months ended March 31, 2018 and 2017 was $900,017 and $229,663, respectively. The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $1.8 million as of March 31, 2018 and December 31, 2017, in conjunction with its current leases.

Stock Incentive Plans

Stock Incentive Plans	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK INCENTIVE PLANS	7. STOCK INCENTIVE PLANS 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 issuance 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 March 31, 2018, there were 453,176 shares subject to outstanding options and no additional shares available for future grant under the 2015 Plan. In March 2018, the Company’s Board of Directors adopted and the Company’s stockholders approved the Homology Medicines, Inc. 2018 Incentive Plan (the “2018 Plan” and, together with the 2015 Plan, the “Plans”), which became effective upon the effectiveness of the registration statement on Form S-1 for the Company’s initial public offering. 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, nonstatutory stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards. The number of shares of common stock initially available for issuance under the 2018 Plan is 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. At March 31, 2018, there were 2,454,493 shares available for future grant under the 2018 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 “ESPP”). The 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 ESPP is 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 ESPP. As of March 31, 2018, there were no purchase rights outstanding under the ESPP. Total stock-based compensation expense recorded as research and development and general and administrative expenses, respectively, for employees, directors and non-employees for the three months ended March 31, 2018 and 2017 is as follows: Three Months Ended March 31, 2018 2017 Research and development $ 133,042 $ 10,861 General and administrative 247,078 16,819 $ 380,120 $ 27,680 As of March 31, 2018, there was $9.5 million of unrecognized compensation expense related to unvested employee and non-employee share-based compensation arrangements granted under the Plans. The unrecognized compensation expense is estimated to be recognized over a period of 3.65 years at March 31, 2018. 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 option was used for the expected life of nonemployees. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk-free rate for periods within the expected life of the option is based upon the U.S. Treasury yield curve in effect at the time of grant. 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 the awards that were granted prior to 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. The assumptions used in Black-Scholes option pricing model for the three months ended March 31, 2018 and 2017 are as follows: Three Months Ended March 31, 2018 2017 Expected volatility 52.80% - 60.12% 53.01% - 59.40% Weighted-average risk-free interest rate 2.33% - 2.69% 2.22% - 2.31% Expected dividend yield — % — % Expected term (in years) 5.5 - 7.6 6.3 - 8.6 Underlying common stock fair value $6.63 - $16.00 $ 0.63 A summary of option activity under the Plans for the three months ended March 31, 2018 is as follows: Number of Options Weighted- Average Exercise Price per Share Weighted Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value Outstanding at January 1, 2018 1,971,711 $ 3.61 9.3 $ 5,977,162 Granted 755,507 $ 15.71 Exercised (26,204 ) $ 2.64 Cancelled/Forfeited (16,609 ) $ 1.28 Outstanding at March 31, 2018 2,684,405 $ 7.03 9.3 $ 31,329,115 Exercisable at March 31, 2018 216,985 $ 0.41 8.0 $ 3,969,527 Vested and expected to vest at March 31, 2018 2,684,405 $ 7.03 9.3 $ 31,329,115 The total intrinsic value of options exercised during the three months ended March 31, 2018 was $391,913. There were no option exercises in 2017. The weighted-average grant date fair value of options granted during the three months ended March 31, 2018 and 2017 was $8.44 and $0.34, 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 (34,137 ) Repurchased — Unvested shares - March 31, 2018 230,961 As of March 31, 2018 and December 31, 2017, 230,961 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 condensed consolidated balance sheets of $107,135 and $122,551, respectively.

Convertible Preferred Stock and

Convertible Preferred Stock and Stockholders' Equity	3 Months Ended
	Mar. 31, 2018
Temporary Equity Disclosure [Abstract]
CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY	8. CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY 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 $151.0 million, after deducting underwriting discounts and commissions and estimated 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.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
NET LOSS PER SHARE	9. 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 March 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: Three Months Ended March 31, 2018 2017 Convertible preferred shares (as converted to common stock) 24,168,656 11,831,489 Unvested common stock from early exercise of options 230,961 618,742 Stock options to purchase common stock 2,684,405 734,218 Total 27,084,022 13,184,449

Collaboration and License Agree

Collaboration and License Agreement	3 Months Ended
	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AND LICENSE AGREEMENT	10. 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. Under the terms of the Collaboration Agreement, 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, the ex vivo in vivo in vivo The Collaboration Agreement consists of a research term, where the Company and Novartis will collaborate to perform research and conduct preclinical development to identify candidates that modulate the ophthalmic targets and sickle cell disease targets. Novartis may select up to four targets, with limited substitution rights. The Company will be 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 will be 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, with the exception of the right to commercialize in the U.S. the in vivo in vivo in vivo In accordance with the Collaboration Agreement, the Company is also eligible to receive up to a total of $960.0 million in milestone payments, including up to $335.0 million in development milestone payments, up to $275.0 million in regulatory milestone payments and up to $350.0 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, which royalties are potentially subject to various reductions and offsets. 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. There are no performance, cancellation, termination or refund provisions in the arrangement that contain material financial consequences to the Company. Revenue Recognition The Company evaluated the terms of the Collaboration Agreement and determined the development and commercialization activities related to the in vivo 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, and allocated the remaining $33.3 million to the Collaboration Agreement. The Company believes the consideration it will receive for the manufacturing services during the commercialization term, when and if it provides such services, is representative of the best estimate of selling price of the services. Therefore, the entire $33.3 million of upfront nonrefundable consideration was allocated to the combined unit of accounting. At the inception of the Collaboration Agreement, the Company could not reasonably estimate 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 $823,000 for the three months ended March 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 amount recorded as deferred revenue under this agreement totaled $33.3 million as of March 31, 2018.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	11. SUBSEQUENT EVENTS Initial Public Offering 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 $151.0 million, after deducting underwriting discounts and commissions and estimated 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.

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation — The accompanying unaudited condensed consolidated financial statements as of March 31, 2018 and for the three months ended March 31, 2018 and 2017 have been prepared by the Company in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and, pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2017 included in the Company's final prospectus that forms a part of the Company’s Registration Statement on Form S-1 (Reg. No. 333-223409), filed with the SEC pursuant to Rule 424(b)(4) on March 29, 2018. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company’s financial position as of March 31, 2018 and consolidated results of operations and cash flows for the three months ended March 31, 2018 and 2017. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018.
Principles of Consolidation	Principles of Consolidation —The Company’s condensed consolidated financial statements include the accounts of the Company and Homology Medicines Securities Corporation, a wholly owned Massachusetts corporation, for the s ole purpose of buying, selling, and holding securities on the Company’s behalf. All intercompany balances and transactions have been eliminated in the condensed 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 condensed consolidated financial statements include, but are not limited to, revenue recognition, 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.
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 maturities of 90 days or less at the time of purchase 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’ 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 condensed 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.
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 10). 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	Multiple Element Arrangements The terms of the Collaboration 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. For arrangements identified with multiple units of accounting, an allocation of the consideration is performed. 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.
Net Loss per Share and Basic net loss per share	Net Loss per Share— Net loss per share and 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 attributable to common stockholders is 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 contains participation rights in any dividend paid by the Company and is deemed to be a participating security. Net loss attributable to common stockholders and participating preferred shares are 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. The Company reported a net loss attributable to common stockholders for the three months ended March 31, 2018 and 2017.
Recent Accounting Pronouncements	Recent Accounting Pronouncements —The Jumpstart Our Business Startups Act of 2012 permits an emerging growth company to t ake 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”) In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), In March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting ASU No. 2016-09 was adopted by the Company on January 1, 2018 and the adoption did not have a material impact on its condensed consolidated financial statements. 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),

Cash, Cash Equivalents and Sh19

Cash, Cash Equivalents and Short-Term Investments (Tables)	3 Months Ended
	Mar. 31, 2018
Cash Cash Equivalents And Available For Sale Securities [Abstract]
Summary of Investments Included in Cash Equivalents and Short Term Investments	The following table summarizes the Company’s investments, which are included in cash equivalents and short-term investments, as of March 31, 2018 and December 31, 2017: March 31, 2018 Amortized Cost Unrealized Gains Unrealized Loss Fair Value Cash equivalents and short-term investments Money market funds $ 28,831,110 $ — $ — $ 28,831,110 Asset-backed securities 8,099,245 — (8,721 ) 8,090,524 Commercial paper 32,277,246 — — 32,277,246 Corporate debt securities 21,433,498 — (53,474 ) 21,380,024 U.S. Treasury securities 24,916,290 — (23,970 ) 24,892,320 Total $ 115,557,389 $ — $ (86,165 ) $ 115,471,224 December 31, 2017 Amortized Cost Unrealized Gains Unrealized Loss Fair Value Cash equivalents and short-term investments Money market funds $ 44,181,756 $ — $ — $ 44,181,756 Asset-backed securities 7,428,021 — (6,318 ) 7,421,703 Commercial paper 34,882,298 — — 34,882,298 Corporate debt securities 26,905,815 — (49,532 ) 26,856,283 U.S. Treasury securities 8,940,778 — (17,458 ) 8,923,320 Total $ 122,338,668 $ — $ (73,308 ) $ 122,265,360

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 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 At March 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Money market funds $ 28,831,110 $ 28,831,110 $ — $ — Asset-backed securities 2,697,840 — 2,697,840 — Commercial paper 13,423,213 — 13,423,213 — Short-term investments: Asset-backed securities 5,392,684 — 5,392,684 — Commercial paper 18,854,033 — 18,854,033 — Corporate debt securities 21,380,024 — 21,380,024 — U.S. treasury securities 24,892,320 — 24,892,320 — Total financial assets $ 115,471,224 $ 28,831,110 $ 86,640,114 $ — Description At December 31, 2017 Quoted Prices in Active Markets for Identical Assets Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Money market funds $ 44,181,756 $ 44,181,756 $ — $ — Short-term investments: Asset-backed securities 7,421,703 — 7,421,703 — Commercial paper 34,882,298 — 34,882,298 — Corporate debt securities 26,856,283 — 26,856,283 — U.S. treasury securities 8,923,320 — 8,923,320 — Total financial assets $ 122,265,360 $ 44,181,756 $ 78,083,604 $ —
Schedule of Reconciliations 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 period ended March 31, 2017 consisted of: Fair value as of January 1, 2017 $ 4,247,000 Change in fair value of convertible preferred stock tranche liability 876,000 Reduction in tranche liability due to preferred stock issuance (5,123,000 ) Fair value as of March 31, 2017 $ —

Accrued Expenses And Other Li21

Accrued Expenses And Other Liabilities (Tables)	3 Months Ended
	Mar. 31, 2018
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities at March 31, 2018 and December 31, 2017 consist of the following: As of March 31, 2018 December 31, 2017 Accrued professional fees $ 1,156,451 $ 1,119,959 Accrued compensation and benefits 844,451 1,435,015 Accrued unvested common stock subject to repurchase 107,135 122,551 Accrued research and development expenses 150,000 182,500 $ 2,258,037 $ 2,860,025

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	Future minimum lease payments as of March 31, 2018 are as follows: Years Ending December 31 Amount 2018 $ 1,312,813 2019 3,505,225 2020 3,750,149 2021 3,696,598 2022 2,928,014 Thereafter 13,177,338 Total future minimum lease payments $ 28,370,137

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	3 Months Ended
	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense	Total stock-based compensation expense recorded as research and development and general and administrative expenses, respectively, for employees, directors and non-employees for the three months ended March 31, 2018 and 2017 is as follows: Three Months Ended March 31, 2018 2017 Research and development $ 133,042 $ 10,861 General and administrative 247,078 16,819 $ 380,120 $ 27,680
Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model	The assumptions used in Black-Scholes option pricing model for the three months ended March 31, 2018 and 2017 are as follows: Three Months Ended March 31, 2018 2017 Expected volatility 52.80% - 60.12% 53.01% - 59.40% Weighted-average risk-free interest rate 2.33% - 2.69% 2.22% - 2.31% Expected dividend yield — % — % Expected term (in years) 5.5 - 7.6 6.3 - 8.6 Underlying common stock fair value $6.63 - $16.00 $ 0.63
Summary of Option Activity under Plans	A summary of option activity under the Plans for the three months ended March 31, 2018 is as follows: Number of Options Weighted- Average Exercise Price per Share Weighted Average Remaining Contractual Term (in Years) Aggregate Intrinsic Value Outstanding at January 1, 2018 1,971,711 $ 3.61 9.3 $ 5,977,162 Granted 755,507 $ 15.71 Exercised (26,204 ) $ 2.64 Cancelled/Forfeited (16,609 ) $ 1.28 Outstanding at March 31, 2018 2,684,405 $ 7.03 9.3 $ 31,329,115 Exercisable at March 31, 2018 216,985 $ 0.41 8.0 $ 3,969,527 Vested and expected to vest at March 31, 2018 2,684,405 $ 7.03 9.3 $ 31,329,115
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 (34,137 ) Repurchased — Unvested shares - March 31, 2018 230,961

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 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 March 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: Three Months Ended March 31, 2018 2017 Convertible preferred shares (as converted to common stock) 24,168,656 11,831,489 Unvested common stock from early exercise of options 230,961 618,742 Stock options to purchase common stock 2,684,405 734,218 Total 27,084,022 13,184,449

Nature of Business and Basis 25

Nature of Business and Basis of Presentation - Additional Information (Details)	Apr. 02, 2018USD ($)$ / sharesshares	Mar. 12, 2018	Mar. 31, 2018USD ($)	May 14, 2018shares
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from issuance of common stock | $			$ 69,263
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
Subsequent Event
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Convertible preferred shares converted to common stock	24,168,656
Initial Public Offering | Subsequent Event
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 | $	$ 165,600,000
Convertible preferred shares converted to common stock	24,168,656
Preferred stock outstanding				0

Summary of Significant Accoun26

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended
	Mar. 31, 2018USD ($)
Accounting Policies [Abstract]
Royalty revenue earned	$ 0

Investments - Summary of Invest

Investments - Summary of Investments Included in Cash Equivalents and Short Term Investments (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost	$ 115,557,389	$ 122,338,668
Unrealized Loss	(86,165)	(73,308)
Fair Value	115,471,224	122,265,360
Asset-backed Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	8,099,245	7,428,021
Cash equivalents and short-term investments, Unrealized Loss	(8,721)	(6,318)
Cash equivalents and short-term investments, Fair Value	8,090,524	7,421,703
Money market funds
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	28,831,110	44,181,756
Cash equivalents and short-term investments, Fair Value	28,831,110	44,181,756
Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	32,277,246	34,882,298
Cash equivalents and short-term investments, Fair Value	32,277,246	34,882,298
Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	21,433,498	26,905,815
Cash equivalents and short-term investments, Unrealized Loss	(53,474)	(49,532)
Cash equivalents and short-term investments, Fair Value	21,380,024	26,856,283
U.S Treasury Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash equivalents and short-term investments, Amortized Cost	24,916,290	8,940,778
Cash equivalents and short-term investments, Unrealized Loss	(23,970)	(17,458)
Cash equivalents and short-term investments, Fair Value	$ 24,892,320	$ 8,923,320

Investments - Additional Inform

Investments - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 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

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 70,519,061	$ 78,083,604
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	115,471,224	122,265,360
Fair Value, Measurements, Recurring | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	28,831,110	44,181,756
Fair Value, Measurements, Recurring | Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	2,697,840
Short-term investments	5,392,684	7,421,703
Fair Value, Measurements, Recurring | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	13,423,213
Short-term investments	18,854,033	34,882,298
Fair Value, Measurements, Recurring | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	21,380,024	26,856,283
Fair Value, Measurements, Recurring | U.S Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	24,892,320	8,923,320
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	28,831,110	44,181,756
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	28,831,110	44,181,756
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total financial assets	86,640,114	78,083,604
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]
Cash equivalents	2,697,840
Short-term investments	5,392,684	7,421,703
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	13,423,213
Short-term investments	18,854,033	34,882,298
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	21,380,024	26,856,283
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	$ 24,892,320	$ 8,923,320

Fair Value Measurements - Sch30

Fair Value Measurements - Schedule of Reconciliations of Changes in the Fair Value of Financial Instruments (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Change in fair value associated with convertible preferred stock tranche liability	$ 876,000
Significant Unobservable Inputs (Level 3)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value as of January 1, 2017	$ 4,247,000	$ 4,247,000
Change in fair value associated with convertible preferred stock tranche liability		876,000
Reduction in tranche liability due to preferred stock issuance		$ (5,123,000)

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Transfers between fair value measure levels	$ 0	$ 0

Accrued Expenses And Other Li32

Accrued Expenses And Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued professional fees	$ 1,156,451	$ 1,119,959
Accrued compensation and benefits	844,451	1,435,015
Accrued unvested common stock subject to repurchase	107,135	122,551
Accrued research and development expenses	150,000	182,500
Accrued expenses	$ 2,258,037	$ 2,860,025

Commitment and Contingencies -

Commitment and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended
	Dec. 31, 2017USD ($)ft²Phase	Sep. 30, 2016	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)
Commitment And Contingencies [Line Items]
Deferred rent incentives			$ 470,000
Rent expense			900,017	$ 229,663
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit	$ 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 expiration year and month	2027-02	2021-10
Operating lease agreements additional term		3 years
Office space leased | ft²	67,000
Lessee, operating lease, lease not yet commenced, renewal term	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
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000

Commitment and Contingencies 34

Commitment and Contingencies - Schedule of Future Minimum Lease Payments (Details)	Mar. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 1,312,813
2,019	3,505,225
2,020	3,750,149
2,021	3,696,598
2,022	2,928,014
Thereafter	13,177,338
Total future minimum lease payments	$ 28,370,137

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jul. 31, 2017	Feb. 28, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total intrinsic value of options exercised	$ 391,913
Stock options exercised	26,204		0
Weighted-average grant date fair value of options granted	$ 8.44	$ 0.34
Remaining shares subject to repurchase	230,961		265,098
Liability for unvested common shares subject to repurchase included in accrued expenses and other liabilities	$ 107,135		$ 122,551
2015 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance				3,225,346	2,446,323
Stock option vesting period	4 years
Stock options expiration period	10 years
Number of shares outstanding available for future grant	453,176
Number of additional available for future grant	0
2018 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance	3,186,205
Number of shares outstanding available for future grant	2,454,493
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
2018 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for issuance	353,980
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	1.00%
Maximum shares allowed to be issued under ESPP	4,778,738
2015 and 2018 Stock Incentive Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense	$ 9,500,000
Unrecognized compensation expense estimated to be recognized over period	3 years 7 months 24 days

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 380,120	$ 27,680
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	133,042	10,861
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 247,078	$ 16,819

Stock Incentive Plans - Sched37

Stock Incentive Plans - Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	52.80%	53.01%
Expected volatility, maximum	60.12%	59.40%
Weighted-average risk-free interest rate, minimum	2.33%	2.22%
Weighted-average risk-free interest rate, maximum	2.69%	2.31%
Underlying common stock fair value		$ 0.63
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 8 months 12 days	6 years 3 months 18 days
Underlying common stock fair value	$ 6.63
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	$ 18.70

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Option Activity under Plans (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 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	755,507
Number of Options, Exercised	(26,204)	0
Number of Options, Cancelled/Forfeited	(16,609)
Number of Options, Outstanding at Ending Balance	2,684,405	1,971,711
Number of Options, Exercisable at March 31, 2018	216,985
Number of Options, Vested and expected to vest at March 31, 2018	2,684,405
Weighted-Average Exercise Price per Share, Outstanding at Beginning Balance	$ 3.61
Weighted-Average Exercise Price per Share, Granted	15.71
Weighted-Average Exercise Price per Share, Exercised	2.64
Weighted-Average Exercise Price per Share, Cancelled/Forfeited	1.28
Weighted-Average Exercise Price per Share, Outstanding at Ending Balance	7.03	$ 3.61
Weighted-Average Exercise Price per Share, Exercisable at March 31, 2018	0.41
Weighted-Average Exercise Price per Share, Vested and expected to vest at March 31, 2018	$ 7.03
Weighted Average Remaining Contractual Term, Outstanding	9 years 3 months 18 days	9 years 3 months 18 days
Weighted Average Remaining Contractual Term, Exercisable at March 31, 2018	8 years
Weighted Average Remaining Contractual Term, Vested and expected to vest at March 31, 2018	9 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding Ending Balance	$ 31,329,115	$ 5,977,162
Aggregate Intrinsic Value, Exercisable at March 31, 2018	3,969,527
Aggregate Intrinsic Value, Vested and expected to vest at March 31, 2018	$ 31,329,115

Stock Incentive Plans - Summa39

Stock Incentive Plans - Summary of Unvested Common Stock from Early Exercises Subject to Repurchase (Details)	3 Months Ended
	Mar. 31, 2018shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested shares - January 1, 2018	265,098
Vested	(34,137)
Unvested shares - March 31, 2018	230,961

Convertible Preferred Stock a40

Convertible Preferred Stock and Stockholders' Equity - Additional Information (Details) - Subsequent Event	Apr. 02, 2018USD ($)$ / sharesshares
Temporary Equity [Line Items]
Convertible preferred shares converted to common stock | shares	24,168,656
Initial Public Offering
Temporary Equity [Line Items]
Sale of common stock in initial public offering | $	$ 10,350,000
Offering price | $ / shares	$ 16
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses | $	$ 151,000,000
Convertible preferred shares converted to common stock | shares	24,168,656

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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	27,084,022	13,184,449
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	11,831,489
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	230,961	618,742
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	2,684,405	734,218

Collaboration and License Agr42

Collaboration and License Agreement - Additional Information (Details)	1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2017USD ($)TargetCandidateorProduct	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from issuance of shares of preferred stock			$ 20,479,488
Collaboration agreement revenue recognized		$ 823,372
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 | Collaborative Arrangement
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		823,000		$ 0
Deferred revenue		$ 33,300,000
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	$ 960,000,000
Collaboration agreement eligible development milestone payments eligible to receive	335,000,000
Collaboration agreement eligible regulatory milestone payments eligible to receive	275,000,000
Collaboration agreement eligible commercial milestone payments eligible to receive	350,000,000
Novartis | Series B Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from issuance of shares of preferred stock	10,000,000
Preferred stock estimated fair value	$ 11,700,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Subsequent Event $ / shares in Units, $ in Millions	Apr. 02, 2018USD ($)$ / sharesshares
Subsequent Event [Line Items]
Net proceeds from initial public offering | $	$ 151
Convertible preferred shares converted to common stock	24,168,656
Initial Public Offering
Subsequent Event [Line Items]
Common stock shares issued	10,350,000
Offering price per share | $ / shares	$ 16
Convertible preferred shares converted to common stock	24,168,656