Dianthus Therapeutics (DNTH) 10-K 3 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Jan. 31, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Registrant Name	Magenta Therapeutics, Inc.
Trading Symbol	MGTA
Entity Central Index Key	0001690585
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Ex Transition Period	false
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Shell Company	false
Entity Tax Identification Number	81-0724163
Entity Public Float			$ 246.4
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	100 Technology Square
Entity Address, City or Town	Cambridge
Entity Address, Postal Zip Code	02139
Entity Address, State or Province	MA
Entity File Number	001-38541
City Area Code	857
Local Phone Number	242-0170
Entity Common Stock, Shares Outstanding		48,556,135
ICFR Auditor Attestation Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 58,152	$ 65,071
Marketable securities	90,683	80,658
Prepaid expenses and other current assets	2,692	4,114
Total current assets	151,527	149,843
Restricted cash	1,780	1,780
Property and equipment, net	8,312	9,891
Total assets	161,619	161,514
Current liabilities:
Accounts payable	3,760	2,812
Accrued expenses and other current liabilities	7,670	11,303
Total current liabilities	11,430	14,115
Deferred rent	6,283	6,206
Total liabilities	17,713	20,321
Commitments and contingencies (Note 8)
Stockholders' Equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares authorized; 48,541,601 shares and 39,466,254 shares issued and 48,533,135 shares and 39,260,532 shares outstanding as of December 31, 2020 and 2019, respectively	49	39
Additional paid-in capital	398,311	320,641
Accumulated other comprehensive income (loss)	(23)	8
Accumulated deficit	(254,431)	(179,495)
Total stockholders' equity	143,906	141,193
Total liabilities and stockholders' equity	$ 161,619	$ 161,514

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, Shares authorized	10,000,000	10,000,000
Preferred stock, Shares issued	0	0
Preferred stock, Shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, Shares issued	48,541,601	39,466,254
Common stock, Shares outstanding	48,533,135	39,260,532

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ 50,615	$ 59,208
General and administrative	28,087	23,761
Total operating expenses	78,702	82,969
Loss from operations	(78,702)	(82,969)
Interest and other income, net	3,766	6,200
Net loss	$ (74,936)	$ (76,769)
Net loss per share, basic and diluted	$ (1.71)	$ (2.07)
Weighted average common shares outstanding, basic and diluted	43,920,121	37,014,875
Comprehensive loss:
Net loss	$ (74,936)	$ (76,769)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities	(31)	16
Total other comprehensive income (loss)	(31)	16
Total comprehensive loss	$ (74,967)	$ (76,753)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 145,648	$ 33	$ 248,349	$ (8)	$ (102,726)
Beginning balance, shares at Dec. 31, 2018		33,305,033
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs	60,276	$ 5	60,271
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs, shares		4,887,500
Vesting of restricted stock		$ 1	(1)
Vesting of restricted stock, shares		773,689
Issuance of common stock upon exercise of stock options	2,021		2,021
Issuance of common stock upon exercise of stock options, shares		294,310
Stock-based compensation expense	10,001		10,001
Unrealized gains (losses) on marketable securities	16			16
Net loss	(76,769)				(76,769)
Ending balance at Dec. 31, 2019	141,193	$ 39	320,641	8	(179,495)
Ending balance, shares at Dec. 31, 2019		39,260,532
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs	64,563	$ 9	64,554
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs, shares		8,625,000
Vesting of restricted stock, shares		184,500
Issuance of common stock upon exercise of stock options	$ 3,072	$ 1	3,071
Issuance of common stock upon exercise of stock options, shares	447,402	447,402
Issuance of common stock under Employee Stock Purchase Plan	$ 104		104
Issuance of common stock under Employee Stock Purchase Plan, shares		15,701
Stock-based compensation expense	9,941		9,941
Unrealized gains (losses) on marketable securities	(31)			(31)
Net loss	(74,936)				(74,936)
Ending balance at Dec. 31, 2020	$ 143,906	$ 49	$ 398,311	$ (23)	$ (254,431)
Ending balance, shares at Dec. 31, 2020		48,533,135

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (74,936)	$ (76,769)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,941	10,001
Depreciation and amortization expense	1,978	1,843
Loss on disposal of property and equipment	1
Net amortization (accretion) of premiums (discounts) on marketable securities	179	(1,009)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,422	(1,363)
Accounts payable	948	145
Accrued expenses and other current liabilities	(3,633)	5,089
Deferred rent	77	4,960
Net cash used in operating activities	(64,023)	(57,103)
Cash flows from investing activities:
Purchases of property and equipment	(400)	(3,060)
Purchases of marketable securities	(95,735)	(144,371)
Maturities of marketable securities	85,500	148,963
Net cash provided by (used in) investing activities	(10,635)	1,532
Cash flows from financing activities:
Proceeds from public offerings, net of underwriting discounts and commissions	64,860	60,874
Payments of public offering costs	(297)	(598)
Proceeds from exercise of common stock options	3,072	2,021
Proceeds from issuance of common stock under Employee Stock Purchase Plan	104
Net cash provided by financing activities	67,739	62,297
Net increase (decrease) in cash, cash equivalents and restricted cash	(6,919)	6,726
Cash, cash equivalents and restricted cash at beginning of period	66,851	60,125
Cash, cash equivalents and restricted cash at end of period	$ 59,932	$ 66,851

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Magenta Therapeutics, Inc. (the “Company”) is a clinical-stage biotechnology company developing novel medicines to bring the curative power of stem cell transplants to more patients with blood cancers, genetic diseases and autoimmune diseases. The Company was incorporated under the laws of the State of Delaware in June 2015 . The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the continuing impact of the novel coronavirus (“COVID-19”) In May 2019, the Company issued and sold 4,887,500 shares of its common stock, including the underwriters’ exercise in full of their option to purchase additional shares of common stock, in a follow-on In June 2020, the Company issued and sold shares of its common stock, including the underwriters’ exercise in full of their option to purchase additional shares of common stock, in a follow-on public offering at a public offering price of $ per share, resulting in net proceeds of $ million after underwriting discounts and commissions and other offering expenses. The Company has a shelf registration statement on Form S-3 “at-the-market” The Company has incurred recurring losses since inception, including net losses of $74.9 million and $76.8 million for the years ended December 31, 2020 and 2019, respectively. As of December 31, 2020, the Company had an accumulated deficit of $254.4 million. The Company expects to continue to generate operating losses for the foreseeable future. The Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of these consolidated financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to fund its operations. The Company expects its expenses to increase substantially in connection with ongoing activities, particularly as the Company advances its preclinical activities and clinical trials for its product candidates in development. Accordingly, the Company will need to obtain substantial additional funding in connection with continuing operations. If the Company is unable to raise capital when needed, or on attractive terms, it could be forced to delay, reduce or eliminate its research or drug development programs or any future commercialization

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company maintains all cash, cash equivalents and marketable securities at two accredited financial institutions in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs. Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents. Marketable Securities The Company’s marketable securities are classified as available-for-sale and Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Lab equipment 5 years Computer equipment 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of life of lease or estimated useful life Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2020 or 2019. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and Deferred Rent The Company’s lease agreements include payment escalations and lease incentives, which are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term. Adjustments for such items, consisting primarily of tenant improvement allowances and payment escalations, are recorded as deferred rent and amortized over the lease term. Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. All of the Company’s tangible assets are held in the United States . Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, manufacturing expenses and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States . These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. The Company measures all stock-based awards granted to employees, directors and non-employees The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in its consolidated financial statements by applying a two-step more-likely-than-not Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2020 and 2019, the Company’s only element of other comprehensive loss was unrealized gain s es Net Loss per Share Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options. For periods in which the Company has reported net losses, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their affect is anti-dilutive. The Company reported a net loss for the years ended December 31, 2020 and 2019. The following potential dilutive securities presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: As of December 31, 2020 2019 Stock options to purchase common stock 5,622,868 4,688,133 Unvested restricted common stock and units 373,466 205,722 Shares of common stock issuable under Employee Stock Purchase Plan 15,443 4,213 6,011,777 4,898,068 Recently Adopted Accounting Pronouncements In December 2019, the Financing Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes 2019-12”). 2019-12 2019-12 Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases 2016-02”). 2016-02 right-of-use No. 2020-05, 2016-02 2018-11, Leases (Topic 842) Targeted Improvements 2016-02 right-of-use In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326) account that is deducted from the amortized cost basis of the financial assets to present the net amount expected to be collected. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2019 and for interim periods within those fiscal years. For nonpublic entities and emerging growth companies that choose to take advantage of the extended transition period, the guidance is effective for annual reporting periods beginning after December 15, 2020. Early adoption is permitted for all entities. In November 2019, the FASB issued ASU No. 2019-10,

Marketable Securities and Fair

Marketable Securities and Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Marketable Securities And Fair Value Measurements	3. Marketable Securities and Fair Value Measurements As of December 31, 2020, marketable securities by security type consisted of (in thousands): Amortized Gross Gross Estimated U . . $ 90,706 $ — $ (23 ) $ 90,683 Total $ 90,706 $ — $ (23 ) $ 90,683 As of December 31, 2019, marketable securities by security type consisted of (in thousands): Amortized Gross Gross Estimated U . S . $ 78,656 $ 25 $ (21 ) $ 78,660 Agency bonds (due within one year) 1,994 4 — 1,998 Total $ 80,650 $ 29 $ (21 ) $ 80,658 Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 43,182 $ — $ — $ 43,182 U.S. — 14,999 — 14,999 Marketable securities: U.S. — 90,683 — 90,683 Total $ 43,182 $ 105,682 $ — $ 148,864 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 64,796 $ — $ — $ 64,796 Marketable securities: U .S. — 78,660 — 78,660 Agency bonds — 1,998 — 1,998 Total $ 64,796 $ 80,658 $ — $ 145,454

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property and Equipment, Net	4. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Laboratory and computer equipment $ 5,477 $ 5,114 Furniture and fixtures 837 805 Leasehold improvements 6,905 6,905 13,219 12,824 Less: Accumulated depreciation and amortization (4,907 ) (2,933 ) $ 8,312 $ 9,891

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Accrued Expenses	5. Accrued Expenses Accrued expenses consisted of the following (in thousands): December 31, 2020 2019 Accrued payroll and related expenses $ 3,107 $ 3,247 Accrued external research and development expenses 2,662 6,516 Deferred rent, current portion 555 601 Accrued professional fees 693 660 Accrued other 653 279 $ 7,670 $ 11,303

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2020
Common Stock	6. Common Stock On May 6, 2019, the Company issued and sold 4,887,500 shares of its common stock, including the underwriters’ exercise in full of their option to purchase additional shares of common stock, in a follow-on In June 2020, the Company issued and sold 8,625,000 shares of its common stock, including the underwriters’ exercise in full of their option to purchase additional shares of common stock, in a follow-on In August 2019, the Company filed a shelf registration statement on Form S-3 “at-the-market” Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are not entitled to receive dividends unless declared by the board of directors.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Stock-Based Compensation	7. Stock-Based Compensation 2018 Stock Option and Incentive Plan The Magenta Therapeutics, Inc. 2018 Stock Option and Incentive Plan (the “2018 Plan”) provides for the grant of incentive stock options, non-statutory The 2018 Plan provides that the number of shares reserved and available for issuance under the 2018 Plan will automatically increase each January 1 by 4% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Company’s compensation committee. This number is subject to adjustment in the event of a stock split, stock dividend or other change in capitalization. The number of shares reserved for issuance under the 2018 Plan was increased by 1,941,325 shares effective January 1, 2021. 2016 Stock Option and Grant Plan The Company also has outstanding stock options and restricted stock awards under the 2016 Plan, but is no longer granting awards under this plan. T is three 2019 Employee Stock Purchase Plan The Magenta Therapeutics, Inc. 2019 Employee Stock Purchase Plan (the “ESPP”) became effective in June 2019. An aggregate of 166,525 shares were reserved for issuance under the ESPP. In addition, on January 1, 2020 and each January 1 thereafter through January 1, 2029, the number of shares available for issuance shall be cumulatively increased by the lesser of (i) 1% of the number of shares issued and outstanding on the immediately preceding December 31, (ii) 1,000,000 shares and (iii) such number of shares as determined by the compensation committee of the Company’s board of directors. The offering periods begin in December and June of each year, with the initial offering period commencing on December 1, 2019. The purchase price of common stock under the ESPP is equal to 85% of the lower of the fair market value of the common stock on the offering date or the exercise date. During the year ended December 31, 2020, 15,701 shares of common stock were purchased under the ESPP at a weighted average purchase price of $6.59 per share. During the year ended December 31, 2019, no shares were purchased under the ESPP. As of December 31, 2020, 150,824 shares remained available for issuance under the ESPP. The Company recognized less than $0.1 million of stock-based compensation during each of the years ended December 31, 2020 and 2019 related to the ESPP. The number of shares reserved for issuance under the ESPP did not increase on January 1, 2021. Common Stock Option Valuation The assumptions that the Company used to determine the fair value of options granted were as follows, presented on a weighted average basis: Year Ended 2020 2019 Risk-free interest rate 1.0 % 2.3 % Expected term (in years) 6.0 6.0 Expected volatility 80.4 % 78.5 % Expected dividend yield 0 % 0 % Common Stock Option Activity The following table summarizes the Company’s option activity since December 31, 2019: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2019 4,688,133 $ 8.88 8.6 $ 29,634 Granted 2,730,023 10.09 Exercised (447,402 ) 6.87 Forfeited (1,347,886 ) 9.28 Outstanding as of December 31, 2020 5,622,868 $ 9.54 8.3 $ 2,136 Options vested and expected to vest as of December 31, 2020 5,622,868 $ 9.54 8.3 $ 2,136 Options exercisable as of December 31, 2020 2,277,399 $ 9.52 7.7 $ 908 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of options exercised during the years ended December 31, 2020 and 2019 was $1.1 million and $2.3 million, respectively. The weighted average grant-date fair value per share of stock options granted during the years ended December 31, 2020 and 2019 was $6.90 and $6.05, respectively. Restricted Stock Activity Unvested shares of restricted stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The table below summarizes the Company’s restricted stock activity for grants issued under the 2016 Plan since December 31, 2019: Number Weighted Average Outstanding as of December 31, 2019 205,722 $ 1.76 Vested (184,500 ) 1.66 Forfeited (12,756 ) 1.13 Outstanding as of December 31, 2020 8,466 $ 4.84 The total fair value of restricted stock vested during the years ended December 31, 2020 and 2019 was $1.7 million and $9.3 million, respectively. Restricted Stock Units During the year ended December 31, 2020, the Company granted service-based restricted stock units to certain employees. Upon vesting, each restricted stock unit entitles the holder to a specified number of shares of common stock. The table below summarizes the Company’s restricted stock unit activity since December 31, 2019: Number Weighted Outstanding as of December 31, 2019 — $ — Granted 220,000 $ 6.80 Vested — — Forfeited (15,000 ) $ 6.80 Outstanding as of December 31, 2020 205,000 $ 6.80 Performance Restricted Stock Units During the year ended December 31, 2020, the Company granted performance-based restricted stock units to certain senior employees which vest upon the occurrence of certain operational and financial events. At the achievement of the performance-based vesting criteria, each performance-based restricted stock unit entitles the holder to a specified number of shares of common stock. The table below summarizes the Company’s performance restricted stock unit activity since December 31, 2019: Number Weighted Outstanding as of December 31, 2019 — $ — Granted 160,000 $ 6.75 Vested — — Forfeited — — Outstanding as of December 31, 2020 160,000 $ 6.75 Stock-Based Compensation Stock-based compensation expense was classified in the consolidated Year Ended 2020 2019 Research and development expenses $ 3,542 $ 4,518 General and administrative expenses 6,399 5,483 $ 9,941 $ 10,001 As of December 31, 2020, total unrecognized compensation cost related to the unvested stock-based awards was $18.7 million, which is expected to be recognized over a weighted average period of 2.6 years. This amount excludes unrecognized of $1.1 million

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies	8. Commitments and Contingencies Leases In May 2018, the Company entered into a sublease for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease was amended effective March 2019 to increase the lease payments over the lease term and to increase the sublandlord-funded tenant improvements. The sublease is subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and expires in February 2028. The sublandlord has the right to terminate the sublease after five years. The Company is also obligated to pay real estate taxes and other costs related to the premises, including costs of operations and management of the leased premises. In connection with the sublease, as amended, the sublandlord funded $5.2 million in tenant improvements to the leased facility during 2019. The Company is required to maintain a cash balance of $1.8 million to secure a letter of credit associated with the sublease. This amount was classified as noncurrent restricted cash in the consolidated balance sheets at December 31, 2020 and 2019. As of December 31, 2020 and 2019, the Company had long-term deferred rent of $6.3 million and $6.2 million, respectively, related to lease incentives and payment escalations. As of December 31, 2020 and 2019, the short-term portion of deferred rent of $0.6 million for each period was included in accrued expenses and other current liabilities. The Company recorded rent expense of $6.2 million and $6.1 million, respectively, during the years ended December 31, 2020 and 2019. As of December 31, 2020, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands): 2021 $ 6,072 2022 6,375 2023 6,734 2024 7,100 2025 7,455 Thereafter 17,444 $ 51,180 In the fourth quarter of 2018, the Company entered into two two-year sub-subleases sub-subleases sub-subleases sub-subleases. Collaboration Agreement In March 2018, the Company entered into a collaboration agreement with Heidelberg Pharma Research GmbH (“HDPR”) whereby the parties agreed to combine the Company’s stem cell platform with proprietary antibodies across up to four exclusive targets with HDPR’s proprietary Antibody Targeted Amanitin Conjugates platform. Under the agreement, the Company may pay upfront technology access fees, research exclusivity fees and payment for research support. Additionally, upon the exercise of certain license rights, the Company may be obligated to pay HDPR development, regulatory and commercial milestone payments of up to $83.5 million per target as well as royalties on net sales of products licensed under the agreement. During the years ended December 31, 2020 and 2019, the Company recorded $0.7 million and $1.9 million, respectively, of research and development expense related to this agreement for upfront technology access fees, research exclusivity fees and research support. Intellectual Property Licenses The Company has a license agreement with the President and Fellows of Harvard College (“Harvard”), entered into in November 2016, for an exclusive, worldwide, royalty-bearing license for certain technologies related to conditioning and mobilization. The Company is obligated to pay Harvard maintenance fees of $0.1 million annually and to reimburse qualified expenses related to the patents. The Company is also obligated to pay milestone payments of up to $7.4 million for the first two product-by-product country-by-country two The Company has a license agreement with Novartis International Pharmaceutical Ltd. (“Novartis”), entered into in April 2017, to use and develop certain patent rights (the “Novartis License”). Under the Novartis License, the Company was granted an exclusive, worldwide, sublicensable license to research, develop and commercialize certain licensed products that contain Novartis compounds for the expansion of cord blood derived non-gene-edited/-modified product-by-product country-by-country The Company has agreements with third parties in the normal course of business, under which it can license certain developed technologies. If the Company exercises its rights to license the respective technologies, it may be subject to additional fees and milestone payments. During the year ended December 31, 2020, the Company recorded research and development expense of $ 0.8 Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and senior management that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements, and it has no Legal Proceedings The Company is not currently a party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses the costs related to its legal proceedings as they are incurred.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2020
401(k) Savings Plan	9. 401(k) Savings Plan The Company has a 401(k) available for participating employees who meet certain eligibility requirements. Eligible employees may defer a portion of their salary as defined by the plan. Company contributions to the plan may be made at the discretion of the board of directors of the Company. To date, the Company has not made any contributions to the plan.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax	10. Income Taxes During the years ended December 31, 2020 and 2019, the Company recorded no income tax benefits for the net operating losses incurred or for the research and orphan drug tax credits generated in each year, due to its uncertainty of realizing a benefit from those items. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 Federal statutory income tax rate 21.0 % 21.0 % State taxes, net of federal benefit 5.8 6.1 Research and orphan drug tax credits 3.4 4.1 Other (1.7 ) (1.2 ) Increase in deferred tax asset valuation allowance (28.5 ) (30.0 ) Effective income tax rate — % — % Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 49,958 $ 20,836 Capitalized research and development expenses 9,506 20,723 Research and orphan drug tax credit carryforwards 9,988 7,416 Stock compensation expense 2,832 2,121 Accrued expense 828 865 Other 1,867 1,860 Total deferred tax assets 74,979 53,821 Valuation allowance (73,600 ) (52,248 ) Net deferred tax assets 1,379 1,573 Deferred tax liabilities: Depreciation and amortization (1,379 ) (1,573 ) Total deferred tax liabilities (1,379 ) (1,573 ) Net deferred tax assets and liabilities $ — $ — As of December 31, 2020, the Company had net operating loss carryforwards for federal income tax purposes of $182.3 million, of which $17.5 million begin to expire in long-term tax-exempt rate, The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. The Company considered its history of cumulative net losses incurred since inception and its lack of commercialization of any products since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2020 and 2019. The Company reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2020 and 2019 related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and orphan drug tax credit carryforwards and were as follows (in thousands): Year Ended 2020 2019 Valuation allowance as of beginning of year $ 52,248 $ 29,243 Net increases recorded to income tax provision 21,352 23,005 Valuation allowance as of end of year $ 73,600 $ 52,248 The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2020 or 2019. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s tax years are open under statute from 201 7

Related Parties

Related Parties	12 Months Ended
	Dec. 31, 2020
Related Parties	11. Related Parties National Marrow Donor Program (as successor in interest to Be The Match BioTherapies, LLC) Effective March 2018, the President of Be The Match BioTherapies, LLC became a member of the Company’s board of directors and subsequently was appointed Chief Executive Officer of the National Marrow Donor Program/Be The Match, or NMDP/Be The Match, organization in June 2020. The Company has collaboration agreements with the National Marrow Donor Program (as successor in interest to Be The Match BioTherapies, LLC) and a research agreement with an affiliated organization, Center for International Blood and Marrow Transplant Research. In addition, in June 2020, the Company entered into a clinical collaboration agreement with NMDP/Be The Match to evaluate the potential utility of MGTA-145 During each of the years ended December 31, 2020 and 2019, the Company recorded $0.4 million of expense related to these agreements. As of December 31, 2020 and 2019, amounts on the balance sheet related to these agreements was less than $0.1 million and $0.2 million, respectively, which were included in accounts payable and accrued expenses and as of December 31, 2020, less than $0.1 million was

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company maintains all cash, cash equivalents and marketable securities at two accredited financial institutions in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less at the date of purchase to be cash equivalents.
Marketable Securities	Marketable Securities The Company’s marketable securities are classified as available-for-sale and
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Lab equipment 5 years Computer equipment 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of life of lease or estimated useful life Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, determined based on discounted cash flows. The Company did not record any impairment losses on long-lived assets during the years ended December 31, 2020 or 2019.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and
Deferred Rent	Deferred Rent The Company’s lease agreements include payment escalations and lease incentives, which are accrued or deferred as appropriate such that rent expense for each lease is recognized on a straight-line basis over the respective lease term. Adjustments for such items, consisting primarily of tenant improvement allowances and payment escalations, are recorded as deferred rent and amortized over the lease term.
Segment Information	Segment Information The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. All of the Company’s tangible assets are held in the United States .
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries, stock-based compensation and benefits, facilities costs, depreciation, manufacturing expenses and external costs of outside vendors engaged to conduct preclinical development activities and clinical trials as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States . These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Stock-Based Compensation	Stock-Based Compensation The Company measures all stock-based awards granted to employees, directors and non-employees The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in its consolidated financial statements by applying a two-step more-likely-than-not
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2020 and 2019, the Company’s only element of other comprehensive loss was unrealized gain s es
Net Loss per Share	Net Loss per Share Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options. For periods in which the Company has reported net losses, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their affect is anti-dilutive. The Company reported a net loss for the years ended December 31, 2020 and 2019. The following potential dilutive securities presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: As of December 31, 2020 2019 Stock options to purchase common stock 5,622,868 4,688,133 Unvested restricted common stock and units 373,466 205,722 Shares of common stock issuable under Employee Stock Purchase Plan 15,443 4,213 6,011,777 4,898,068
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In December 2019, the Financing Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes 2019-12”). 2019-12 2019-12 Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases 2016-02”). 2016-02 right-of-use No. 2020-05, 2016-02 2018-11, Leases (Topic 842) Targeted Improvements 2016-02 right-of-use In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326) account that is deducted from the amortized cost basis of the financial assets to present the net amount expected to be collected. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2019 and for interim periods within those fiscal years. For nonpublic entities and emerging growth companies that choose to take advantage of the extended transition period, the guidance is effective for annual reporting periods beginning after December 15, 2020. Early adoption is permitted for all entities. In November 2019, the FASB issued ASU No. 2019-10,

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Estimated Useful Life of Asset	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: Estimated Useful Life Lab equipment 5 years Computer equipment 3 years Furniture and fixtures 5 years Leasehold improvements Shorter of life of lease or estimated useful life
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following potential dilutive securities presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact: As of December 31, 2020 2019 Stock options to purchase common stock 5,622,868 4,688,133 Unvested restricted common stock and units 373,466 205,722 Shares of common stock issuable under Employee Stock Purchase Plan 15,443 4,213 6,011,777 4,898,068

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Marketable Securities by Security Type	As of December 31, 2020, marketable securities by security type consisted of (in thousands): Amortized Gross Gross Estimated U . . $ 90,706 $ — $ (23 ) $ 90,683 Total $ 90,706 $ — $ (23 ) $ 90,683 As of December 31, 2019, marketable securities by security type consisted of (in thousands): Amortized Gross Gross Estimated U . S . $ 78,656 $ 25 $ (21 ) $ 78,660 Agency bonds (due within one year) 1,994 4 — 1,998 Total $ 80,650 $ 29 $ (21 ) $ 80,658
Schedule of Financial Assets Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands): Fair Value Measurements at December 31, 2020 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 43,182 $ — $ — $ 43,182 U.S. — 14,999 — 14,999 Marketable securities: U.S. — 90,683 — 90,683 Total $ 43,182 $ 105,682 $ — $ 148,864 Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 64,796 $ — $ — $ 64,796 Marketable securities: U .S. — 78,660 — 78,660 Agency bonds — 1,998 — 1,998 Total $ 64,796 $ 80,658 $ — $ 145,454

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Laboratory and computer equipment $ 5,477 $ 5,114 Furniture and fixtures 837 805 Leasehold improvements 6,905 6,905 13,219 12,824 Less: Accumulated depreciation and amortization (4,907 ) (2,933 ) $ 8,312 $ 9,891

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Accrued Expenses	Accrued expenses consisted of the following (in thousands): December 31, 2020 2019 Accrued payroll and related expenses $ 3,107 $ 3,247 Accrued external research and development expenses 2,662 6,516 Deferred rent, current portion 555 601 Accrued professional fees 693 660 Accrued other 653 279 $ 7,670 $ 11,303

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Assumptions Used to Determine the Fair Value of Option Grants	The assumptions that the Company used to determine the fair value of options granted were as follows, presented on a weighted average basis: Year Ended 2020 2019 Risk-free interest rate 1.0 % 2.3 % Expected term (in years) 6.0 6.0 Expected volatility 80.4 % 78.5 % Expected dividend yield 0 % 0 %
Schedule of Common Stock Option Activity	The following table summarizes the Company’s option activity since December 31, 2019: Number of Weighted Weighted Aggregate (in years) (in thousands) Outstanding as of December 31, 2019 4,688,133 $ 8.88 8.6 $ 29,634 Granted 2,730,023 10.09 Exercised (447,402 ) 6.87 Forfeited (1,347,886 ) 9.28 Outstanding as of December 31, 2020 5,622,868 $ 9.54 8.3 $ 2,136 Options vested and expected to vest as of December 31, 2020 5,622,868 $ 9.54 8.3 $ 2,136 Options exercisable as of December 31, 2020 2,277,399 $ 9.52 7.7 $ 908
Schedule of Activity Relating to Restricted Stock Units	The table below summarizes the Company’s restricted stock activity for grants issued under the 2016 Plan since December 31, 2019: Number Weighted Average Outstanding as of December 31, 2019 205,722 $ 1.76 Vested (184,500 ) 1.66 Forfeited (12,756 ) 1.13 Outstanding as of December 31, 2020 8,466 $ 4.84
Schedule of Stock Based Compensation Expense	Stock-based compensation expense was classified in the consolidated Year Ended 2020 2019 Research and development expenses $ 3,542 $ 4,518 General and administrative expenses 6,399 5,483 $ 9,941 $ 10,001
Restricted Stock Units (RSUs) [Member]
Schedule of Activity Relating to Restricted Stock Units	The table below summarizes the Company’s restricted stock unit activity since December 31, 2019: Number Weighted Outstanding as of December 31, 2019 — $ — Granted 220,000 $ 6.80 Vested — — Forfeited (15,000 ) $ 6.80 Outstanding as of December 31, 2020 205,000 $ 6.80
Performance Restricted Stock Units [Member]
Schedule of Activity Relating to Restricted Stock Units	The table below summarizes the Company’s performance restricted stock unit activity since December 31, 2019: Number Weighted Outstanding as of December 31, 2019 — $ — Granted 160,000 $ 6.75 Vested — — Forfeited — — Outstanding as of December 31, 2020 160,000 $ 6.75

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease	As of December 31, 2020, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands): 2021 $ 6,072 2022 6,375 2023 6,734 2024 7,100 2025 7,455 Thereafter 17,444 $ 51,180

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Schedule of Reconciliation of U.S. Federal Statutory Rate to Effective Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2020 2019 Federal statutory income tax rate 21.0 % 21.0 % State taxes, net of federal benefit 5.8 6.1 Research and orphan drug tax credits 3.4 4.1 Other (1.7 ) (1.2 ) Increase in deferred tax asset valuation allowance (28.5 ) (30.0 ) Effective income tax rate — % — %
Company's Deferred Tax Assets	Net deferred tax assets as of December 31, 2020 and 2019 consisted of the following (in thousands): December 31, 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 49,958 $ 20,836 Capitalized research and development expenses 9,506 20,723 Research and orphan drug tax credit carryforwards 9,988 7,416 Stock compensation expense 2,832 2,121 Accrued expense 828 865 Other 1,867 1,860 Total deferred tax assets 74,979 53,821 Valuation allowance (73,600 ) (52,248 ) Net deferred tax assets 1,379 1,573 Deferred tax liabilities: Depreciation and amortization (1,379 ) (1,573 ) Total deferred tax liabilities (1,379 ) (1,573 ) Net deferred tax assets and liabilities $ — $ —
Changes in Deferred Tax Asset Valuation Allowance	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2020 and 2019 related primarily to the increase in net operating loss carryforwards, capitalized research and development expenses and research and orphan drug tax credit carryforwards and were as follows (in thousands): Year Ended 2020 2019 Valuation allowance as of beginning of year $ 52,248 $ 29,243 Net increases recorded to income tax provision 21,352 23,005 Valuation allowance as of end of year $ 73,600 $ 52,248

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2020	Aug. 08, 2019	May 06, 2019	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date					Jun. 30, 2020
Entity incorporation state code					DE
Net loss					$ (74,936)	$ (76,769)
Accumulated deficit					(254,431)	(179,495)
Net proceeds from common stock	$ 64,600		$ 60,300	$ 60,300	$ 64,860	$ 60,874
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shelf offering value		$ 350,000
Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares	8,625,000		4,887,500	4,887,500	8,625,000	4,887,500
Stock issued, price per shares	$ 8		$ 13.25	$ 13.25	$ 8
Net proceeds from common stock	$ 64,600
Cowen and Company LLC [Member] | Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
ATM offering value		$ 100,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Estimated Useful Life of Asset (Detail)	12 Months Ended
	Dec. 31, 2020
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	5 years
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	3 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	Shorter of life of lease or estimated useful life

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	6,011,777	4,898,068
Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	5,622,868	4,688,133
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	373,466	205,722
Shares of common stock issuable under the ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	15,443	4,213

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Summary Of Significant Accounting Policies [Line Items]
Impairment losses on long-lived assets	$ 0	$ 0

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities by Security Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Marketable Securities [Line Items]
Amortized Cost	$ 90,706	$ 80,650
Gross Unrealized Gains	0	29
Gross Unrealized Losses	(23)	(21)
Estimated Fair Value	90,683	80,658
US Treasury Notes [Member]
Marketable Securities [Line Items]
Amortized Cost	90,706	78,656
Gross Unrealized Gains	0	25
Gross Unrealized Losses	(23)	(21)
Estimated Fair Value	$ 90,683	78,660
Agency Bonds [Member]
Marketable Securities [Line Items]
Amortized Cost		1,994
Gross Unrealized Gains		4
Gross Unrealized Losses		0
Estimated Fair Value		$ 1,998

Marketable Securities and Fai_4

Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 90,683	$ 80,658
Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	148,864	145,454
Level 1 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	43,182	64,796
Level 2 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	105,682	80,658
Money Market Funds [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	43,182	64,796
Money Market Funds [Member] | Level 1 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	43,182	64,796
US Treasury Notes [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	14,999
US Treasury Notes [Member] | Level 2 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	14,999
US Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	90,683	78,660
US Treasury Notes [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	90,683	78,660
US Treasury Notes [Member] | Level 2 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 90,683	78,660
Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		1,998
Agency Bonds [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		1,998
Agency Bonds [Member] | Level 2 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		$ 1,998

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 13,219	$ 12,824
Less: Accumulated depreciation and amortization	(4,907)	(2,933)
Property and equipment, net	8,312	9,891
Laboratory and computer equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	5,477	5,114
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	837	805
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 6,905	$ 6,905

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 1,978	$ 1,843

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accrued payroll and related expenses	$ 3,107	$ 3,247
Accrued external research and development expenses	2,662	6,516
Deferred rent, current portion	555	601
Accrued professional fees	693	660
Accrued other	653	279
Total	$ 7,670	$ 11,303

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2020	Aug. 08, 2019	May 06, 2019	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Net proceeds from common stock	$ 64,600		$ 60,300	$ 60,300	$ 64,860	$ 60,874
Common Stock [Member]
Class of Stock [Line Items]
Stock issued, shares	8,625,000		4,887,500	4,887,500	8,625,000	4,887,500
Stock issued, price per shares	$ 8		$ 13.25	$ 13.25	$ 8
Net proceeds from common stock	$ 64,600
Maximum [Member]
Class of Stock [Line Items]
Shelf offering value		$ 350,000
Cowen and Company LLC [Member] | Maximum [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance		$ 100,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value of stock options granted		$ 6.90	$ 6.05
Total unrecognized stock-based compensation expense		$ 18,700
Period for recognition of unrecognized expense		2 years 7 months 6 days
Stock based compensation		$ 9,941	$ 10,001
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of award		10 years
Intrinsic value of stock options exercised		$ 1,100	2,300
Maximum [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Minimum [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		3 years
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense		$ 1,100
Total fair value of restricted stock vested		$ 1,700	9,300
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance under Stock Option and Incentive Plan		2,432,666
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance		4.00%
Increase in shares reserved for future issuance		1,941,325
2019 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance under Stock Option and Incentive Plan	166,525	150,824
Annual Increase In Aggregate Number Of Shares Reserved For Future Issuance	1,000,000
Minimum percentage of shares issued and outstanding	1.00%
Stock based compensation		$ 100	$ 100
Percentage of purchase price of common stock under the ESPP		85.00%
Common stock purchased		15,701	0
Stock issued, price per shares		$ 6.59

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Assumptions Used to Determine the Fair Value of Option Grants (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk-free interest rate	1.00%	2.30%
Expected term (in years)	6 years	6 years
Expected volatility	80.40%	78.50%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Common Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning Balance	4,688,133
Number of Shares, Granted	2,730,023
Number of Shares, Exercised	(447,402)
Number of Shares, Forfeited	(1,347,886)
Number of Shares, Ending Balance	5,622,868	4,688,133
Number of Shares, Options vested and expected to vest as of December 31, 2020	5,622,868
Number of Shares, Options exercisable as of December 31, 2020	2,277,399
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 8.88
Weighted-Average Exercise Price, Granted	10.09
Weighted-Average Exercise Price, Exercised	6.87
Weighted-Average Exercise Price, Forfeited	9.28
Weighted-Average Exercise Price, Outstanding, Ending balance	9.54	$ 8.88
Weighted-Average Exercise Price, Options vested and expected to vest as of December 31, 2020	9.54
Weighted-Average Exercise Price, Options exercisable as of December 31, 2020	$ 9.52
Weighted Average Remaining Contractual Term	8 years 3 months 18 days	8 years 7 months 6 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest as of December 31, 2020	8 years 3 months 18 days
Weighted Average Remaining Contractual Term, Options exercisable as of December 31, 2020	7 years 8 months 12 days
Aggregate Intrinsic Value, Beginning Balance	$ 29,634
Aggregate Intrinsic Value, Ending Balance	2,136	$ 29,634
Aggregate Intrinsic Value, Options vested and expected to vest as of December 31, 2020	2,136
Aggregate Intrinsic Value, Options exercisable as of December 31, 2020	$ 908

Stock-Based Compensation - Sc_3

Stock-Based Compensation - Schedule of Activity Relating to Restricted Stock Units (Detail)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted | shares	220,000
Number of Shares, Forfeited | shares	(15,000)
Number of Shares Outstanding, Ending Balance | shares	205,000
Weighted Average Grant Date Fair Value, Granted | $ / shares	$ 6.80
Weighted Average Grant Date Fair Value,Forfeited | $ / shares	6.80
Weighted Average Grant Date Fair Value Outstanding, Ending Balance | $ / shares	$ 6.80
Performance Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Granted | shares	160,000
Number of Shares Outstanding, Ending Balance | shares	160,000
Weighted Average Grant Date Fair Value, Granted | $ / shares	$ 6.75
Weighted Average Grant Date Fair Value Outstanding, Ending Balance | $ / shares	$ 6.75
2016 Stock Option and Grant Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Outstanding, Beginning Balance | shares	205,722
Number of Shares, Vested | shares	(184,500)
Number of Shares, Forfeited | shares	(12,756)
Number of Shares Outstanding, Ending Balance | shares	8,466
Weighted Average Grant Date Fair Value Outstanding, Beginning Balance | $ / shares	$ 1.76
Weighted Average Grant Date Fair Value, Vested | $ / shares	1.66
Weighted Average Grant Date Fair Value,Forfeited | $ / shares	1.13
Weighted Average Grant Date Fair Value Outstanding, Ending Balance | $ / shares	$ 4.84

Stock-Based Compensation - Sc_4

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 9,941	$ 10,001
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	3,542	4,518
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 6,399	$ 5,483

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2017USD ($)	Dec. 31, 2018ft²Sublease	Dec. 31, 2020USD ($)Milestone	Dec. 31, 2019USD ($)	May 31, 2018USD ($)ft²	Nov. 30, 2016USD ($)
Other Commitments [Line Items]
Sublease commencement period			2018-06
Sublease expiration period			2028-02
Property sublease, description			In May 2018, the Company entered into a sublease for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease was amended effective March 2019 to increase the lease payments over the lease term and to increase the sublandlord-funded tenant improvements. The sublease is subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and expires in February 2028. The sublandlord has the right to terminate the sublease after five years.
Sublease termination period			5 years
Long-term deferred rent			$ 6,283	$ 6,206
Deferred rent, current portion			555	601
Rent expense			6,200	6,100
Rental payments receivable			2,300
Research and development expense			50,615	59,208
Research and Development Expenses [Member]
Other Commitments [Line Items]
Research and development expense			100	100
Long Term Deferred Rent [Member]
Other Commitments [Line Items]
Long-term deferred rent			6,300	6,200
Short Term Deferred Rent [Member]
Other Commitments [Line Items]
Deferred rent, current portion			600	600
Maximum [Member] | Office and Lab Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease | ft²					69,000
Tenant improvement allowance					$ 5,200
Sub-Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease | ft²		27,000
Lease agreement maturity period		2 years
Number of Sub-sublease | Sublease		2
Other income from sub-sublease			$ 2,900	2,800
License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Other Commitments [Line Items]
Number of milestones met | Milestone			0
License Agreement with Novartis International Pharmaceutical Ltd. [Member] | Maximum [Member]
Other Commitments [Line Items]
License agreement royalty percentage	20.00%
License Agreement with Novartis International Pharmaceutical Ltd. [Member] | Achievement of Clinical and Regulatory Milestones [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation	$ 177,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] | Development and Regulatory Milestones [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation	$ 125,000
Collaboration Agreement [Member] | Maximum [Member]
Other Commitments [Line Items]
Research and development expense			$ 700	$ 1,900
Collaboration Agreement [Member] | Development, Regulatory and Commercial Milestone [Member] | Per Target [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation			83,500
License Agreement With Harvard [Member] | Maintenance [Member] | Annually Through 2019 [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation						$ 100
License Agreement With Harvard [Member] | Achievement of Development and Regulatory Milestones [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation						$ 7,400
License Agreement With Third Parties [Member] | License [Member]
Other Commitments [Line Items]
Research and development expense			$ 800
Letter of Credit [Member] | Office and Lab Sublease [Member]
Other Commitments [Line Items]
Cash balance to secure letter of credit associated with sublease					$ 1,800

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Operating Leased Assets [Line Items]
2021	$ 6,072
2022	6,375
2023	6,734
2024	7,100
2025	7,455
Thereafter	17,444
Operating leases future minimum payments due	$ 51,180

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Line Items]
Income tax expense (benefit)	$ 0	$ 0
Research and orphan drug tax credit carryforwards	9,988,000	7,416,000
Unrecognized tax benefits	0	$ 0
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	182,300,000
Research and orphan drug tax credit carryforwards	$ 8,400,000
Research and orphan drug tax credit carryforwards, expiration period	2035
Federal [Member] | Expires 2035 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 17,500,000
Operating loss carry forwards expiration period	2035
Federal [Member] | Indefinitely [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 164,800,000
State [Member]
Income Tax Disclosure [Line Items]
Operating loss carry forwards expiration period	2035
Research and orphan drug tax credit carryforwards	$ 2,000,000
Research and orphan drug tax credit carryforwards, expiration period	2030
State [Member] | Expires 2035 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 184,600,000

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of U.S. Federal Statutory Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Federal statutory income tax rate	21.00%	21.00%
State taxes, net of federal benefit	5.80%	6.10%
Research and orphan drug tax credits	3.40%	4.10%
Other	(1.70%)	(1.20%)
Increase in deferred tax asset valuation allowance	(28.50%)	(30.00%)
Effective income tax rate	0.00%	0.00%

Income Taxes - Company's Deferr

Income Taxes - Company's Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 49,958	$ 20,836
Capitalized research and development expenses	9,506	20,723
Research and orphan drug tax credit carryforwards	9,988	7,416
Stock compensation expense	2,832	2,121
Accrued expense	828	865
Other	1,867	1,860
Total deferred tax assets	74,979	53,821
Valuation allowance	(73,600)	(52,248)	$ (29,243)
Net deferred tax assets	1,379	1,573
Deferred tax liabilities:
Depreciation and amortization	(1,379)	(1,573)
Total deferred tax liabilities	(1,379)	(1,573)
Net deferred tax assets and liabilities	$ 0	$ 0

Income Taxes - Changes in Defer

Income Taxes - Changes in Deferred Tax Asset Valuation Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Valuation allowance as of beginning of year	$ 52,248	$ 29,243
Net increases recorded to income tax provision	21,352	23,005
Valuation allowance as of end of year	$ 73,600	$ 52,248

Related Parties - Additional In

Related Parties - Additional Information (Detail) - Match Bio Therapies [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Expenses paid to related party	$ 0.4	$ 0.4
Accounts Payable, Accrued Expenses and Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Amounts owed to related party	0.1	$ 0.2
Prepaid Expenses and Other Current Assets [Member]
Related Party Transaction [Line Items]
Amounts owed to related party	$ 0.1