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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Feb. 28, 2019	Jun. 29, 2018
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	MGTA
Entity Registrant Name	MAGENTA THERAPEUTICS, INC.
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 Common Stock, Shares Outstanding		33,466,526
Entity Public Float			$ 209.1

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 58,345	$ 51,402
Marketable securities	84,225
Restricted cash		165
Prepaid expenses and other current assets	2,751	936
Total current assets	145,321	52,503
Restricted cash	1,780
Property and equipment, net	10,212	1,956
Other assets		4
Total assets	157,313	54,463
Current liabilities:
Accounts payable	3,492	167
Accrued expenses and other current liabilities	6,927	3,975
Total current liabilities	10,419	4,142
Long-term liabilities	1,246
Total liabilities	11,665	4,142
Commitments and contingencies (Note 11)
Redeemable convertible preferred stock (Series A and B), $0.001 par value; no shares and 49,178,527 shares authorized, issued and outstanding as of December 31, 2018 and 2017, respectively		92,439
Stockholders' Equity (Deficit):
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of December 31, 2018; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares and 70,000,000 shares authorized as of December 31, 2018 and 2017, respectively; 34,327,554 shares and 4,458,547 shares issued and 33,305,033 shares and 2,351,247 shares outstanding as of December 31, 2018 and 2017, respectively	33	2
Additional paid-in capital	248,349	3,091
Accumulated other comprehensive loss	(8)
Accumulated deficit	(102,726)	(45,211)
Total stockholders' equity (deficit)	145,648	(42,118)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 157,313	$ 54,463

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Redeemable convertible preferred stock (Series A and B ), Par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock (Series A and B), Shares authorized	0	49,178,527
Redeemable convertible preferred stock (Series A and B), Shares issued	0	49,178,527
Redeemable convertible preferred stock (Series A and B), Shares outstanding	0	49,178,527
Preferred stock, par value	$ 0.001	$ 0.000
Preferred stock, Shares authorized	10,000,000	0
Preferred stock, Shares issued	0	0
Preferred stock, Shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	70,000,000
Common stock, Shares issued	34,327,554	4,458,547
Common stock, Shares outstanding	33,305,033	2,351,247

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue	$ 0	$ 0	$ 0
Operating expenses:
Research and development	41,340	27,899	5,782
General and administrative	18,623	7,828	3,486
Total operating expenses	59,963	35,727	9,268
Loss from operations	(59,963)	(35,727)	(9,268)
Other income (expense):
Interest expense			(163)
Interest and other income, net	2,448	236
Total other income (expense), net	2,448	236	(163)
Net loss	(57,515)	(35,491)	(9,431)
Accretion of redeemable convertible preferred stock to redemption value	(88)	(213)	(107)
Cumulative dividends on redeemable convertible preferred stock		(437)	(197)
Reversal of cumulative dividends on redeemable convertible preferred stock		634
Net loss attributable to common stockholders	$ (57,603)	$ (35,507)	$ (9,735)
Net loss per share attributable to common stockholders, basic and diluted	$ (3.13)	$ (19.12)	$ (65.15)
Weighted average common shares outstanding, basic and diluted	18,389,576	1,856,907	149,414
Comprehensive loss:
Net loss	$ (57,515)	$ (35,491)	$ (9,431)
Other comprehensive loss:
Unrealized loss on marketable securities	(8)
Total other comprehensive loss	(8)
Total comprehensive loss	$ (57,523)	$ (35,491)	$ (9,431)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Consultant Fees [Member]	License Agreement [Member]	Series A Preferred Stock [Member]	Series B Preferred Stock [Member]	Series A and B Convertible Preferred Stock [Member] License Agreement [Member]	Series C Preferred Stock [Member]	Series A B C Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Consultant Fees [Member]	Common Stock [Member] License Agreement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Consultant Fees [Member]	Additional Paid-in Capital [Member] License Agreement [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Issuance of redeemable convertible preferred stock, net of issuance cost				$ 10,448
Issuance of redeemable convertible preferred stock, net of issuance cost, shares				10,554,990
Conversion of notes payable and accrued interest into Series A redeemable convertible preferred stock, amount				$ 7,164
Conversion of notes payable and accrued interest into Series A redeemable convertible preferred stock, shares				7,163,984
Accretion of redeemable convertible preferred stock to redemption value				$ 107
Cumulative dividends on redeemable convertible preferred stock				197
Ending balances at Dec. 31, 2016	$ 17,916
Ending balance, shares at Dec. 31, 2016	17,718,974
Beginning balance at Dec. 31, 2015	$ (289)															$ (289)
Vesting of restricted stock	13								$ 1			$ 12
Vesting of restricted stock, shares									773,433
Issuance of common stock		$ 157	$ 318										$ 157	$ 318
Issuance of common stock, shares										189,629	385,063
Accretion of redeemable convertible preferred stock to redemption value	(107)											(107)
Cumulative dividends on redeemable convertible preferred stock	$ (197)											(197)
Issuance of common stock upon exercise of stock options, shares	0
Stock-based compensation expense	$ 662											662
Net loss	(9,431)															(9,431)
Ending balance at Dec. 31, 2016	(8,874)								$ 1			845				(9,720)
Ending balance, shares at Dec. 31, 2016									1,348,125
Issuance of redeemable convertible preferred stock, net of issuance cost				$ 15,445	$ 49,787	$ 9,275
Issuance of redeemable convertible preferred stock, net of issuance cost, shares				15,445,000	12,871,003	3,143,550
Accretion of redeemable convertible preferred stock to redemption value					$ 213
Cumulative dividends on redeemable convertible preferred stock				$ 437
Reversal of Series A redeemable convertible preferred stock dividend				(634)
Ending balances at Dec. 31, 2017	$ 92,439			$ 39,939	$ 52,500
Ending balance, shares at Dec. 31, 2017	49,178,527			35,663,974	13,514,553
Vesting of restricted stock	$ 4								$ 1			3
Vesting of restricted stock, shares									999,253
Accretion of redeemable convertible preferred stock to redemption value	(213)											(213)
Cumulative dividends on redeemable convertible preferred stock	(437)											(437)
Reversal of Series A redeemable convertible preferred stock dividend	$ 634											634
Issuance of common stock upon exercise of stock options, shares	0
Stock-based compensation expense	$ 2,259											2,259
Net loss	(35,491)															(35,491)
Ending balance at Dec. 31, 2017	$ (42,118)								$ 2			3,091				(45,211)
Ending balance, shares at Dec. 31, 2017									2,351,247
Conversion of preferred stock to common stock								$ (144,739)
Conversion of preferred stock to common stock, shares								(60,401,629)
Issuance of redeemable convertible preferred stock, net of issuance cost							$ 52,212
Issuance of redeemable convertible preferred stock, net of issuance cost, shares							11,223,102
Accretion of redeemable convertible preferred stock to redemption value							$ 88
Ending balance, shares at Dec. 31, 2018	0
Vesting of restricted stock									$ 1			(1)
Vesting of restricted stock, shares									907,562
Accretion of redeemable convertible preferred stock to redemption value	$ (88)											(88)
Conversion of preferred stock to common stock	144,739								$ 23			144,716
Conversion of preferred stock to common, shares stock									23,375,405
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	89,906								$ 7			89,899
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares									6,666,667
Issuance of common stock upon exercise of stock options	$ 29											29
Issuance of common stock upon exercise of stock options, shares	4,152								4,152
Stock-based compensation expense	$ 10,703											10,703
Unrealized loss on marketable securities	(8)														$ (8)
Net loss	(57,515)															(57,515)
Ending balance at Dec. 31, 2018	$ 145,648								$ 33			$ 248,349			$ (8)	$ (102,726)
Ending balance, shares at Dec. 31, 2018									33,305,033

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Payments of redeemable convertible preferred stock issuance costs	$ 88	$ 213	$ 107
Series A Preferred Stock [Member]
Payments of redeemable convertible preferred stock issuance costs			$ 107
Series B Preferred Stock [Member]
Payments of redeemable convertible preferred stock issuance costs		$ 213
Series C Preferred Stock [Member]
Payments of redeemable convertible preferred stock issuance costs	$ 88

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (57,515)	$ (35,491)	$ (9,431)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	10,703	2,259	662
Depreciation and amortization expense	875	376	6
Loss on disposal of property and equipment	67
Accretion of discounts on marketable securities	(190)
Non-cash interest expense			163
License fees paid in common and preferred stock		9,275	318
Consultant fees paid in common stock			157
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,811)	(895)	(41)
Accounts payable	2,500	(1,125)	1,006
Accrued expenses and other current liabilities	2,239	3,342	631
Other assets		(4)
Other long-term liabilities	1,246
Net cash used in operating activities	(41,886)	(22,263)	(6,529)
Cash flows from investing activities:
Purchases of property and equipment	(7,672)	(2,199)	(139)
Proceeds from sale of property and equipment	12
Purchase of marketable securities	(84,043)
Net cash used in investing activities	(91,703)	(2,199)	(139)
Cash flows from financing activities:
Proceeds from initial public offering, net of underwriting discounts and commissions	93,000
Proceeds from issuance of redeemable convertible preferred stock	52,300	71,695	4,305
Proceeds from issuance of convertible notes payable			6,500
Payments of initial public offering costs	(3,094)
Payments of redeemable convertible preferred stock issuance costs	(88)	(213)	(107)
Proceeds from issuance of common and restricted stock		4	13
Proceeds from exercise of common stock options	29
Net cash provided by financing activities	142,147	71,486	10,711
Net increase in cash, cash equivalents and restricted cash	8,558	47,024	4,043
Cash, cash equivalents and restricted cash at beginning of period	51,567	4,543	500
Cash, cash equivalents and restricted cash at end of period	60,125	51,567	4,543
Supplemental disclosure of non-cash investing and financing activities:
Conversion of redeemable convertible preferred stock to common stock	144,739
Purchase of property and equipment included in accounts payable and accrued expenses	1,538		375
Accretion of redeemable convertible preferred stock to redemption value	$ 88	213	107
Cumulative dividends on Series A redeemable convertible preferred stock		437	197
Reversal of cumulative dividends on Series A redeemable convertible preferred stock		$ (634)
Conversion of notes payable and accrued interest into redeemable convertible preferred stock			7,164
Other receivable from investor for redeemable convertible preferred stock			$ 6,250

Nature of the Business and Basis of Presentation

12 Months Ended

Dec. 31, 2018

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 biopharmaceutical company developing novel medicines to bring the curative power of stem cell transplant to more patients. The Company was incorporated under the laws of the State of Delaware in June 2015 as HSCTCo Therapeutics, Inc. In February 2016, the Company changed its name to Magenta Therapeutics, Inc.

The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

On June 25, 2018, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold 6,666,667 shares of common stock at a public offering price of $15.00 per share, resulting in net proceeds of $89.9 million, after deducting underwriting discounts and commissions and other offering expenses. Upon the closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock.

Prior to its IPO, the Company had funded its operations primarily with proceeds from the sales of redeemable convertible preferred stock and issuance of convertible notes. The Company has incurred recurring losses since inception, including net losses of $57.5 million and $35.5 million, respectively, for the years ended December 31, 2018 and 2017. As of December 31, 2018, the Company had an accumulated deficit of $102.7 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 of $142.6 million as of December 31, 2018 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 efforts. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany balances and transactions have been eliminated.

The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

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, the valuation of stock-based awards and prior to the IPO, the valuation of common and preferred stock. 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 are carried at fair value with the unrealized gains and losses reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of interest and other income, net based on the specific identification method. The Company classifies its marketable securities with maturities beyond one year as short-term, based on their highly liquid nature and because such marketable securities are available for current operations.

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, 2018, 2017 or 2016.

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 accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.

Classification and Accretion of Redeemable Convertible Preferred Stock

The Company classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contain certain redemption features that were not solely within the control of the Company. The Company immediately accreted the carrying value of its redeemable convertible preferred stock to redemption value at the end of each reporting period as if the end of the reporting period were the redemption date.

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.

Stock-Based Compensation

The Company measures all stock-based awards granted to employees and directors based on the fair value on the date of grant. Compensation expense of those awards is recognized over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues awards with either service-only vesting conditions and records the expense using the straight-line method or service and performance vesting conditions and records the expense using the graded-vesting method. The Company accounts for forfeitures as they occur.

The Company measures the fair value of stock-based awards granted to non-employees on the date at which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such non-employee consultants until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of its common stock and updated assumption inputs in the Black-Scholes option-pricing model for options or the then-current fair value of its common stock for restricted stock.

The Company classifies stock-based compensation expense in its statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

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 process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2018, the Company’s only element of other comprehensive loss was unrealized gain or (loss) on marketable securities. For the years ended December 31, 2017 and 2016, there was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements.

Net Loss per Share

Prior to the closing of its IPO, the Company followed the two-class method when computing net income (loss) per share, as the Company had issued shares that met the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reported a net loss, such losses were not allocated to such participating securities, and as a result, basic and diluted net loss per share were the same.

Subsequent to the closing of its IPO, basic net income (loss) per common share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per common share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding 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.

In June 2018, upon the closing of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock automatically converted into 23,375,405 shares of the Company’s common stock.

The Company reported a net loss attributable to common stockholders for the years ended December 31, 2018, 2017 and 2016.

Recently Adopted Accounting Pronouncements

In May 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. For public and non-public entities, the standard was effective for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The Company’s adoption of ASU 2017-09 on January 1, 2018 did not have an impact on the Company’s financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230) (“ASU 2016-18”), which requires that amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. For public entities, ASU 2016-18 is effective for fiscal years beginning after December 15, 2017 and interim periods within those fiscal years. For non-public entities and emerging growth companies that choose to take advantage of the extended transition period, the standard is effective for fiscal years beginning after December 15, 2018. Early adoption is permitted for all entities. As early adoption was permitted, the Company adopted this standard retrospectively as of December 31, 2018. Restricted cash is now included as a component of cash, cash equivalents and restricted cash on the Company’s consolidated statement of cash flows. Upon the adoption of ASU 2016-18, the amount of cash and cash equivalents previously presented on the consolidated statements of cash flows for the year ended December 31, 2017 increased by less than $0.1 million as of beginning of the year and by $0.1 million as of end of the year and for the year ended December 31, 2016 increased by less than $0.1 as of the end of the year to reflect the inclusion of restricted cash in the amount reported for changes in cash, cash equivalents and restricted cash.

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (“ASU 2016-02”). ASU 2016-02 will require lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability. Leases will be classified as either operating or finance, and classification will be based on criteria similar to current lease accounting, but without explicit bright lines. For public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. For non-public 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, 2019. Early adoption is permitted for all entities. ASU 2016-02 initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its financial statements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities and emerging growth companies that choose to take advantage of the extended transition period, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. Early adoption is permitted for all entities but no earlier than the Company’s adoption of ASU 2014-09. The Company is currently evaluating whether to early adopt ASU 2018-07 and the impact that the adoption will have on its financial statements.

Marketable Securities and Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Marketable Securities and Fair Value Measurements

3. Marketable Securities and Fair Value Measurements

As of December 31, 2018, marketable securities by security type consisted of (in thousands):


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
United States Treasury Notes (due within one year)	$	71,944	$	5	$	(9	)	$	71,940
Agency Bonds (due within one year)		12,289		1		(5	)		12,285

Total	$	84,233	$	6	$	(14	)	$	84,225

The Company had no marketable securities as of December 31, 2017.

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, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	45,725	$	—	$	—	$	45,725
United States Treasury Notes		—		12,488		—		12,488
Marketable securities:
United States Treasury Notes		—		71,940		—		71,940
Agency Bonds		—		12,285		—		12,285

Total	$	45,725	$	96,713	$	—	$	142,438


	Fair Value Measurements at December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	51,147	$	—	$	—	$	51,147

Total	$	51,147	$	—	$	—	$	51,147

During the years ended December 31, 2018 and 2017, there were no transfers between Level 1, Level 2 and Level 3.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property and Equipment, Net

4. Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2018			2017
Laboratory and computer equipment	$	4,160		$	2,120
Furniture and fixtures		743			77
Leasehold improvements		6,399			141

		11,302			2,338
Less: Accumulated depreciation and amortization		(1,090	)		(382	)

	$	10,212		$	1,956

Depreciation and amortization expense was $0.9 million, $0.4 million and less than $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Accrued Expenses

5. Accrued Expenses

Accrued expenses consisted of the following (in thousands):


	December 31,
	2018		2017
Accrued external research and development expenses	$	2,619	$	1,531
Accrued payroll and related expenses		2,526		1,330
Accrued professional fees		787		963
Accrued leasehold improvements		501		—
Accrued other		494		151

	$	6,927	$	3,975

Convertible Promissory Notes

12 Months Ended

Dec. 31, 2018

Convertible Promissory Notes

6. Convertible Promissory Notes

In December 2015, the Company issued convertible promissory notes in the aggregate principal amount of $0.5 million. In 2016, the Company issued convertible promissory notes in the amount of $6.5 million. The notes accrued interest at an annual rate of 6% payable upon maturity at one year after issuance. In November 2016, the total outstanding principal and accrued interest on the notes of $7.2 million converted into 7,163,984 shares of the Company’s Series A redeemable convertible preferred stock (the “Series A Preferred Stock”) at a price of $1.00 per share. The convertible promissory notes did not contain any features which were required to be bifurcated and accounted for separately.

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Redeemable Convertible Preferred Stock

7. Redeemable Convertible Preferred Stock

In June 2018, the Company effected a one-for-2.58398 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s redeemable convertible preferred stock (“Preferred Stock”). Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios.

As of December 31, 2017, the Company’s certificate of incorporation, as amended and restated (the “Certificate of Incorporation”), authorized the Company to issue 49,178,527 shares of $0.001 par value preferred stock, of which 35,663,974 shares were designated as Series A Preferred Stock and 13,514,553 shares were designated as Series B redeemable convertible preferred stock (the “Series B Preferred Stock”). In April 2018, in connection with the sale of Series C redeemable convertible preferred stock (the “Series C Preferred Stock”), the Company’s Certificate of Incorporation was amended and restated to authorize the Company to issue 11,226,000 shares of Series C Preferred Stock. The Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock are collectively referred to as the “Preferred Stock”.

Series A Preferred Stock Purchase Agreement

In November 2016, the Company entered into a Series A Preferred Stock purchase agreement and issued 17,718,974 shares of Series A Preferred Stock at $1.00 per share for gross proceeds of $10.6 million and the conversion of principal and accrued interest of $7.2 million on convertible promissory notes (see Note 6). Cash proceeds of $6.3 million were received in January 2017. The Company incurred issuance costs of $0.1 million in connection with the issuance and sale of the Series A Preferred Stock.

The Series A Preferred Stock purchase agreement obligated the Company to sell and the Purchasers (as defined) to purchase at $1.00 per share an aggregate of 15,445,000 shares of Series A Preferred Stock at a subsequent closing (the “First Subsequent Closing”) and at the Company’s sole discretion an aggregate of 15,500,000 shares of Series A Preferred Stock at a second subsequent closing (the “Second Subsequent Closing”), upon the achievement or waiver of certain milestones. In April 2017, in connection with the First Subsequent Closing and the waiver of the applicable milestones, the Company received gross proceeds of $15.4 million for the issuance and sale of 15,445,000 shares of Series A Preferred Stock. The Company determined that the future tranche obligations of the Series A Preferred Stock purchase agreement did not meet the definition of a freestanding financial instrument because, while separately exercisable, it was not legally detachable. Further, the Company determined that the embedded future tranche obligation did not require bifurcation for accounting purposes as it was clearly and closely related to the economic characteristics and risks of the initial preferred shares and would not meet the definition of a derivative on a standalone basis. There was no Second Subsequent Closing.

Series B Preferred Stock Purchase Agreement

In April 2017, the Company entered into a Series B Preferred Stock purchase agreement and issued and sold 12,871,003 shares of Series B Preferred Stock at $3.8847 per share for gross proceeds of $50.0 million in two separate closings. The Company incurred issuance costs of $0.2 million in connection with the issuance and sale of the Series B Preferred Stock.

Series C Preferred Stock Purchase Agreement

In April 2018, the Company entered into a Series C Preferred Stock purchase agreement, pursuant to which the Company issued and sold 11,223,102 shares of Series C redeemable convertible Preferred Stock at a price of $4.66 per share for total gross proceeds of $52.3 million.

Issuance of Preferred Stock for License

In consideration for the grant of rights to the Company pursuant to a license agreement between the Company and Novartis International Pharmaceutical Ltd., (“Novartis”), dated April 3, 2017 (see Note 11), the Company issued 2,500,000 shares of Series A Preferred Stock and 643,550 shares of Series B Preferred Stock to Novartis.

Upon issuance of each class of Preferred Stock the Company assessed the embedded conversion and liquidation features of the securities and determined that such features did not require the Company to separately account for these features. The Company also concluded that no beneficial conversion feature existed upon the issuance date of each class of Preferred Stock. As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts):


	December 31, 2017
	Preferred Stock Authorized		Preferred Stock Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A Preferred Stock		35,663,974		35,663,974	$	39,939	$	35,664		13,801,936
Series B Preferred Stock		13,514,553		13,514,553		52,500		52,500		5,230,130

		49,178,527		49,178,527	$	92,439	$	88,164		19,032,066

Upon closing of the IPO in June 2018, all of the outstanding shares of Preferred Stock, including the shares of Series C Preferred Stock issued in April 2018, were converted into 23,375,405 shares of common stock. Prior to the conversion, the holders of the Preferred Stock had the following rights and preferences:

Voting Rights

The holders of Preferred Stock were entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote. Each preferred stockholder was entitled to the number of votes equal to the number of shares of common stock into which each Preferred Stock was convertible at the time of such vote.

Dividends

There were no stated dividends on the Preferred Stock.

At the time of issuance, the holders of Series A Preferred Stock were entitled to receive cumulative dividends at an annual rate of 8% of the Original Issue Price (as described below). However, in connection with the Series B Preferred Stock financing in April 2017, holders of Series A Preferred Stock waived their right to dividends. During the years ended December 31, 2017 and 2016, the Company accrued dividends of $0.4 million and $0.2 million, respectively, and recorded an increase to the carrying value of the Series A Preferred Stock with an offset to additional paid in capital. In April 2017, in connection with the elimination of dividends on the Series A Preferred Stock, the Company reduced the carrying value of the Series A Preferred Stock by $0.6 million with an offset to additional paid in capital.

The Company may not declare, pay or set aside any dividends on shares of any other class or series of capital stock of the Company unless the holders of Preferred Stock then outstanding shall first receive, or simultaneously receive, dividends on each outstanding share of Preferred Stock. To date, no dividends had been approved or paid.

Liquidation

In the event of any liquidation, dissolution or winding up of the Company or Liquidating Event (as described below), the holders of Preferred Stock then outstanding were entitled, on a pari passu basis, to be paid out of the assets of the Company prior to any payments made to the holders of common stock by reason of their ownership thereof, an amount per share equal to the greater of the Original Issue Price (as described below) plus any dividends declared but unpaid or such amount per share as would have been payable had all shares of Preferred Stock been converted into common stock. If upon liquidation the assets of the Company available for distribution to its stockholders were insufficient to pay the holders of Preferred Stock the full amount, the holders of the shares of Preferred Stock would share ratably in any distribution in proportion to the respective amount of Preferred Stock.

Upon completion of the liquidation preference distribution to the holders of Preferred Stock, the remaining assets of the Company would be distributed among holders of common stock, pro rata, based on the number of shares held by each such holder. Unless the holders of a majority of the outstanding Preferred Stock, voting together as a single class, and in certain circumstances, holders of a majority of the outstanding Series B Preferred Stock, voting as a separate class, elect otherwise, Liquidating Event would include a merger or consolidation (other than one in which shareholders of the Company own a majority of the voting power of the outstanding shares of the surviving or acquiring corporation) or a sale, lease, transfer, exclusive license or other disposition of all or substantially all of the assets of the Company.

Conversion

Each share of Preferred Stock was convertible at the option of the stockholder at any time without the payment of additional consideration by the holder thereof, into a number of shares of common stock at the applicable conversion ratio determined by dividing the Original Issue Price by the Conversion Price of each series. The Original Issue Price was $1.00 per share for Series A Preferred Stock, $3.8847 per share for Series B Preferred Stock and $4.66 per share for Series C Preferred Stock. The Conversion Price is $2.58398 per share for Series A Preferred Stock, $10.03799 per share for Series B Preferred Stock and $12.04135 per share for Series C Preferred Stock, subject to appropriate adjustment in the event of any stock split, stock dividend, combination or other similar recapitalization and other adjustments as set forth in the Company’s certificate of incorporation, as amended and restated.

Each share of Preferred Stock automatically converts into shares of common stock at the then effective conversion ratio (i) upon the closing of an initial public offering with proceeds to the Company of at least $50.0 million, after deducting underwriting discounts and commissions or (ii) upon the vote or written consent of the holders of a majority of the then outstanding shares of the Preferred Stock, voting together as a single class, and holders of a majority of the outstanding Series B Preferred Stock, voting as a separate class.

As of December 31, 2017, all outstanding shares of Preferred Stock were convertible into shares of common stock on a 2.58398-for-one basis.

Redemption Rights

On or after the fifth anniversary of the Series C original issue date, or April 2, 2023, shares of the Preferred Stock were subject to mandatory redemption by the Company in three equal annual installments beginning 60 days after receipt of a notice of redemption from the holders of a majority of the voting power of the holders of the outstanding Preferred Stock, voting as a single class. As of December 31, 2017, the redemption price for the Preferred Stock was equal to the Original Issuance Price per share, plus any accruing dividends accrued but unpaid therein.

Reissuance

Shares of any Preferred Stock that were redeemed or converted will be retired or canceled and may not be reissued by the Company.

Common Stock

12 Months Ended

Dec. 31, 2018

Common Stock

8. Common Stock

On June 6, 2018, the Company effected a one-for-2.58398 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Preferred Stock. Accordingly, all share and per share amounts for all periods presented in the accompanying consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios.

On June 25, 2018, the Company completed its IPO pursuant to which it issued and sold 6,666,667 shares of common stock at a public offering price of $15.00 per share, resulting in net proceeds of $89.9 million, after deducting underwriting discounts and commissions and other offering expenses. Upon the closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock.

In 2017 and 2016, the Company issued 218,654 and 4,377,280 shares, respectively, of restricted stock to employees and consultants of the Company. Unvested shares of restricted stock may not be sold or transferred by the holder. These restrictions lapse according to the vesting conditions of each award (see Note 9). The Company issued 193,498 shares of common stock to two investors in exchange for consulting services (see Note 14). In 2016, the Company issued 385,063 shares of common stock in connection with a license agreement (see Note 11).

As of December 31, 2018, the Company’s amended and restated certificate of incorporation authorizes the Company to issue 150,000,000 shares of common stock, par value $0.001 per share, and 10,000,000 shares of preferred stock, par value $0.001 per share.

As of December 31, 2017, the Company’s Certificate of Incorporation authorized the Company to issue 70,000,000 shares of common stock, $0.001 par value. In connection with the Series C Preferred Stock purchase agreement, the Company’s Certificate of Incorporation was amended and restated to increase the number of common stock authorized from 70,000,000 to 85,000,000 shares.

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

12 Months Ended

Dec. 31, 2018

Stock-Based Compensation

9. Stock-Based Compensation

2016 Stock Option and Grant Plan

In April 2018, the Company’s 2016 Stock Option and Grant Plan (the “2016 Plan”) was amended to increase the number of shares reserved for issuance under the 2016 Plan by 1,884,850 shares to 5,900,917 shares. Upon effectiveness of the Company’s 2018 Stock Option and Incentive Plan, (the “2018 Plan”) in June 2018, the remaining 1,142,136 shares available under the 2016 Plan became available for issuance under the 2018 Plan and no future issuance will be made under the 2016 Plan. Additionally, shares of common stock underlying any awards under the 2016 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by the Company prior to vesting, satisfied without any issuance of stock, expire or are otherwise terminated (other than by exercise) will be available for future awards under the 2018 Plan.

2018 Stock Option and Incentive Plan

The 2018 Plan was approved in May 2018 and became effective on June 19, 2018. The 2018 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares to employees, directors and consultants. The Company initially reserved 1,765,162 shares of common stock plus the 1,142,136 shares of common stock remaining available for issuance under the 2016 Plan for the issuance of awards under the 2018 Plan. As of December 31, 2018, 2,693,929 shares remained available for future grants under the 2018 Plan.

The 2018 Plan provides that the number of shares reserved and available for issuance under the 2018 Plan will automatically increase each January 1, beginning on January 1, 2019, 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 1,332,201 shares effective January 1, 2019.

Both the 2016 Plan and the 2018 Plan are administered by the board of directors, or at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the term of awards may not be greater than ten years. Vesting periods are determined at the discretion of the board of directors. Awards granted to employees and directors typically vest over three or four years. The exercise price for stock options granted may not be less than the fair value of common stock as of the date of grant. Subsequent to the IPO, the fair value of common stock is based on quoted market prices. Prior to the IPO, the Company’s board of directors valued the Company’s common stock, taking into consideration its most recently available valuation of common stock performed by third parties as well as additional factors which may have changed since the date of the most recent contemporaneous valuation through the date of grant.

Common Stock Option Valuation

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 information. Therefore, the Company estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The assumptions that the Company used to determine the fair value of options granted were as follows, presented on a weighted average basis:


	Year Ended December 31,
	2018			2017
Risk-free interest rate		2.8	%		2.0	%
Expected term (in years)		6.2			6.0
Expected volatility		78.4	%		78.1	%
Expected dividend yield		0	%		0	%

There were no common stock options granted during the year ended December 31, 2016.

Common Stock Option Activity

The following table summarizes the Company’s option activity since December 31, 2017:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding as of December 31, 2017		624,406		$	4.84		9.8	$	1,791
Granted		3,123,026			9.40
Exercised		(4,152	)		7.71
Forfeited		(233,451	)		8.29

Outstanding as of December 31, 2018		3,509,829		$	8.67		9.2	$	526

Options vested and expected to vest as of December 31, 2018		3,509,829		$	8.67		9.2	$	526

Options exercisable as of December 31, 2018		573,669		$	7.51		9.1	$	179

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 year ended December 31, 2018 was less than $0.1 million. There were no options exercised during the years ended December 31, 2017 and 2016.

The weighted average grant-date fair value per share of stock options granted during the years ended December 31, 2018 and 2017 was $6.56 and $3.31, respectively.

As of December 31, 2018, there was 187,686 stock options held by non-employees. There were no stock options held by non-employees as of December 31, 2017.

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, 2017:


	Number of Shares			Weighted Average Grant Date Fair Value
Outstanding as of December 31, 2017		2,107,300		$	1.25
Vested		(907,562	)		1.23
Forfeited		(177,217	)		1.89

Outstanding as of December 31, 2018		1,022,521		$	1.16

The total fair value of restricted stock vested during the years ended December 31, 2018, 2017 and 2016 was $9.3 million, $1.9 million and $0.6 million, respectively.

As of December 31, 2018 and 2017, there were 326,017 and 626,376, respectively, of unvested shares of restricted stock held by non-employees.

Stock-Based Compensation

Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Research and development expenses	$	6,032	$	1,626	$	427
General and administrative expenses		4,671		633		235

	$	10,703	$	2,259	$	662

As of December 31, 2018, total unrecognized compensation cost related to the unvested stock-based awards was $16.8 million, which is expected to be recognized over a weighted average period of 2.3 years.

Net Loss per Share

12 Months Ended

Dec. 31, 2018

Net Loss per Share

10. Net Loss per Share

Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):


	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(57,515	)	$	(35,491	)	$	(9,431	)
Accretion of redeemable convertible preferred stock to redemption value		(88	)		(213	)		(107	)
Cumulative dividends on redeemable convertible preferred stock		—			(437	)		(197	)
Reversal of cumulative dividends on redeemable convertible preferred stock		—			634			—

Net loss attributable to common stockholders	$	(57,603	)	$	(35,507	)	$	(9,735	)

Denominator:
Weighted average common shares outstanding, basic and diluted		18,389,576			1,856,907			149,414

Net loss per share attributable to common stockholders, basic and diluted	$	(3.13	)	$	(19.12	)	$	(65.15	)

Common Stock Equivalents

The following common stock equivalents 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:


	December 31,
	2018		2017		2016
Stock options to purchase common stock		3,509,829		624,406		—
Unvested restricted common stock		1,022,521		2,107,300		3,603,847
Redeemable convertible preferred stock (as converted to common stock)		—		19,032,066		6,857,241

		4,532,350		21,763,772		10,461,088

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies

11. 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 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 new leased premises. In connection with the sublease, the sublandlord agreed to fund up to $0.7 million in tenant improvements to the leased facility which was included in prepaid expenses and other current assets at December 31, 2018. 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 balance sheet at December 31, 2018.

The tenant improvement allowance and payment escalations specified in the lease agreement are accrued or deferred as appropriate such that rent expense per square foot is recognized on a straight-line basis over the terms of occupancy. The Company recorded rent expense of $4.4 million, $1.1 million and less than $0.1 million during the years ended December 31, 2018, 2017 and 2016, respectively.

In the fourth quarter of 2018, with the landlord’s consent, the Company entered into two two-year sub-subleases of approximately 27,000 square feet of office space in Cambridge, Massachusetts under which it will receive $4.5 million of payments over the sub-sublease term.

In August 2018, in connection with the sublease agreement entered into in May 2018, the Company entered into an agreement for the build-out and customization of this space. Under the agreement, the Company is obligated to make payments of up to $6.8 million, of which $6.1 million has been incurred through December 31, 2018.

The Company had a sublease for office space in Cambridge, Massachusetts that expired in September 2018. The Company was required to maintain a cash balance of $0.2 million to secure a letter of credit associated with this sublease. This amount was classified as restricted cash in the balance sheet at December 31, 2017.

As of December 31, 2018, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands):


2019	$	5,434
2020		5,597
2021		5,765
2022		5,938
2023		6,116
Thereafter		27,516

	$	56,366

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 year ended December 31, 2018, the Company recorded $1.7 million of research and development expense related to this agreement for upfront technology access fees, research exclusivity fees and research support.

Intellectual Property Licenses

In November 2016, the Company entered into a license agreement with the President and Fellows of Harvard College (“Harvard”) for an exclusive, worldwide, royalty-bearing license for certain technologies related to conditioning and mobilization. In consideration for these rights the Company paid Harvard an upfront fee of $0.1 million and reimbursed Harvard $0.3 million for costs incurred by Harvard related to the patented technology, which amounts were recorded as research and development expense during the year ended December 31, 2016. The Company also issued 385,063 shares of common stock in connection with the license agreement. The fair value of the shares of $0.3 million as of the issuance date was recorded as research and development expense during the year ended December 31, 2016. The Company is obligated to pay Harvard maintenance fees of less than $0.1 million annually through 2019 and $0.1 million annually thereafter 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 licensed products upon the achievement of certain development and regulatory milestones and to pay royalties on a product-by-product and country-by-country basis on net sales of products licensed under the agreement. As of December 31, 2018, no milestones related to this agreement have been met.

In April 2017, the Company entered into a license agreement with Novartis International Pharmaceutical Ltd. (“Novartis”) 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 hematopoietic stem cells. In consideration for these rights, the Company issued 2,500,000 shares of Series A redeemable convertible preferred stock and 643,550 shares of Series B redeemable convertible preferred stock to Novartis. The total fair value of the shares of $9.3 million as of the issuance date was recorded as research and development expense during the year ended December 31, 2017. The Company is obligated to make payments of up to $177.0 million upon the achievement of specified clinical and regulatory milestones and up to $125.0 million upon the achievement of specified commercial milestones and to pay tiered royalties, on a product-by-product and country-by-country basis, up to a maximum of 20% on net sales of products licensed under the agreement. As of December 31, 2018, no milestones related to the Novartis License have been met.

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, 2018, the Company expensed $0.8 million related to the license of certain developed technologies under these agreements.

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 not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2018 or 2017.

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	12 Months Ended
401(k) Savings Plan	Dec. 31, 2018
401(k) Savings Plan	12. 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

12 Months Ended

Dec. 31, 2018

Income Taxes

13. Income Taxes

During the years ended December 31, 2018, 2017 and 2016, 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,
	2018			2017			2016
Federal statutory income tax rate		21.0	%		34.0	%		34.0	%
State taxes, net of federal benefit		5.6			4.9			4.6
Research and orphan drug tax credits		4.7			3.4			1.8
Impact of 2018 tax rate change on temporary differences		—			(13.4	)		—
Other		(2.1	)		(3.7	)		(4.2	)
Increase in deferred tax asset valuation allowance		(29.2	)		(25.2	)		(36.2	)

Effective income tax rate		—	%		—	%		—	%

Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands):


	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	11,106		$	5,158
Capitalized research and development expenses		12,030			5,925
Research and orphan drug tax credit carryforwards		4,108			1,400
Stock compensation expense		1,336			9
Accrued expense		664			20
Other		360			—

Total deferred tax assets		29,604			12,512
Valuation allowance		(29,243	)		(12,466	)

Net deferred tax assets		361			46

Deferred tax liabilities:
Depreciation and amortization		(361	)		(46	)

Total deferred tax liabilities		(361	)		(46	)

Net deferred tax assets and liabilities	$	—		$	—

On December 22, 2017, the Tax Cuts and Jobs Act (the “TCJA”) was signed into United States law. The TCJA included a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 35% to 21%, effective as of January 1, 2018, as well as limitation of the deduction for net operating losses to 80% of annual taxable income and elimination of net operating loss carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such net operating losses may be carried forward indefinitely). The tax rate change resulted in (i) a reduction in the gross amount of the Company’s deferred tax assets recorded as of December 31, 2017, without an impact on the net amount of its deferred tax assets, which are recorded with a full valuation allowance, and (ii) no income tax expense or benefit being recognized as of the enactment date of the TCJA. In connection with the TCJA, the Company remeasured its deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21% for federal tax purposes. The remeasurement of the Company’s deferred tax assets and liabilities resulted in a provision of $4.8 million to income tax expense which was offset by a corresponding decrease in the valuation allowance.

As of December 31, 2018, the Company had net operating loss carryforwards for federal income tax purposes of $40.6 million, of which $17.5 million begin to expire in 2035 and $23.1 million can be carried forward indefinitely. As of December 31, 2018, the Company had net operating loss carryforwards for state income tax purposes of $40.8 million which begin to expire in 2035. As of December 31, 2018, the Company also had available research and orphan drug tax credit carryforwards for federal and state income tax purposes of $3.4 million and $0.8 million, respectively, which begin to expire in 2035 and 2030, respectively. Utilization of the net operating loss carryforwards and research and orphan drug tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended (the “Code”) due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. The Company has not conducted a formal study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382 and 383 of the Code, at any time since inception, utilization of the net operating loss carryforwards or research and orphan drug tax credit carryforwards may be subject to an annual limitation under Section 382 and 383 of the Code, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and orphan drug tax credit carryforwards before utilization.

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, 2018 and 2017. 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, 2018, 2017 and 2016 related primarily to the increase in net operating loss carryforwards and research and orphan drug tax credit carryforwards, partially offset in 2017 by a decrease in deferred tax assets resulting from the decreased federal corporate tax rate, and were as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Valuation allowance as of beginning of year	$	12,466	$	3,540	$	129
Net increases recorded to income tax provision		16,777		8,926		3,411

Valuation allowance as of end of year	$	29,243	$	12,466	$	3,540

The Company has not recorded any amounts for unrecognized tax benefits as of December 31, 2018 or 2017.

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 2015, the inception of the Company, to the present. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

Related Parties

12 Months Ended

Dec. 31, 2018

Related Parties

14. Related Parties

Atlas Venture Life Science Advisors, LLC and Third Rock Ventures LLC

During the years ended December 31, 2017 and 2016, the Company received consulting, advisory and related services from Atlas Venture Life Science Advisors, LLC and Third Rock Ventures LLC, holders, together with their affiliates, of more than 5% of the Company’s common stock outstanding. For the years ended December 31, 2017 and 2016, the Company recorded $0.2 million and $1.1 million, respectively, of general and administrative expenses for management and advisory services and other related services provided by these investors. The Company also issued 96,749 shares of common stock to each investor and recorded $0.2 million of general and administrative expense for the year ended December 31, 2016 related to these shares. There were no services provided during the year ended December 31, 2018 and no amounts owed to these investors as of December 31, 2018 or 2017.

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. The Company has a collaboration agreement with Be The Match BioTherapies, LLC and a research agreement with an affiliated organization, Center for International Blood and Marrow Transplant Research. For the year ended December 31, 2018, the Company recorded $0.6 million to research and development expenses and $0.2 million to general and administrative expenses and made payments of $0.6 million related to these agreements. As of December 31, 2018, amounts on the balance sheet related to these agreements was $0.4 million included in accounts payable and accrued expenses and other current liabilities.

In April 2018, the Company sold 246,781 shares of Series C Preferred Stock to Be The Match Bio Therapies, LLC for $1.1 million (see Note 7).

Novartis

In April 2017, the Company entered into a license agreement with Novartis, a beneficial owner of 5% or more of its outstanding shares of capital stock as of December 31, 2017, to use and develop certain patent rights. In consideration for these rights, the Company issued 2,500,000 shares of Series A Preferred Stock and 643,550 shares of Series B Preferred Stock to Novartis. The fair value of the shares of $9.3 million as of the issuance date was recorded as research and development expense during the year ended December 31, 2017 (see Note 11). As of December 31, 2018, Novartis is no longer an owner of 5% or more of the Company’s outstanding stock.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Selected Quarterly Financial Data (Unaudited)

15. Selected Quarterly Financial Data (Unaudited)

The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information (in thousands except per share data):


	Three Months Ended
	Dec. 31, 2018			Sept. 30, 2018			June 30, 2018			March 31, 2018			Dec. 31, 2017			Sept. 30, 2017			June 30, 2017			March 31, 2017
Statements of Operations Data:
Revenue	$	—		$	—		$	—		$	—		$	—		$	—		$	—		$	—
Loss from operations		(17,930	)		(16,702	)		(14,025	)		(11,306	)		(8,170	)		(7,015	)		(15,731	)		(4,811	)
Net loss		(16,679	)		(16,015	)		(13,660	)		(11,161	)		(8,036	)		(6,913	)		(15,731	)		(4,811	)
Net loss attributable to common stockholders		(16,679	)		(16,015	)		(13,748	)		(11,161	)		(8,036	)		(6,919	)		(15,387	)		(5,165	)
Net loss per share attributable to common stockholders, basic and diluted		(0.50	)		(0.49	)		(3.13	)		(4.53	)		(3.59	)		(3.44	)		(8.90	)		(3.59	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Use of Estimates

Use of Estimates

Concentrations of Credit Risk and of Significant Suppliers

Concentrations of Credit Risk and of Significant Suppliers

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

Property and Equipment

Property and Equipment


		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

Impairment of Long-Lived Assets

Impairment of Long-Lived Assets

Fair Value Measurements

Fair Value Measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Classification and Accretion of Redeemable Convertible Preferred Stock

Classification and Accretion of Redeemable Convertible Preferred Stock

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 Contract Costs and Accruals

Research Contract Costs and Accruals

Patent Costs

Patent Costs

Stock-Based Compensation

Stock-Based Compensation

Income Taxes

Income Taxes

Comprehensive Loss

Comprehensive Loss

Net Loss per Share

Net Loss per Share

The Company reported a net loss attributable to common stockholders for the years ended December 31, 2018, 2017 and 2016.

Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncments

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Estimated Useful Life of Asset


		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

Marketable Securities and Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Marketable Securities by Security Type

As of December 31, 2018, marketable securities by security type consisted of (in thousands):


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
United States Treasury Notes (due within one year)	$	71,944	$	5	$	(9	)	$	71,940
Agency Bonds (due within one year)		12,289		1		(5	)		12,285

Total	$	84,233	$	6	$	(14	)	$	84,225

Schedule of Financial Assets Measured at Fair Value on Recurring Basis


	Fair Value Measurements at December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	45,725	$	—	$	—	$	45,725
United States Treasury Notes		—		12,488		—		12,488
Marketable securities:
United States Treasury Notes		—		71,940		—		71,940
Agency Bonds		—		12,285		—		12,285

Total	$	45,725	$	96,713	$	—	$	142,438


	Fair Value Measurements at December 31, 2017 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	51,147	$	—	$	—	$	51,147

Total	$	51,147	$	—	$	—	$	51,147

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2018			2017
Laboratory and computer equipment	$	4,160		$	2,120
Furniture and fixtures		743			77
Leasehold improvements		6,399			141

		11,302			2,338
Less: Accumulated depreciation and amortization		(1,090	)		(382	)

	$	10,212		$	1,956

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Accrued Expenses

Accrued expenses consisted of the following (in thousands):


	December 31,
	2018		2017
Accrued external research and development expenses	$	2,619	$	1,531
Accrued payroll and related expenses		2,526		1,330
Accrued professional fees		787		963
Accrued leasehold improvements		501		—
Accrued other		494		151

	$	6,927	$	3,975

Redeemable Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Preferred Stock

As of December 31, 2017, the Preferred Stock consisted of the following (in thousands, except share amounts):


	December 31, 2017
	Preferred Stock Authorized		Preferred Stock Issued and Outstanding		Carrying Value		Liquidation Preference		Common Stock Issuable Upon Conversion
Series A Preferred Stock		35,663,974		35,663,974	$	39,939	$	35,664		13,801,936
Series B Preferred Stock		13,514,553		13,514,553		52,500		52,500		5,230,130

		49,178,527		49,178,527	$	92,439	$	88,164		19,032,066

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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 December 31,
	2018			2017
Risk-free interest rate		2.8	%		2.0	%
Expected term (in years)		6.2			6.0
Expected volatility		78.4	%		78.1	%
Expected dividend yield		0	%		0	%

Schedule of Common Stock Option Activity

The following table summarizes the Company’s option activity since December 31, 2017:


	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding as of December 31, 2017		624,406		$	4.84		9.8	$	1,791
Granted		3,123,026			9.40
Exercised		(4,152	)		7.71
Forfeited		(233,451	)		8.29

Outstanding as of December 31, 2018		3,509,829		$	8.67		9.2	$	526

Options vested and expected to vest as of December 31, 2018		3,509,829		$	8.67		9.2	$	526

Options exercisable as of December 31, 2018		573,669		$	7.51		9.1	$	179

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, 2017:


	Number of Shares			Weighted Average Grant Date Fair Value
Outstanding as of December 31, 2017		2,107,300		$	1.25
Vested		(907,562	)		1.23
Forfeited		(177,217	)		1.89

Outstanding as of December 31, 2018		1,022,521		$	1.16

Schedule of Stock Based Compensation Expense

Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands):


	Year Ended December 31,
	2018		2017		2016
Research and development expenses	$	6,032	$	1,626	$	427
General and administrative expenses		4,671		633		235

	$	10,703	$	2,259	$	662

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders

Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts):


	Year Ended December 31,
	2018			2017			2016
Numerator:
Net loss	$	(57,515	)	$	(35,491	)	$	(9,431	)
Accretion of redeemable convertible preferred stock to redemption value		(88	)		(213	)		(107	)
Cumulative dividends on redeemable convertible preferred stock		—			(437	)		(197	)
Reversal of cumulative dividends on redeemable convertible preferred stock		—			634			—

Net loss attributable to common stockholders	$	(57,603	)	$	(35,507	)	$	(9,735	)

Denominator:
Weighted average common shares outstanding, basic and diluted		18,389,576			1,856,907			149,414

Net loss per share attributable to common stockholders, basic and diluted	$	(3.13	)	$	(19.12	)	$	(65.15	)

Schedule of Antidilutive Common Stock Equivalents Excluded from Computation of Earnings Per Share


	December 31,
	2018		2017		2016
Stock options to purchase common stock		3,509,829		624,406		—
Unvested restricted common stock		1,022,521		2,107,300		3,603,847
Redeemable convertible preferred stock (as converted to common stock)		—		19,032,066		6,857,241

		4,532,350		21,763,772		10,461,088

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease

As of December 31, 2018, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands):


2019	$	5,434
2020		5,597
2021		5,765
2022		5,938
2023		6,116
Thereafter		27,516

	$	56,366

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
Federal statutory income tax rate		21.0	%		34.0	%		34.0	%
State taxes, net of federal benefit		5.6			4.9			4.6
Research and orphan drug tax credits		4.7			3.4			1.8
Impact of 2018 tax rate change on temporary differences		—			(13.4	)		—
Other		(2.1	)		(3.7	)		(4.2	)
Increase in deferred tax asset valuation allowance		(29.2	)		(25.2	)		(36.2	)

Effective income tax rate		—	%		—	%		—	%

Company's Deferred Tax Assets

Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following (in thousands):


	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	11,106		$	5,158
Capitalized research and development expenses		12,030			5,925
Research and orphan drug tax credit carryforwards		4,108			1,400
Stock compensation expense		1,336			9
Accrued expense		664			20
Other		360			—

Total deferred tax assets		29,604			12,512
Valuation allowance		(29,243	)		(12,466	)

Net deferred tax assets		361			46

Deferred tax liabilities:
Depreciation and amortization		(361	)		(46	)

Total deferred tax liabilities		(361	)		(46	)

Net deferred tax assets and liabilities	$	—		$	—

Changes in Deferred Tax Asset Valuation Allowance


	Year Ended December 31,
	2018		2017		2016
Valuation allowance as of beginning of year	$	12,466	$	3,540	$	129
Net increases recorded to income tax provision		16,777		8,926		3,411

Valuation allowance as of end of year	$	29,243	$	12,466	$	3,540

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Selected Quarterly Financial Data


	Three Months Ended
	Dec. 31, 2018			Sept. 30, 2018			June 30, 2018			March 31, 2018			Dec. 31, 2017			Sept. 30, 2017			June 30, 2017			March 31, 2017
Statements of Operations Data:
Revenue	$	—		$	—		$	—		$	—		$	—		$	—		$	—		$	—
Loss from operations		(17,930	)		(16,702	)		(14,025	)		(11,306	)		(8,170	)		(7,015	)		(15,731	)		(4,811	)
Net loss		(16,679	)		(16,015	)		(13,660	)		(11,161	)		(8,036	)		(6,913	)		(15,731	)		(4,811	)
Net loss attributable to common stockholders		(16,679	)		(16,015	)		(13,748	)		(11,161	)		(8,036	)		(6,919	)		(15,387	)		(5,165	)
Net loss per share attributable to common stockholders, basic and diluted		(0.50	)		(0.49	)		(3.13	)		(4.53	)		(3.59	)		(3.44	)		(8.90	)		(3.59	)

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jun. 25, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date										Jun. 30, 2015
Entity incorporation state name										State of Delaware
Net proceeds from initial public offering										$ 93,000
Net loss		$ (16,679)	$ (16,015)	$ (13,660)	$ (11,161)	$ (8,036)	$ (6,913)	$ (15,731)	$ (4,811)	(57,515)	$ (35,491)	$ (9,431)
Accumulated deficit		(102,726)				$ (45,211)				(102,726)	$ (45,211)
Cash and cash equivalents		$ 142,600								$ 142,600
Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares										6,666,667
Common Stock [Member] \| IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares	6,666,667
Stock issued, price per shares	$ 15.00
Net proceeds from initial public offering	$ 89,900

Summary of Significant Accounting Policies - Schedule of Estimated Useful Life of Asset (Detail)	12 Months Ended
	Dec. 31, 2018
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 Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 25, 2018
Summary Of Significant Accounting Policies [Line Items]
Impairment losses on long-lived assets	$ 0	$ 0	$ 0
Redeemable convertible preferred stock converted into common stock shares		19,032,066		23,375,405
Maximum [Member] \| Accounting Standards Update 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Net increase in cash and cash equivalents		$ 100,000	$ 100,000

Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities by Security Type (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Marketable Securities [Line Items]
Amortized Cost	$ 84,233
Gross Unrealized Gains	6
Gross Unrealized Losses	(14)
Estimated Fair Value	84,225
United States Treasury Notes [Member]
Marketable Securities [Line Items]
Amortized Cost	71,944
Gross Unrealized Gains	5
Gross Unrealized Losses	(9)
Estimated Fair Value	71,940
Agency Bonds [Member]
Marketable Securities [Line Items]
Amortized Cost	12,289
Gross Unrealized Gains	1
Gross Unrealized Losses	(5)
Estimated Fair Value	$ 12,285

Marketable Securities and Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		$ 0
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	0
Fair value, assets, level 2 to level 1 transfers, amount	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset transfers into level 3	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers out of level 3	$ 0	$ 0

Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	$ 142,438	$ 51,147
United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	71,940
Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	12,285
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	45,725	51,147
United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	12,488
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	45,725	51,147
Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	45,725	$ 51,147
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	96,713
Level 2 [Member] \| United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	71,940
Level 2 [Member] \| Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	12,285
Level 2 [Member] \| United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets Measured at Fair Value on Recurring Basis	$ 12,488

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 11,302	$ 2,338
Less: Accumulated depreciation and amortization	(1,090)	(382)
Property and equipment, net	10,212	1,956
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	6,399	141
Laboratory and Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,160	2,120
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 743	$ 77

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 875	$ 376	$ 6

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 2,619	$ 1,531
Accrued payroll and related expenses	2,526	1,330
Accrued professional fees	787	963
Accrued leasehold improvements	501
Accrued other	494	151
Total	$ 6,927	$ 3,975

Convertible Promissory Notes - Additional Information (Detail) - Convertible Promissory Notes [Member] - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Convertible promissory notes issued			$ 6.5	$ 0.5
Accrued interest rate		6.00%
Total outstanding principal and accrued interest on notes	$ 7.2
Series A- Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Shares issued in lieu of outstanding principal and accrued interest on notes	7,163,984
Conversion price per share	$ 1.00

Redeemable Convertible Preferred Stock - Additional Information (Detail)		1 Months Ended					12 Months Ended
	Jun. 06, 2018	Jun. 30, 2018	Apr. 30, 2018 USD ($) $ / shares shares	Apr. 30, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($)	Nov. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($)	Jun. 25, 2018 shares	Apr. 03, 2017 shares
Temporary Equity [Line Items]
Reverse stock split, conversion ratio	0.387	0.387						0.387
Preferred stock, shares authorized							0	49,178,527
Preferred stock, par value \| $ / shares							$ 0.001	$ 0.001
Preferred stock issued, shares							0	49,178,527
Proceeds from issuance of redeemable convertible preferred stock \| $							$ 52,300,000	$ 71,695,000	$ 4,305,000
Payments of stock issuance costs \| $							$ 3,094,000
Preferred stock, Shares issued							0	0
Conversion of preferred stock into common stock								19,032,066		23,375,405
IPO [Member] \| Minimum [Member]
Temporary Equity [Line Items]
Proceeds from issuance of common stock \| $							$ 50,000,000
Series C Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, shares authorized			11,226,000
Proceeds from issuance of redeemable convertible preferred stock \| $			$ 52,300,000
Stock issued, shares			11,223,102
Stock issued, price per shares \| $ / shares			$ 4.66				$ 4.66
Conversion of preferred stock into common stock			23,375,405
Conversion price of preferred stock \| $ / shares							$ 12.04135
Original date of preferred stock issuance							Apr. 02, 2023
Preferred stock redemption description							On or after the fifth anniversary of the Series C original issue date, or April 2, 2023, shares of the Preferred Stock were subject to mandatory redemption by the Company in three equal annual installments beginning 60 days after receipt of a notice of redemption from the holders of a majority of the voting power of the holders of the outstanding Preferred Stock, voting as a single class.
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, shares authorized								35,663,974
Preferred stock, par value \| $ / shares						$ 1.00
Preferred stock issued, shares						17,718,974		35,663,974
Proceeds from issuance of redeemable convertible preferred stock \| $				$ 15,400,000	$ 6,300,000	$ 10,600,000
Total outstanding principal and accrued interest on notes \| $						7,200,000
Payments of stock issuance costs \| $						$ 100,000
Preferred stock share purchase, per share \| $ / shares							$ 1.00
Stock issued, shares				15,445,000
Stock issued, price per shares \| $ / shares							$ 1.00
Conversion of preferred stock into common stock								13,801,936
Dividend on preferred stock \| $							$ 0
Dividend on preferred stock, percentage							8.00%
Preferred stock accrued dividends \| $								$ 400,000	$ 200,000
Reduction in preferred stock carrying value \| $				$ (600,000)
Conversion price of preferred stock \| $ / shares							$ 2.58398
Series A Preferred Stock [Member] \| License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Temporary Equity [Line Items]
Preferred stock, Shares issued											2,500,000
Series A Preferred Stock [Member] \| First Closing [Member]
Temporary Equity [Line Items]
Preferred stock, Shares issued							15,445,000
Series A Preferred Stock [Member] \| Second Closing [Member]
Temporary Equity [Line Items]
Preferred stock, Shares issued							15,500,000
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock, shares authorized								13,514,553
Preferred stock issued, shares								13,514,553
Proceeds from issuance of redeemable convertible preferred stock \| $				50,000,000
Payments of stock issuance costs \| $				$ 200,000
Stock issued, shares				12,871,003
Stock issued, price per shares \| $ / shares				$ 3.8847			$ 3.8847
Conversion of preferred stock into common stock								5,230,130
Conversion price of preferred stock \| $ / shares							$ 10.03799
Series B Preferred Stock [Member] \| License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Temporary Equity [Line Items]
Preferred stock, Shares issued											643,550

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Jun. 25, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2016
Temporary Equity [Line Items]
Preferred Stock Authorized	0		49,178,527
Preferred Stock Issued	0		49,178,527
Preferred Stock Outstanding	0		49,178,527	17,718,974
Carrying Value			$ 92,439	$ 17,916
Liquidation Preference			$ 88,164
Common Stock Issuable Upon Conversion		23,375,405	19,032,066
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized			35,663,974
Preferred Stock Issued			35,663,974		17,718,974
Preferred Stock Outstanding			35,663,974
Carrying Value			$ 39,939
Liquidation Preference			$ 35,664
Common Stock Issuable Upon Conversion			13,801,936
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred Stock Authorized			13,514,553
Preferred Stock Issued			13,514,553
Preferred Stock Outstanding			13,514,553
Carrying Value			$ 52,500
Liquidation Preference			$ 52,500
Common Stock Issuable Upon Conversion			5,230,130

Common Stock - Additional Information (Detail) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	Jun. 25, 2018 USD ($) $ / shares shares	Jun. 06, 2018	Jun. 30, 2018	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 shares	Nov. 30, 2016 shares
Class of Stock [Line Items]
Reverse stock split, conversion ratio		0.387	0.387		0.387
Proceeds from initial public offering, net of underwriting discounts and commissions \| $				$ 93,000
Common stock, Shares issued				34,327,554	4,458,547
Common stock, shares authorized				150,000,000	70,000,000
Common stock, Par value \| $ / shares				$ 0.001	$ 0.001
Preferred stock, Shares authorized				10,000,000	0
Preferred stock, par value \| $ / shares				$ 0.001	$ 0.000
License Agreement With Harvard [Member]
Class of Stock [Line Items]
Common stock, Shares issued						385,063	385,063
Employees And Consultants [Member] \| Restricted Stock [Member]
Class of Stock [Line Items]
Common stock, Shares issued					218,654	4,377,280
Two Investors [Member]
Class of Stock [Line Items]
Common stock, Shares issued					193,498
Common Stock [Member]
Class of Stock [Line Items]
Stock issued, shares				6,666,667
Common Stock [Member] \| IPO [Member]
Class of Stock [Line Items]
Stock issued, shares	6,666,667
Stock issued, price per shares \| $ / shares	$ 15.00
Proceeds from initial public offering, net of underwriting discounts and commissions \| $	$ 89,900

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Jun. 30, 2018	Apr. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, grants in period, gross			3,123,026
Number of shares, exercised			4,152	0	0
Weighted average fair value of stock options granted			$ 6.56	$ 3.31
Total unrecognized stock-based compensation expense			$ 16.8
Period for recognition of unrecognized expense			2 years 3 months 18 days
Minimum [Member] \| Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			3 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of award			10 years
Intrinsic value of stock options exercised			$ 0.1
Maximum [Member] \| Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of non-vested options outstanding			187,686	0
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted stock vested			$ 9.3	$ 1.9	$ 0.6
Unvested shares of restricted stock			1,022,521	2,107,300
Restricted Stock [Member] \| Non Employee [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares of restricted stock			326,017	626,376
2016 Stock Option and Grant Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in shares reserved for future issuance		1,884,850
Authorized shares reserved for issuance		5,900,917	1,142,136
Shares available for future grants	1,142,136
Share-based compensation arrangement by share-based payment award, options, grants in period, gross	0
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in shares reserved for future issuance			1,332,201
Authorized shares reserved for issuance			1,765,162
Shares available for future grants			2,693,929
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance			4.00%

Stock-Based Compensation - Schedule of Assumptions Used to Determine the Fair Value of Option Grants (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate	2.80%	2.00%
Expected term (in years)	6 years 2 months 12 days	6 years
Expected volatility	78.40%	78.10%
Expected dividend yield	0.00%	0.00%

Stock-Based Compensation - Schedule of Common Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning Balance	624,406
Number of Shares, Granted	3,123,026
Number of Shares, Exercised	(4,152)	0	0
Number of Shares, Forfeited	(233,451)
Number of Shares, Ending Balance	3,509,829	624,406
Number of Shares, Options vested and expected to vest as of December 31, 2018	3,509,829
Number of Shares, Options exercisable as of December 31, 2018	573,669
Weighted-Average Exercise Price, Outstanding, Beginning balance	$ 4.84
Weighted-Average Exercise Price, Granted	9.40
Weighted-Average Exercise Price, Exercised	7.71
Weighted-Average Exercise Price, Forfeited	8.29
Weighted-Average Exercise Price, Outstanding, Ending balance	8.67	$ 4.84
Weighted-Average Exercise Price, Options vested and expected to vest as of December 31, 2018	8.67
Weighted-Average Exercise Price, Options exercisable as of December 31, 2018	$ 7.51
Weighted Average Remaining Contractual Term	9 years 2 months 12 days	9 years 9 months 18 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest as of December 31, 2018	9 years 2 months 12 days
Weighted Average Remaining Contractual Term, Options exercisable as of December 31, 2018	9 years 1 month 6 days
Aggregate Intrinsic Value, Beginning Balance	$ 1,791
Aggregate Intrinsic Value, Ending Balance	526	$ 1,791
Aggregate Intrinsic Value, Options vested and expected to vest as of December 31, 2018	526
Aggregate Intrinsic Value, Options exercisable as of December 31, 2018	$ 179

Stock-Based Compensation - Schedule of Activity Relating to Restricted Stock Units (Detail) - Restricted Stock [Member]	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares Outstanding, Beginning Balance \| shares	2,107,300
Number of Shares, Vested \| shares	(907,562)
Number of Shares, Forfeited \| shares	(177,217)
Number of Shares Outstanding, Ending Balance \| shares	1,022,521
Weighted Average Grant Date Fair Value Outstanding, Beginning Balance \| $ / shares	$ 1.25
Weighted Average Grant Date Fair Value, Vested \| $ / shares	1.23
Weighted Average Grant Date Fair Value,Forfeited \| $ / shares	1.89
Weighted Average Grant Date Fair Value Outstanding, Ending Balance \| $ / shares	$ 1.16

Stock-Based Compensation - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 10,703	$ 2,259	$ 662
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	6,032	1,626	427
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 4,671	$ 633	$ 235

Net Loss per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (16,679)	$ (16,015)	$ (13,660)	$ (11,161)	$ (8,036)	$ (6,913)	$ (15,731)	$ (4,811)	$ (57,515)	$ (35,491)	$ (9,431)
Accretion of redeemable convertible preferred stock to redemption value									(88)	(213)	(107)
Cumulative dividends on redeemable convertible preferred stock										(437)	(197)
Reversal of cumulative dividends on redeemable convertible preferred stock										634
Net loss attributable to common stockholders	$ (16,679)	$ (16,015)	$ (13,748)	$ (11,161)	$ (8,036)	$ (6,919)	$ (15,387)	$ (5,165)	$ (57,603)	$ (35,507)	$ (9,735)
Denominator:
Weighted average common shares outstanding, basic and diluted									18,389,576	1,856,907	149,414
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.49)	$ (3.13)	$ (4.53)	$ (3.59)	$ (3.44)	$ (8.90)	$ (3.59)	$ (3.13)	$ (19.12)	$ (65.15)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss Per Share - Schedule of Antidilutive Common Stock Equivalents Excluded from Computation of Earnings Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	4,532,350	21,763,772	10,461,088
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount		19,032,066	6,857,241
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	3,509,829	624,406
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	1,022,521	2,107,300	3,603,847

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended	12 Months Ended
	May 31, 2018 USD ($) ft²	Apr. 30, 2017 USD ($) shares	Nov. 30, 2016 USD ($) shares	Dec. 31, 2018 USD ($) ft² Sublease shares	Dec. 31, 2018 USD ($) ft² Milestone shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Sep. 30, 2018 USD ($)	Aug. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Other Commitments [Line Items]
Square feet of property subject to sublease \| ft²	69,000
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 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
Rent expense					$ 4,400,000	$ 1,100,000
Research and development expense					$ 41,340,000	$ 27,899,000	$ 5,782,000
Common stock issued \| shares				34,327,554	34,327,554	4,458,547
Redeemable convertible preferred stock issued \| shares				0	0	0
Maximum [Member]
Other Commitments [Line Items]
Rent expense							$ 100,000
Leasehold Improvements [Member] \| Maximum [Member]
Other Commitments [Line Items]
Deferred rent				$ 700,000	$ 700,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Other Commitments [Line Items]
Fair value of shares as of issuance date						$ 9,300,000
Number of milestones met \| Milestone					0
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Achievement of Clinical and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation		$ 177,000,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Achievement of Commercial Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation		$ 125,000,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Sales Revenue, Net [Member] \| Product Concentration Risk [Member] \| Maximum [Member]
Other Commitments [Line Items]
Maximum prcentage of net sales of products licensed		20.00%
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Series A- Redeemable Convertible Preferred Stock [Member]
Other Commitments [Line Items]
Redeemable convertible preferred stock issued \| shares		2,500,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Series B Redeemable Convertible Preferred Stock [Member]
Other Commitments [Line Items]
Redeemable convertible preferred stock issued \| shares		643,550
License Agreement With Harvard [Member]
Other Commitments [Line Items]
Payment of upfront fee			$ 100,000
Common stock issued \| shares			385,063				385,063
Fair value of shares as of issuance date							$ 300,000
Number of milestones met \| Milestone					0
License Agreement With Harvard [Member] \| Maintenance [Member] \| Annually Thereafter [Member]
Other Commitments [Line Items]
Payment obligation			$ 100,000
License Agreement With Harvard [Member] \| Maintenance [Member] \| Maximum [Member] \| Annually Through 2019 [Member]
Other Commitments [Line Items]
Payment obligation			100,000
License Agreement With Harvard [Member] \| Patents [Member]
Other Commitments [Line Items]
Reimbursed costs			300,000
License Agreement With Harvard [Member] \| Achievement of Development and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation			$ 7,400,000
Sublease Agreement [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease \| ft²				27,000	27,000
Sublease expiration period					2018-09
Rental income				$ 4,500,000
Lease agreement maturity period				2 years
Number of Sub-sublease \| Sublease				2
Contractual obligation incurred					$ 6,100,000
Sublease Agreement [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation									$ 6,800,000
Collaboration Agreement [Member]
Other Commitments [Line Items]
Research and development expense					1,700,000
Collaboration Agreement [Member] \| Development, Regulatory and Commercial Milestone [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation										$ 83,500,000
License Agreement With Third Parties [Member] \| License [Member]
Other Commitments [Line Items]
Research and development expense					800,000
Letter of Credit [Member]
Other Commitments [Line Items]
Cash balance to secure letter of credit associated with sublease	$ 1,800,000
Letter of Credit [Member] \| Sublease Agreement [Member]
Other Commitments [Line Items]
Cash balance to secure letter of credit associated with sublease				$ 0	$ 0			$ 200,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
2019	$ 5,434
2020	5,597
2021	5,765
2022	5,938
2023	6,116
Thereafter	27,516
Operating leases future minimum payments due	$ 56,366

Income Taxes - Additional Information (Detail) - USD ($)		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Income tax expense (benefit)	$ 0	$ 0	$ 0	$ 0
Federal statutory income tax rate		21.00%	34.00%	34.00%
Deduction for net operating loss, percentage of annual taxable income			80.00%
Provision for income tax expense			$ 4,800,000
Research and orphan drug tax credit carryforwards		$ 4,108,000	1,400,000
Unrecognized tax benefits		0	$ 0
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards		$ 40,600,000
Operating loss carry forwards expiration period		2035
Research and orphan drug tax credit carryforwards		$ 3,400,000
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
Federal [Member] \| Indefinitely [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards		$ 23,100,000
State [Member]
Income Tax Disclosure [Line Items]
Operating loss carry forwards expiration period		2035
Research and orphan drug tax credit carryforwards		$ 800,000
Research and orphan drug tax credit carryforwards, expiration period		2030
State [Member] \| Expires 2035 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards		$ 40,800,000

Income Taxes - Schedule of Reconciliation of U.S. Federal Statutory Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Federal statutory income tax rate	21.00%	34.00%	34.00%
State taxes, net of federal benefit	5.60%	4.90%	4.60%
Research and orphan drug tax credits	4.70%	3.40%	1.80%
Impact of 2018 tax rate change on temporary differences		(13.40%)
Other	(2.10%)	(3.70%)	(4.20%)
Increase in deferred tax asset valuation allowance	(29.20%)	(25.20%)	(36.20%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Company's Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 11,106	$ 5,158
Capitalized research and development expenses	12,030	5,925
Research and orphan drug tax credit carryforwards	4,108	1,400
Stock compensation expense	1,336	9
Accrued expense	664	20
Other	360
Total deferred tax assets	29,604	12,512
Valuation allowance	(29,243)	(12,466)	$ (3,540)	$ (129)
Net deferred tax assets	361	46
Deferred tax liabilities:
Depreciation and amortization	(361)	(46)
Total deferred tax liabilities	(361)	(46)
Net deferred tax assets and liabilities	$ 0	$ 0

Income Taxes - Changes in Deferred Tax Asset Valuation Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation allowance as of beginning of year	$ 12,466	$ 3,540	$ 129
Net increases recorded to income tax provision	16,777	8,926	3,411
Valuation allowance as of end of year	$ 29,243	$ 12,466	$ 3,540

Related Parties - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Preferred stock issued			0	0
Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Stock issued, shares	11,223,102
Series A Preferred Stock [Member]
Related Party Transaction [Line Items]
Stock issued, shares		15,445,000
Series B Preferred Stock [Member]
Related Party Transaction [Line Items]
Stock issued, shares		12,871,003
Atlas Venture Life Science Advisors And Third Rock Ventures [Member]
Related Party Transaction [Line Items]
Administrative expenses related party				$ 200,000	$ 1,100,000
Issuance of common stock in connection with payment of consultant fees					96,749
Amounts owed to related party			$ 0	$ 0
Atlas Venture Life Science Advisors And Third Rock Ventures [Member] \| Minimum [Member]
Related Party Transaction [Line Items]
Ownership percentage				5.00%	5.00%
License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Related Party Transaction [Line Items]
Ownership percentage		5.00%
Fair value of shares as of issuance date		$ 9,300,000
License agreement, description			Novartis is no longer an owner of 5% or more of the Company's outstanding stock.
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Series A Preferred Stock [Member]
Related Party Transaction [Line Items]
Preferred stock issued		2,500,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Series B Preferred Stock [Member]
Related Party Transaction [Line Items]
Preferred stock issued		643,550
Match Bio Therapies [Member]
Related Party Transaction [Line Items]
Administrative expenses related party			$ 200,000
Expenses paid to related party			600,000
Match Bio Therapies [Member] \| Accounts Payable, Accrued Expenses and Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Amounts owed to related party			400,000
Match Bio Therapies [Member] \| Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Stock issued, shares	246,781
Proceeds From Issuance Of Preferred shares	$ 1,100,000
Match Bio Therapies [Member] \| Research and Development Expenses [Member]
Related Party Transaction [Line Items]
Research and development expense			$ 600,000

Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Loss from operations	(17,930)	(16,702)	(14,025)	(11,306)	(8,170)	(7,015)	(15,731)	(4,811)	(59,963)	(35,727)	(9,268)
Net loss	(16,679)	(16,015)	(13,660)	(11,161)	(8,036)	(6,913)	(15,731)	(4,811)	(57,515)	(35,491)	(9,431)
Net loss attributable to common stockholders	$ (16,679)	$ (16,015)	$ (13,748)	$ (11,161)	$ (8,036)	$ (6,919)	$ (15,387)	$ (5,165)	$ (57,603)	$ (35,507)	$ (9,735)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.49)	$ (3.13)	$ (4.53)	$ (3.59)	$ (3.44)	$ (8.90)	$ (3.59)	$ (3.13)	$ (19.12)	$ (65.15)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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