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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2018	Oct. 31, 2018
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Trading Symbol	MGTA
Entity Registrant Name	MAGENTA THERAPEUTICS, INC.
Entity Central Index Key	0001690585
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		33,151,539

Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 159,674	$ 51,402
Restricted cash	165	165
Prepaid expenses and other current assets	3,025	936
Total current assets	162,864	52,503
Restricted cash	1,780
Property and equipment, net	8,149	1,956
Other assets		4
Total assets	172,793	54,463
Current liabilities:
Accounts payable	4,897	167
Accrued expenses and other current liabilities	7,032	3,975
Total current liabilities	11,929	4,142
Long-term liabilities	1,080
Total liabilities	13,009	4,142
Commitments and contingencies (Note 9)
Redeemable convertible preferred stock (Series A and B), $0.001 par value; no shares and 49,178,527 shares authorized as of September 30, 2018 and December 31, 2017, respectively; no shares and 49,178,527 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively		92,439
Stockholders' Equity (Deficit):
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of September 30, 2018; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares and 70,000,000 shares authorized as of September 30, 2018 and December 31, 2017, respectively; 34,327,554 shares and 4,458,547 shares issued and 33,102,167 shares and 2,351,247 shares outstanding as of September 30, 2018 and December 31, 2017, respectively	33	2
Additional paid-in capital	245,798	3,091
Accumulated deficit	(86,047)	(45,211)
Total stockholders' equity (deficit)	159,784	(42,118)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 172,793	$ 54,463

Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 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	33,102,167	2,351,247

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	11,418	5,210	28,950	22,348
General and administrative	5,284	1,805	13,083	5,209
Total operating expenses	16,702	7,015	42,033	27,557
Loss from operations	(16,702)	(7,015)	(42,033)	(27,557)
Interest and other income, net	687	102	1,197	102
Net loss and comprehensive loss	(16,015)	(6,913)	(40,836)	(27,455)
Accretion of redeemable convertible preferred stock to redemption value		(6)	(88)	(213)
Cumulative dividends on redeemable convertible preferred stock				(437)
Reversal of cumulative dividends on redeemable convertible preferred stock				634
Net loss attributable to common stockholders	$ (16,015)	$ (6,919)	$ (40,924)	$ (27,471)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.49)	$ (3.44)	$ (3.05)	$ (15.90)
Weighted average common shares outstanding, basic and diluted	32,997,346	2,009,880	13,396,856	1,727,622

Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (40,836)	$ (27,455)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,152	1,486
Depreciation and amortization expense	516	247
License fees paid in preferred stock		9,275
Loss on disposal of property and equipment	67
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,085)	(321)
Accounts payable	2,104	1,167
Accrued expenses and other current liabilities	1,740	1,578
Other assets		(8)
Other long-term liabilities	1,080
Net cash used in operating activities	(29,262)	(14,031)
Cash flows from investing activities:
Purchases of property and equipment	(3,321)	(1,882)
Proceeds from sale of property and equipment	12
Change in restricted cash	(1,780)	(135)
Net cash used in investing activities	(5,089)	(2,017)
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
Payments of initial public offering costs	(2,618)
Payments of redeemable convertible preferred stock issuance costs	(88)	(213)
Proceeds from issuance of restricted stock		4
Proceeds from exercise of common stock options	29
Net cash provided by financing activities	142,623	71,486
Net increase in cash and cash equivalents	108,272	55,438
Cash and cash equivalents at beginning of period	51,402	4,513
Cash and cash equivalents at end of period	159,674	59,951
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 or accrued expenses	3,467
Offering costs included in accounts payable or accrued expenses	476
Accretion of redeemable convertible preferred stock to redemption value	$ 88	213
Cumulative dividends on Series A redeemable convertible preferred stock		437
Reversal of cumulative dividends on Series A redeemable convertible preferred stock		$ (634)

Nature of the Business and Basis of Presentation

9 Months Ended

Sep. 30, 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 bone marrow 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 $40.8 million for the nine months ended September 30, 2018 and $35.5 million for the year ended December 31, 2017. As of September 30, 2018, the Company had an accumulated deficit of $86.0 million. The Company expects to continue to generate operating losses for the foreseeable future. As of the issuance date of the financial statements, the Company expects that its cash and cash equivalents of $159.7 million as of September 30, 2018 will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of these 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 financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 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 revenue and 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 shares. 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.

Unaudited Interim Financial Information

The balance sheet at December 31, 2017 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited financial statements as of September 30, 2018 and for the three and nine months ended September 30, 2018 and 2017 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The Company believes, however, that the disclosures are adequate to make the information presented not misleading. These financial statements should be read in conjunction with the Company’s audited financial statements included in the Company’s Registration Statement on Form S-1 (File No. 333-225178) on file with SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of September 30, 2018 and results of operations for the three and nine months ended September 30, 2018 and 2017 and cash flows for the nine months ended September 30, 2018 and 2017 have been made. The results of operations for the nine months ended September 30, 2018 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2018.

Deferred Offering Costs

The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering.

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

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 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 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 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’ equity that result from transactions and economic events other than those with shareholders. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying financial statements.

Net Loss per Share

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. Prior to this conversion, 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.

Basic net income (loss) per share attributable to common stockholders is computed by dividing the net income (loss) attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) attributable to common stockholders is computed by adjusting income (loss) attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including outstanding stock options and unvested restricted common stock. Diluted net income (loss) per share attributable to common stockholders is computed by dividing the diluted net income (loss) attributable to common stockholders 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 and unvested restricted common stock.

The Company’s outstanding redeemable convertible preferred stock contractually entitled the holders of such shares to participate in distributions but contractually did not require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

The Company reported a net loss attributable to common stockholders for the three and nine months ended September 30, 2018 and 2017.

Classification and Accretion of Redeemable Convertible Preferred Stock

Prior to the conversion of the redeemable convertible preferred stock to common stock in connection with the IPO, the Company had classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contained 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.

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.

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 August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”), to address diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. For public entities, the standard is effective for annual periods beginning after December 15, 2017, 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, the standard is effective for annual periods beginning after December 15, 2018. Early adoption is permitted for all entities. The Company is currently evaluating the impact that the adoption of ASU 2016-15 will have on its financial statements.

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. The Company is currently evaluating the impact that the adoption of ASU 2016-18 will have on its financial statements.

In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public entities, ASU 2017-11 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 2017-11 is effective for annual periods beginning after December 15, 2019. Early adoption is permitted for all entities. The Company does not expect the adoption of ASU 2017-11 will have a material impact 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 the impact that the adoption of ASU 2018-07 will have on its financial statements.

Fair Value of Financial Assets

9 Months Ended

Sep. 30, 2018

Fair Value of Financial Assets

3. Fair Value of Financial Assets

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 September 30, 2018 Using
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	159,687	$	—	$	—	$	159,687

Total	$	159,687	$	—	$	—	$	159,687


	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 nine months ended September 30, 2018, there were no transfers between Level 1, Level 2 and Level 3.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Accrued Expenses and Other Current Liabilities

9 Months Ended

Sep. 30, 2018

Accrued Expenses and Other Current Liabilities

4. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	September 30, 2018		December 31, 2017
Accrued external research and development expenses	$	2,907	$	1,531
Accrued payroll and related expenses		1,909		1,330
Accrued leasehold improvements		1,232		—
Accrued professional fees		563		963
Accrued other		421		151

	$	7,032	$	3,975

Redeemable Convertible Preferred Stock

9 Months Ended

Sep. 30, 2018

Redeemable Convertible Preferred Stock

5. 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 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.

In April 2018, the Company entered into a Series C preferred stock purchase agreement, pursuant to which the Company sold 11,223,102 shares of Series C redeemable convertible preferred stock (the “Series C Preferred Stock”) at a price of $4.66 per share for total gross proceeds of $52.3 million. In connection with the Series C Preferred Stock purchase agreement, 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 and to increase the number of common stock authorized from 70,000,000 to 85,000,000 shares. In addition, the redemption date of the Preferred Stock was changed from any time on or after April 21, 2022, to the fifth anniversary of the Series C Original Issue Date, or April 2, 2023.

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 the closing of the IPO in June 2018, the Company’s Preferred Stock automatically converted into 23,375,405 shares of common stock.

Equity

9 Months Ended

Sep. 30, 2018

Equity

6. Equity

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.

As of September 30, 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.

Stock-Based Awards

9 Months Ended

Sep. 30, 2018

Stock-Based Awards

7. Stock-Based Awards

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 September 30, 2018, 2,645,476 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.

Grant of Stock Options

During the nine months ended September 30, 2018, the Company granted options to certain employees, directors and consultants for the purchase of 3,033,636 shares of common stock with a weighted average grant date fair value of $6.57 per share.

Stock-Based Compensation

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development expenses	$	2,022	$	417	$	4,612	$	1,070
General and administrative expenses		1,654		148		3,540		416

	$	3,676	$	565	$	8,152	$	1,486

As of September 30, 2018, total unrecognized compensation cost related to unvested share-based awards was $22.8 million, which is expected to be recognized over a weighted average period of 2.4 years. As of September 30, 2018, there were 398,777 unvested shares of restricted stock and 187,686 options held by non-employees.

Net Loss per Share

9 Months Ended

Sep. 30, 2018

Net Loss per Share

8. 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):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(16,015	)	$	(6,913	)	$	(40,836	)	$	(27,455	)
Accretion of redeemable convertible preferred stock to redemption value		—			(6	)		(88	)		(213	)
Cumulative dividends on redeemable convertible preferred stock		—			—			—			(437	)
Reversal of cumulative dividends on redeemable convertible preferred stock		—			—			—			634

Net loss attributable to common stockholders	$	(16,015	)	$	(6,919	)	$	(40,924	)	$	(27,471	)

Denominator:
Weighted average common shares outstanding—basic and diluted		32,997,346			2,009,880			13,396,856			1,727,622

Net loss per share attributable to common stockholders—basic and diluted	$	(0.49	)	$	(3.44	)	$	(3.05	)	$	(15.90	)

Common Stock Equivalents

The following potential dilutive securities, presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact:


	As of September 30,
	2018		2017
Stock options to purchase common stock		3,558,282		522,822
Unvested restricted common stock		1,225,387		2,928,548
Redeemable convertible preferred stock (as converted to common stock)		—		19,032,066

		4,783,669		22,483,436

Commitments and Contingencies

9 Months Ended

Sep. 30, 2018

Commitments and Contingencies

9. 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. 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 September 30, 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 $1.8 million and $0.3 million during the three months ended September 30, 2018 and 2017, respectively, and $2.9 million and $0.8 million during the nine months ended September 30, 2018 and 2017, respectively.

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 $5.2 million has been incurred through September 30, 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 September 30, 2018 and December 31, 2017.

As of September 30, 2018, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands):


Remainder of 2018 (3 months)	$	1,335
2019		5,434
2020		5,597
2021		5,765
2022		5,938
Thereafter		33,632

	$	57,701

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 three and nine months ended September 30, 2018, the Company recorded less than $0.1 million and $0.8 million, respectively, of research and development expense related to this agreement for upfront technology access fees, research exclusivity fees and research support. In October 2018, the Company exercised one of its options under the agreement for an exclusive development and commercialization license with respect to a target, which resulted in a low single digit million dollar payment to HDPR.

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 September 30, 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 September 30, 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. As of September 30, 2018, the Company has not exercised its rights to license such technologies.

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 financial statements as of September 30, 2018 or December 31, 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	9 Months Ended
401(k) Savings Plan	Sep. 30, 2018
401(k) Savings Plan	10. 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

9 Months Ended

Sep. 30, 2018

Income Taxes

11. Income Taxes

The Company did not provide for any income taxes in the three and nine months ended September 30, 2018 and 2017. The Company had net deferred tax assets of $12.5 million at December 31, 2017. The Company has provided a valuation allowance for the full amount of its net deferred tax assets because, at each of September 30, 2018 and December 31, 2017, it was more likely than not that any future benefit from deductible temporary differences and net operating loss and tax credit carryforwards would not be realized.

The Company has not recorded any amounts for unrecognized tax benefits as of September 30, 2018 or December 31, 2017. As of September 30, 2018 and December 31, 2017, the Company had no accrued interest or tax penalties recorded. The Company’s income tax return reporting periods since 2015 are open to income tax audit examination by the federal and state tax authorities.

Related Parties

9 Months Ended

Sep. 30, 2018

Related Parties

12. Related Parties

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

During the three and nine months ended September 2017, 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 three months ended September 30, 2017, the Company recorded less than $0.1 million of general and administrative expenses for management and advisory services and other related services provided by these investors, of which $0.1 million was paid. For the nine months ended September 30, 2017, the Company recorded $0.1 million of general and administrative expenses for management and advisory services and other related services provided by these investors, and made payments of $0.2 million. There were no services provided during the nine months ended September 30, 2018. There were no amounts owed to these investors as of September 30, 2018 or December 31, 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 three months ended September 30, 2018, the Company recorded $0.1 million to research and development expenses and less than $0.1 million to general and administrative expenses and made payments of $0.1 million related to these agreements. For the nine months ended September 30, 2018, the Company recorded $0.3 million to research and development expenses and $0.1 million to general and administrative expenses and made payments of $0.6 million related to these agreements. As of September 30, 2018, amounts on the balance sheet related to these agreements was less than $0.1 million included in accounts payable and other current liabilities and $0.1 million included in prepaid expenses.

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 5).

Subsequent Event	9 Months Ended
Subsequent Event	Sep. 30, 2018
Subsequent Event	13. Subsequent Event In October 2018, with the landlord’s consent, the Company entered into a two-year sub-sublease of 13,643 square feet of office space in Cambridge, Massachusetts beginning in October 2018 under which it will receive $2.3 million of payments over the sub-sublease term.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2018

Use of Estimates

Use of Estimates

Unaudited Interim Financial Information

Unaudited Interim Financial Information

Deferred Offering Costs

Deferred Offering Costs

Fair Value Measurements

Fair Value Measurements

•

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

•

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 three and nine months ended September 30, 2018 and 2017.

Classification and Accretion of Redeemable Convertible Preferred Stock

Classification and Accretion of Redeemable Convertible Preferred Stock

Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncments

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

- Definition

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

+ References + Details

Fair Value of Financial Assets (Tables)

9 Months Ended

Sep. 30, 2018

Schedule of Financial Assets Measured at Fair Value on Recurring Basis


	Fair Value Measurements at September 30, 2018 Using
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	159,687	$	—	$	—	$	159,687

Total	$	159,687	$	—	$	—	$	159,687


	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

Accrued Expenses and Other Current Liabilities (Tables)

9 Months Ended

Sep. 30, 2018

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	September 30, 2018		December 31, 2017
Accrued external research and development expenses	$	2,907	$	1,531
Accrued payroll and related expenses		1,909		1,330
Accrued leasehold improvements		1,232		—
Accrued professional fees		563		963
Accrued other		421		151

	$	7,032	$	3,975

Redeemable Convertible Preferred Stock (Tables)

9 Months Ended

Sep. 30, 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 Awards (Tables)

9 Months Ended

Sep. 30, 2018

Schedule of Stock Based Compensation Expense

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


	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development expenses	$	2,022	$	417	$	4,612	$	1,070
General and administrative expenses		1,654		148		3,540		416

	$	3,676	$	565	$	8,152	$	1,486

Net Loss per Share (Tables)

9 Months Ended

Sep. 30, 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):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2017			2018			2017
Numerator:
Net loss	$	(16,015	)	$	(6,913	)	$	(40,836	)	$	(27,455	)
Accretion of redeemable convertible preferred stock to redemption value		—			(6	)		(88	)		(213	)
Cumulative dividends on redeemable convertible preferred stock		—			—			—			(437	)
Reversal of cumulative dividends on redeemable convertible preferred stock		—			—			—			634

Net loss attributable to common stockholders	$	(16,015	)	$	(6,919	)	$	(40,924	)	$	(27,471	)

Denominator:
Weighted average common shares outstanding—basic and diluted		32,997,346			2,009,880			13,396,856			1,727,622

Net loss per share attributable to common stockholders—basic and diluted	$	(0.49	)	$	(3.44	)	$	(3.05	)	$	(15.90	)

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share


	As of September 30,
	2018		2017
Stock options to purchase common stock		3,558,282		522,822
Unvested restricted common stock		1,225,387		2,928,548
Redeemable convertible preferred stock (as converted to common stock)		—		19,032,066

		4,783,669		22,483,436

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2018

Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease

As of September 30, 2018, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands):


Remainder of 2018 (3 months)	$	1,335
2019		5,434
2020		5,597
2021		5,765
2022		5,938
Thereafter		33,632

	$	57,701

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 25, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	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		$ (16,015)	$ (6,913)	(40,836)	$ (27,455)	$ 35,500
Accumulated deficit		(86,047)		(86,047)		(45,211)
Cash and cash equivalents		$ 159,674	$ 59,951	$ 159,674	$ 59,951	$ 51,402	$ 4,513
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 - Additional Information (Detail) - shares	Jun. 25, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Redeemable convertible preferred stock converted into common stock shares	23,375,405	19,032,066

Fair Value of Financial Assets - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Sep. 30, 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	$ 159,687	$ 51,147
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	159,687	51,147
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	159,687	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	$ 159,687	$ 51,147

Fair Value of Financial Assets - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset transfers into level 3	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers out of level 3	$ 0

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 2,907	$ 1,531
Accrued payroll and related expenses	1,909	1,330
Accrued leasehold improvements	1,232
Accrued professional fees	563	963
Accrued other	421	151
Total	$ 7,032	$ 3,975

Redeemable Convertible Preferred Stock - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended
	Jun. 30, 2018	Apr. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Jun. 25, 2018 shares	Apr. 29, 2018 shares	Dec. 31, 2017 shares
Temporary Equity [Line Items]
Reverse stock split, conversion ratio	0.387
Proceeds from issuance of redeemable convertible preferred stock \| $			$ 52,300	$ 71,695
Preferred stock, shares authorized			0				49,178,527
Common stock, shares authorized		85,000,000	150,000,000			70,000,000	70,000,000
Conversion of preferred stock into common stock					23,375,405		19,032,066
Series C Preferred Stock [Member]
Temporary Equity [Line Items]
Stock issued, shares		11,223,102
Stock issued, price per shares \| $ / shares		$ 4.66
Proceeds from issuance of redeemable convertible preferred stock \| $		$ 52,300
Preferred stock, shares authorized		11,226,000

Redeemable Convertible Preferred Stock - Summary of Preferred Stock (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 25, 2018	Dec. 31, 2017
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
Carrying Value			$ 92,439
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
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

Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Jun. 25, 2018	Sep. 30, 2018	Apr. 30, 2018	Apr. 29, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Proceeds from initial public offering, net of underwriting discounts and commissions		$ 93,000
Common stock, shares authorized		150,000,000	85,000,000	70,000,000	70,000,000
Common stock, par value		$ 0.001			$ 0.001
Preferred stock, Shares authorized		10,000,000			0
Preferred stock, par value		$ 0.001			$ 0.000
Common Stock [Member] \| IPO [Member]
Class of Stock [Line Items]
Stock issued, shares	6,666,667
Stock issued, price per shares	$ 15.00
Proceeds from initial public offering, net of underwriting discounts and commissions	$ 89,900

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		9 Months Ended
	Jun. 30, 2018	Apr. 30, 2018	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense			$ 22.8
Period for recognition of unrecognized expense			2 years 4 months 24 days
Employees, Directors And Consultants [Member]
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			3,033,636
Weighted average fair value of stock options granted			$ 6.57
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares of restricted stock			398,777
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of non-vested options outstanding			187,686
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]
Authorized shares reserved for issuance			1,765,162
Shares available for future grants			2,645,476
Percentage applied to the outstanding shares as annual increase in the number of shares authorized for issuance			4.00%

Stock-Based Awards - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 3,676	$ 565	$ 8,152	$ 1,486
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	2,022	417	4,612	1,070
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 1,654	$ 148	$ 3,540	$ 416

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		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Numerator:
Net loss	$ (16,015)	$ (6,913)	$ (40,836)	$ (27,455)	$ 35,500
Accretion of redeemable convertible preferred stock to redemption value		(6)	(88)	(213)
Cumulative dividends on redeemable convertible preferred stock				(437)
Reversal of cumulative dividends on redeemable convertible preferred stock				634
Net loss attributable to common stockholders	$ (16,015)	$ (6,919)	$ (40,924)	$ (27,471)
Denominator:
Weighted average common shares outstanding-basic and diluted	32,997,346	2,009,880	13,396,856	1,727,622
Net loss per share attributable to common stockholders-basic and diluted	$ (0.49)	$ (3.44)	$ (3.05)	$ (15.90)

- 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 Securities Excluded from Computation of Earnings Per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	4,783,669	22,483,436
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
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	3,558,282	522,822
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	1,225,387	2,928,548

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2018 USD ($) ft²	Apr. 30, 2017 USD ($) shares	Nov. 30, 2016 USD ($) shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Milestone shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Aug. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Commitments And Contingencies [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				$ 1,800,000	$ 300,000	$ 2,900,000	$ 800,000
Research and development expense				$ 11,418,000	$ 5,210,000	$ 28,950,000	$ 22,348,000
Common stock issued \| shares				34,327,554		34,327,554		4,458,547
Redeemable convertible preferred stock issued \| shares				0		0		0
Leasehold Improvements [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Deferred rent				$ 700,000		$ 700,000
License Agreement With Harvard [Member]
Commitments And Contingencies [Line Items]
Payment of upfront fee			$ 100,000
Common stock issued \| shares			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]
Commitments And Contingencies [Line Items]
Payment obligation			$ 100,000
License Agreement With Harvard [Member] \| Maintenance [Member] \| Maximum [Member] \| Annually Through 2019 [Member]
Commitments And Contingencies [Line Items]
Payment obligation			100,000
License Agreement With Harvard [Member] \| Patents [Member]
Commitments And Contingencies [Line Items]
Reimbursed costs			300,000
License Agreement With Harvard [Member] \| Achievement of Development and Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Payment obligation			$ 7,400,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Commitments And Contingencies [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]
Commitments And Contingencies [Line Items]
Payment obligation		$ 177,000,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Achievement of Commercial Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [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]
Commitments And Contingencies [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]
Commitments And Contingencies [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]
Commitments And Contingencies [Line Items]
Redeemable convertible preferred stock issued \| shares		643,550
Sublease Agreement [Member]
Commitments And Contingencies [Line Items]
Sublease expiration period						2018-09
Contractual obligation incurred						$ 5,200,000
Sublease Agreement [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Payment obligation										$ 6,800,000
Collaboration Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense						800,000
Collaboration Agreement [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Research and development expense				100,000
Collaboration Agreement [Member] \| Development, Regulatory and Commercial Milestone [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Payment obligation											$ 83,500,000
Letter of Credit [Member]
Commitments And Contingencies [Line Items]
Cash balance to secure letter of credit associated with sublease	$ 1,800,000
Letter of Credit [Member] \| Sublease Agreement [Member]
Commitments And Contingencies [Line Items]
Cash balance to secure letter of credit associated with sublease				$ 200,000		$ 200,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Operating Leased Assets [Line Items]
Remainder of 2018 (3 months)	$ 1,335
2019	5,434
2020	5,597
2021	5,765
2022	5,938
Thereafter	33,632
Operating leases future minimum payments due	$ 57,701

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Income taxes	$ 0	$ 0	$ 0	$ 0
Deferred tax assets, net					$ 12,500,000
Unrecognized tax benefits	0		0		0
Accrued interest or tax penalties	$ 0		$ 0		$ 0

Related Parties - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Stock issued, shares	11,223,102
Atlas Venture Life Science Advisors And Third Rock Ventures [Member]
Related Party Transaction [Line Items]
Administrative expenses related party				$ 100,000	$ 100,000
Expenses paid to related party					$ 200,000
Amounts owed to related party		$ 0		0		$ 0
Match Bio Therapies [Member]
Related Party Transaction [Line Items]
Administrative expenses related party		100,000		100,000
Expenses paid to related party		100,000		600,000
Match Bio Therapies [Member] \| Prepaid Expenses and Other Current Assets [Member]
Related Party Transaction [Line Items]
Amounts owed to related party		100,000		100,000
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				300,000
Minimum [Member] \| Atlas Venture Life Science Advisors And Third Rock Ventures [Member]
Related Party Transaction [Line Items]
Ownership percentage			5.00%		5.00%
Maximum [Member] \| Atlas Venture Life Science Advisors And Third Rock Ventures [Member]
Related Party Transaction [Line Items]
Administrative expenses related party			$ 100,000
Maximum [Member] \| Match Bio Therapies [Member] \| Accounts Payable [Member]
Related Party Transaction [Line Items]
Amounts owed to related party		100,000		$ 100,000
Maximum [Member] \| Match Bio Therapies [Member] \| Research and Development Expenses [Member]
Related Party Transaction [Line Items]
Research and development expense		$ 100,000

Subsequent Event - Additional Information (Detail) $ in Millions	1 Months Ended
	Oct. 31, 2018 USD ($) ft²	May 31, 2018 ft²
Subsequent Event [Line Items]
Square feet of property subject to sublease		69,000
Subsequent Event [Member] \| Sublease Agreement [Member]
Subsequent Event [Line Items]
Square feet of property subject to sublease	13,643
Rental income \| $	$ 2.3
Lease agreement maturity period	2 years