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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Jul. 31, 2019
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Trading Symbol	MGTA
Entity Registrant Name	Magenta Therapeutics, Inc.
Entity Central Index Key	0001690585
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Common Stock, Shares Outstanding		38,767,292
Entity Ex Transition Period	false
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Entity Address, State or Province	MA

Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 33,111	$ 58,345
Marketable securities	139,162	84,225
Prepaid expenses and other current assets	10,740	2,751
Total current assets	183,013	145,321
Restricted cash	1,780	1,780
Property and equipment, net	10,648	10,212
Total assets	195,441	157,313
Current liabilities:
Accounts payable	2,349	3,492
Accrued expenses and other current liabilities	7,591	6,927
Total current liabilities	9,940	10,419
Deferred rent	5,945	1,246
Total liabilities	15,885	11,665
Commitments and contingencies (Note 7)
Stockholders' Equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares authorized; 39,338,100 shares and 34,327,554 shares issued and 38,710,858 shares and 33,305,033 shares outstanding as of June 30, 2019 and December 31, 2018, respectively	39	33
Additional paid-in capital	314,646	248,349
Accumulated other comprehensive income (loss)	139	(8)
Accumulated deficit	(135,268)	(102,726)
Total stockholders' equity	179,556	145,648
Total liabilities and stockholders' equity	$ 195,441	$ 157,313

Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, Shares authorized	10,000,000	10,000,000
Preferred stock, Shares issued	0	0
Preferred stock, Shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, Shares issued	39,338,100	34,327,554
Common stock, Shares outstanding	38,710,858	33,305,033

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	13,433	9,683	23,970	17,532
General and administrative	5,905	4,342	11,718	7,799
Total operating expenses	19,338	14,025	35,688	25,331
Loss from operations	(19,338)	(14,025)	(35,688)	(25,331)
Interest and other income, net	1,630	365	3,146	510
Net loss	(17,708)	(13,660)	(32,542)	(24,821)
Accretion of redeemable convertible preferred stock to redemption value		(88)		(88)
Net loss attributable to common stockholders	$ (17,708)	$ (13,748)	$ (32,542)	$ (24,909)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.48)	$ (3.13)	$ (0.93)	$ (7.25)
Weighted average common shares outstanding, basic and diluted	36,662,562	4,391,363	35,051,371	3,434,175
Comprehensive loss:
Net loss	$ (17,708)	$ (13,660)	$ (32,542)	$ (24,821)
Other comprehensive income:
Unrealized gains (losses) on marketable securities	98		147
Total other comprehensive income	98		147
Total comprehensive loss	$ (17,610)	$ (13,660)	$ (32,395)	$ (24,821)

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A, B and C Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	$ (42,118)		$ 2	$ 3,091		$ (45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Vesting of restricted stock			$ 1	(1)
Vesting of restricted stock, shares			269,032
Stock-based compensation expense	1,425			1,425
Net loss	(11,161)					(11,161)
Ending balance at Mar. 31, 2018		$ 92,439
Ending balance at Mar. 31, 2018	(51,854)		$ 3	4,515		(56,372)
Ending balance, shares at Mar. 31, 2018		49,178,527
Ending balance, shares at Mar. 31, 2018			2,620,279
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	(42,118)		$ 2	3,091		(45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Accretion of Series C redeemable convertible preferred stock to redemption value	(88)
Net loss	(24,821)
Ending balance at Jun. 30, 2018	172,217		$ 33	242,216		(70,032)
Ending balance, shares at Jun. 30, 2018			32,889,056
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	(42,118)		$ 2	3,091		(45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Net loss	(57,500)
Ending balance at Dec. 31, 2018	145,648		$ 33	248,349	$ (8)	(102,726)
Ending balance, shares at Dec. 31, 2018			33,305,033
Beginning balance at Mar. 31, 2018		$ 92,439
Beginning balance at Mar. 31, 2018	(51,854)		$ 3	4,515		(56,372)
Beginning balance, shares at Mar. 31, 2018		49,178,527
Beginning balance, shares at Mar. 31, 2018			2,620,279
Issuance of Series C redeemable convertible preferred stock, net of issuance cost of $88		$ 52,212
Issuance of Series C redeemable convertible preferred stock, net of issuance cost of $88, shares		11,223,102
Accretion of Series C redeemable convertible preferred stock to redemption value	(88)	$ 88		(88)
Conversion of preferred stock to common stock	144,739	$ (144,739)	$ 23	144,716
Conversion of preferred stock to common stock, shares		(60,401,629)	23,375,405
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	90,029		$ 7	90,022
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares			6,666,667
Vesting of restricted stock, shares			226,705
Stock-based compensation expense	3,051			3,051
Net loss	(13,660)					(13,660)
Ending balance at Jun. 30, 2018	172,217		$ 33	242,216		(70,032)
Ending balance, shares at Jun. 30, 2018			32,889,056
Beginning balance at Dec. 31, 2018	145,648		$ 33	248,349	(8)	(102,726)
Beginning balance, shares at Dec. 31, 2018			33,305,033
Vesting of restricted stock			$ 1	(1)
Vesting of restricted stock, shares			197,715
Issuance of common stock upon exercise of stock options	563			563
Issuance of common stock upon exercise of stock options, shares			79,227
Stock-based compensation expense	2,577			2,577
Unrealized loss on marketable securities	49				49
Net loss	(14,834)					(14,834)
Ending balance at Mar. 31, 2019	134,003		$ 34	251,488	41	(117,560)
Ending balance, shares at Mar. 31, 2019			33,581,975
Beginning balance at Dec. 31, 2018	145,648		$ 33	248,349	(8)	(102,726)
Beginning balance, shares at Dec. 31, 2018			33,305,033
Unrealized loss on marketable securities	147
Net loss	(32,542)
Ending balance at Jun. 30, 2019	179,556		$ 39	314,646	139	(135,268)
Ending balance, shares at Jun. 30, 2019			38,710,858
Beginning balance at Mar. 31, 2019	134,003		$ 34	251,488	41	(117,560)
Beginning balance, shares at Mar. 31, 2019			33,581,975
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	60,276		$ 5	60,271
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares			4,887,500
Vesting of restricted stock, shares			197,564
Issuance of common stock upon exercise of stock options	287			287
Issuance of common stock upon exercise of stock options, shares			43,819
Stock-based compensation expense	2,600			2,600
Unrealized loss on marketable securities	98				98
Net loss	(17,708)					(17,708)
Ending balance at Jun. 30, 2019	$ 179,556		$ 39	$ 314,646	$ 139	$ (135,268)
Ending balance, shares at Jun. 30, 2019			38,710,858

Consolidated Statements of Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	3 Months Ended
	Jun. 30, 2018 USD ($)
Series C Preferred Stock [Member]
Payments of redeemable convertible preferred stock issuance costs	$ 88

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (32,542)	$ (24,821)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	5,177	4,476
Depreciation and amortization expense	875	307
Loss on disposal of property and equipment		2
Net accretion of discounts on marketable securities	(614)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(7,989)	(1,392)
Accounts payable	(430)	995
Accrued expenses and other current liabilities	855	924
Deferred rent	4,699	796
Net cash used in operating activities	(29,969)	(18,713)
Cash flows from investing activities:
Purchases of property and equipment	(2,377)	(846)
Proceeds from sale of property and equipment		12
Purchases of marketable securities	(94,176)
Maturities of marketable securities	40,000
Net cash used in investing activities	(56,553)	(834)
Cash flows from financing activities:
Proceeds from public offerings, net of underwriting discounts and commissions	60,874	93,000
Proceeds from issuance of redeemable convertible preferred stock		52,300
Payments of public offering costs	(436)	(1,858)
Payments of redeemable convertible preferred stock issuance costs		(88)
Proceeds from exercise of common stock options	850
Net cash provided by financing activities	61,288	143,354
Net increase (decrease) in cash, cash equivalents and restricted cash	(25,234)	123,807
Cash, cash equivalents and restricted cash at beginning of period	60,125	51,567
Cash, cash equivalents and restricted cash at end of period	34,891	175,374
Supplemental disclosure of non-cash investing and financing activities:
Purchase of property and equipment included in accounts payable and accrued expenses	472	431
Deferred offering and redeemable convertible preferred share issuance costs included in accounts payable and accrued expenses	$ 162	1,113
Conversion of redeemable convertible preferred stock to common stock		144,739
Accretion of redeemable convertible preferred stock to redemption value		$ 88

Nature of the Business and Basis of Presentation

6 Months Ended

Jun. 30, 2019

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.

In June 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.

In May 2019, the Company issued and sold 4,887,500 shares of its common stock, including the underwriters’ exercise in full of their option to purchase additional shares of common stock, in a

follow-on

public offering at a public offering price of $13.25 per share, resulting in net proceeds of $60.3 million after underwriting discounts and commission and other offering expenses.

The Company has incurred recurring losses since inception, including net losses of $32.5 million and $57.5 million for the six months ended June 30, 2019 and the year ended December 31, 2018, respectively. As of June 30, 2019, the Company had an accumulated deficit of $135.3 million. The Company expects to continue to generate operating losses for the foreseeable future. The Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of these consolidated financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to fund its operations.

The Company expects its expenses to increase substantially in connection with ongoing activities, particularly as the Company advances its preclinical activities and clinical trials for its product candidates in development. Accordingly, the Company will need to obtain substantial additional funding in connection with continuing operations. If the Company is unable to raise capital when needed, or on attractive terms, it could be forced to delay, reduce or eliminate its research or drug development programs or any future commercialization 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

6 Months Ended

Jun. 30, 2019

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.

Unaudited Interim Financial Information

The consolidated balance sheet at December 31, 2018 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of June 30, 2019 and for the three and six months ended June 30, 2019 and 2018 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 consolidated financial statements should be read in conjunction with the Company’s audited financial statements for the year ended December 31, 2018 included in the Company’s Annual Report on Form

10-K,

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 consolidated financial position as of June 30, 2019 and consolidated results of operations for the three and six months ended June 30, 2019 and 2018 and consolidated cash flows for the six months ended June 30, 2019 and 2018, have been made. The results of operations for the six months ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019 or any other interim period.

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.

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’ equity that result from transactions and economic events other than those with stockholders. For the three and six months ended June 30, 2019, the Company’s only element of other comprehensive loss was unrealized gains on marketable securities. For the three and six months ended June 30, 2018, there was no difference between net loss and comprehensive loss.

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.

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. Subsequent to the 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.

The Company reported a net loss attributable to common stockholders for the three and six months ended June 30, 2019 and 2018.

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 June 2018, the Financing Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No.

2018-07,

Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment

Accounting

(“ASU

2018-07”).

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

No. 2014-09,

Revenue from Contracts with Customers (Topic 606)

, which supersedes existing revenue recognition guidance under GAAP and which the Company adopted on January 1, 2019. The Company early adopted ASU

2018-07

effective January 1, 2019 by remeasuring outstanding equity-classified awards issued to

non-employees

for which a measurement date had not been established as of January 1, 2019. The adoption of ASU

2018-07

did not have a material impact on the Company’s consolidated financial statements. The Company elected to estimate the expected term of options utilizing the simplified method for

non-employee

options that qualify as plain-vanilla options. The Company elected to account for forfeitures for

non-employee

options as they occur rather than apply an estimated forfeiture rate to stock-based compensation expense.

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) Targeted Improvements

, which added an optional transition method to the existing requirements whereby an entity could adopt the provisions of ASU

2016-02

by recognizing a cumulative-effective adjustment to the opening balance of retained earnings in the period of adoption without adjustment to the financial statements for periods prior to adoption. The Company expects that the adoption of the new leasing standards will result in the recognition of material

right-of-use

assets and lease liabilities on the consolidated balance sheets but does not expect it to have a material impact on its results of operations or cash flows.

Fair Value of Financial Assets

6 Months Ended

Jun. 30, 2019

Fair Value of Financial Assets

3. Fair Value of Financial Assets

As of June 30, 2019, marketable securities by security type consisted of (in thousands):


			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
United States Treasury Notes (due within one year)	$	45,895	$	27	$	—	$	45,922
Agency Bonds (due within one year)		93,128		112		—		93,240

Total	$	139,023	$	139	$	—	$	139,162

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


			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			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 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 June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	33,506	$	—	$	—	$	33,506
Marketable securities:
United States Treasury Notes		—		45,922		—		45,922
Agency Bonds		—		93,240		—		93,240

Total	$	33,506	$	139,162	$	—	$	172,668


	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

During the three and six months ended June 30, 2019 and 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

6 Months Ended

Jun. 30, 2019

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):


	June 30, 2019		December 31, 2018
Accrued external research and development expenses	$	4,011	$	2,619
Accrued payroll and related expenses		1,567		2,526
Accrued professional fees		762		787
Deferred rent, current portion		601		71
Accrued property and equipment		396		713
Accrued other		254		211

	$	7,591	$	6,927

Stock-Based Awards

6 Months Ended

Jun. 30, 2019

Stock-Based Awards

5. Stock-Based Awards

2018 Stock Option and Incentive Plan

The Company grants stock-based awards under its 2018 Stock Option and Incentive Plan (the “2018 Plan”). The Company also has outstanding stock options under its 2016 Stock Option and Grant Plan (the “2016 Plan”) but is no longer granting awards under this plan. As of June 30, 2019, 2,499,293 shares of common stock were available for issuance under the 2018 Plan.

2019 Employee Stock Purchase Plan

The Magenta Therapeutics, Inc. 2019 Employee Stock Purchase Plan (the “ESPP”) was adopted by the Company on April 18, 2019, subject to the approval of stockholders, and became effective following stockholder approval on June 7, 2019. An aggregate of 166,525 shares were reserved for issuance under the ESPP. In addition, on January 1, 2020 and each January 1 thereafter through January 1, 2029, the number of shares available for issuance shall be cumulatively increased by the least of (i) one percent (1%) of the number of shares issued and outstanding on the immediately preceding December 31, (ii) 1,000,000 shares and (iii) such number of shares as determined by the Compensation Committee. As of June 30, 2019, no shares have been issued under the ESPP and 166,525 shares remained available for issuance.

Grant of Stock Options

During the six months ended June 30, 2019, the Company granted options to certain employees and directors with service-based vesting for the purchase of 1,610,566 shares of common stock with a weighted average grant date fair value of $5.94 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development expenses	$	1,268	$	1,699	$	2,524	$	2,590
General and administrative expenses		1,332		1,352		2,653		1,886

	$	2,600	$	3,051	$	5,177	$	4,476

As of June 30, 2019, total unrecognized compensation cost related to unvested share-based awards was $20.6 million, which is expected to be recognized over a weighted average period of 2.4 years.

Prior to the adoption of ASU

2018-07

on January 1, 2019, as described in Note 2, the Company measured the fair value of stock-based awards granted to

non-employees

on the date at which the related service was complete. Compensation expense was 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 was remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model for options. Upon the adoption of ASU

2018-07,

the Company valued the remaining unvested restricted stock and options issued to

non-employees

as of January 1, 2019 and is recognizing stock-based compensation over the remaining vesting period.

Net Loss per Share

6 Months Ended

Jun. 30, 2019

Net Loss per Share

6. 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 June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(17,708	)	$	(13,660	)	$	(32,542	)	$	(24,821	)
Accretion of redeemable convertible preferred stock to redemption value		—			(88	)		—			(88	)

Net loss attributable to common stockholders	$	(17,708	)	$	(13,748	)	$	(32,542	)	$	(24,909	)

Denominator:
Weighted average common shares outstanding, basic and diluted		36,662,562			4,391,363			35,051,371			3,434,175

Net loss per share attributable to common stockholders, basic and diluted	$	(0.48	)	$	(3.13	)	$	(0.93	)	$	(7.25	)

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 June 30,
	2019		2018
Stock options to purchase common stock		4,913,620		3,247,996
Unvested restricted common stock		627,242		1,481,211
Options to purchase additional shares of common stock granted to the underwriters in connection with the IPO		—		1,000,000

		5,540,862		5,729,207

Commitments and Contingencies

6 Months Ended

Jun. 30, 2019

Commitments and Contingencies

7. Commitments and Contingencies

Leases

In May 2018, the Company entered into a sublease for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease was amended effective March 2019 to increase the lease payments over the lease term and to increase the sublandlord-funded tenant improvements. The sublease is subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and expires in February 2028. The sublandlord has the right to terminate the sublease after five years. The Company is also obligated to pay real estate taxes and other costs related to the premises, including costs of operations and management of the leased premises. In connection with the sublease, as amended, the sublandlord agreed to fund up to $5.5 million in tenant improvements to the leased facility and a receivable of $5.5 million and $0.7 million was included in prepaid expenses and other current assets at June 30, 2019 and December 31, 2018, respectively. The Company is required to maintain a cash balance of $1.8 million to secure a letter of credit associated with the sublease. This amount was classified as noncurrent restricted cash in the consolidated balance sheets at June 30, 2019 and December 31, 2018.

The tenant improvement allowance and payment escalations specified in the sublease 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. As of June 30, 2019 and December 31, 2018, the Company had long-term deferred rent of $5.9 million and $1.2 million, respectively, related to lease incentives and payment escalations. As of June 30, 2019 and December 31, 2018, the short-term portion of deferred rent of $0.6 million and $0.1 million, respectively, was included in accrued expenses and other current liabilities. The Company recorded rent expense of $1.6 million and $0.8 million during the three months ended June 30, 2019 and 2018, respectively, and $3.1 million and $1.1 million during the six months ended June 30, 2019 and 2018, 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 rent payments over the

sub-sublease

term.

As of June 30, 2019, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands):


Remainder of 2019 (six months)	$	2,809
2020		5,782
2021		6,072
2022		6,375
2023		6,734
Thereafter		32,000

	$	59,772

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 months ended June 30, 2019 and 2018, the Company recorded less than $0.1 million and $0.1 million, respectively, of research and development expense related to this agreement for upfront technology access fees, research exclusivity fees and research support. During the six months ended June 30, 2019 and 2018, the Company recorded $0.4 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.

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. During the six months ended June 30, 2019, the Company recorded $0.1 million of research and development expenses related to the achievement of one of these milestones.

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 June 30, 2019, 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 three months and six months ended June 30, 2019 and 2018, the Company did not exercise its rights 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 June 30, 2019 and December 31, 2018.

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

6 Months Ended

Jun. 30, 2019

401(k) Savings Plan

8. 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.

Related Parties

6 Months Ended

Jun. 30, 2019

Related Parties

9. Related Parties

National Marrow Donor Program (as successor in interest to Be The Match BioTherapies, LLC)

Effective March 2018, the President of Be The Match BioTherapies, LLC became a member of the Company’s board of directors. The Company has a collaboration agreement with the National Marrow Donor Program (as successor in interest to Be The Match BioTherapies, LLC) and a research agreement with an affiliated organization, Center for International Blood and Marrow Transplant Research. During the three months ended June 30, 2019, the Company recorded less than $0.1 million to research and development expenses and during the three months ended June 30, 2019 and 2018 recorded less than $0.1 million and $0.1 million, respectively, to general and administrative expenses and made payments of less than $0.1 million and $0.2 million, respectively, related to these agreements. There were no amounts recorded to research and development during the three months ended June 30, 2018 related to these agreements. During the six months ended June 30, 2019 and 2018, the Company recorded less than $0.1 million and $0.2 million to research and development expenses, respectively, and during each of the six months ended June 30, 2019 and 2018, recorded $0.1 million to general and administrative expenses and made payments of $0.4 million related to these agreements. As of June 30, 2019, there were no amounts on the balance sheet related to these agreements. As of December 31, 2018, amounts on the balance sheet related to these agreements were $0.4 million, which were included in accounts payable and accrued expenses and other current liabilities.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2019

Use of Estimates

Unaudited Interim Financial Information

10-K,

Fair Value Measurements

•

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

•

Income Taxes

The Company accounts for uncertainty in income taxes recognized in its consolidated financial statements by applying a

two-step

more-likely-than-not

Comprehensive Loss

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

The Company reported a net loss attributable to common stockholders for the three and six months ended June 30, 2019 and 2018.

Classification and Accretion of Redeemable Convertible Preferred Stock

Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

In June 2018, the Financing Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No.

2018-07,

Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment

Accounting

(“ASU

2018-07”).

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

No. 2014-09,

Revenue from Contracts with Customers (Topic 606)

, which supersedes existing revenue recognition guidance under GAAP and which the Company adopted on January 1, 2019. The Company early adopted ASU

2018-07

effective January 1, 2019 by remeasuring outstanding equity-classified awards issued to

non-employees

for which a measurement date had not been established as of January 1, 2019. The adoption of ASU

2018-07

did not have a material impact on the Company’s consolidated financial statements. The Company elected to estimate the expected term of options utilizing the simplified method for

non-employee

options that qualify as plain-vanilla options. The Company elected to account for forfeitures for

non-employee

options as they occur rather than apply an estimated forfeiture rate to stock-based compensation expense.

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

non-public

2016-02

2018-11,

Leases (Topic 842) Targeted Improvements

, which added an optional transition method to the existing requirements whereby an entity could adopt the provisions of ASU

2016-02

right-of-use

assets and lease liabilities on the consolidated balance sheets but does not expect it to have a material impact on its results of operations or cash flows.

- 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)

6 Months Ended

Jun. 30, 2019

Schedule of Marketable Securities by Security Type

As of June 30, 2019, marketable securities by security type consisted of (in thousands):


			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
United States Treasury Notes (due within one year)	$	45,895	$	27	$	—	$	45,922
Agency Bonds (due within one year)		93,128		112		—		93,240

Total	$	139,023	$	139	$	—	$	139,162

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


			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			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 June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents:
Money market funds	$	33,506	$	—	$	—	$	33,506
Marketable securities:
United States Treasury Notes		—		45,922		—		45,922
Agency Bonds		—		93,240		—		93,240

Total	$	33,506	$	139,162	$	—	$	172,668


	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

Accrued Expenses and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2019

Schedule of Accrued Expenses and Other Current Liabilities

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


	June 30, 2019		December 31, 2018
Accrued external research and development expenses	$	4,011	$	2,619
Accrued payroll and related expenses		1,567		2,526
Accrued professional fees		762		787
Deferred rent, current portion		601		71
Accrued property and equipment		396		713
Accrued other		254		211

	$	7,591	$	6,927

Stock-Based Awards (Tables)

6 Months Ended

Jun. 30, 2019

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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development expenses	$	1,268	$	1,699	$	2,524	$	2,590
General and administrative expenses		1,332		1,352		2,653		1,886

	$	2,600	$	3,051	$	5,177	$	4,476

Net Loss per Share (Tables)

6 Months Ended

Jun. 30, 2019

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 June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Numerator:
Net loss	$	(17,708	)	$	(13,660	)	$	(32,542	)	$	(24,821	)
Accretion of redeemable convertible preferred stock to redemption value		—			(88	)		—			(88	)

Net loss attributable to common stockholders	$	(17,708	)	$	(13,748	)	$	(32,542	)	$	(24,909	)

Denominator:
Weighted average common shares outstanding, basic and diluted		36,662,562			4,391,363			35,051,371			3,434,175

Net loss per share attributable to common stockholders, basic and diluted	$	(0.48	)	$	(3.13	)	$	(0.93	)	$	(7.25	)

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share


	As of June 30,
	2019		2018
Stock options to purchase common stock		4,913,620		3,247,996
Unvested restricted common stock		627,242		1,481,211
Options to purchase additional shares of common stock granted to the underwriters in connection with the IPO		—		1,000,000

		5,540,862		5,729,207

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 30, 2019

Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease

As of June 30, 2019, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands):


Remainder of 2019 (six months)	$	2,809
2020		5,782
2021		6,072
2022		6,375
2023		6,734
Thereafter		32,000

	$	59,772

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended				6 Months Ended		12 Months Ended
	May 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
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							$ 60,874	$ 93,000
Net loss			$ (17,708)	$ (14,834)	$ (13,660)	$ (11,161)	(32,542)	$ (24,821)	$ (57,500)
Accumulated deficit			$ (135,268)				$ (135,268)		$ (102,726)
Net proceeds from common stock	$ 60,300
Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares	4,887,500		4,887,500		6,666,667
Stock issued, price per shares	$ 13.25
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			$ 15.00			$ 15.00
Net proceeds from initial public offering		$ 89,900

Summary of Significant Accounting Policies - Additional Information (Detail)	Jun. 25, 2018 shares
Summary Of Significant Accounting Policies [Line Items]
Redeemable convertible preferred stock converted into common stock shares	23,375,405

Fair Value of Financial Assets - Schedule of Marketable Securities by Security Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 139,023	$ 84,233
Gross Unrealized Gains	139	6
Gross Unrealized Losses		(14)
Estimated Fair Value	139,162	84,225
United States Treasury Notes [Member]
Marketable Securities [Line Items]
Amortized Cost	45,895	71,944
Gross Unrealized Gains	27	5
Gross Unrealized Losses		(9)
Estimated Fair Value	45,922	71,940
Agency Bonds [Member]
Marketable Securities [Line Items]
Amortized Cost	93,128	12,289
Gross Unrealized Gains	112	1
Gross Unrealized Losses		(5)
Estimated Fair Value	$ 93,240	$ 12,285

Fair Value of Financial Assets - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 139,162	$ 84,225
Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	172,668	142,438
Level 1 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	33,506	45,725
Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	139,162	96,713
Money Market Funds [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	33,506	45,725
Money Market Funds [Member] \| Level 1 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	33,506	45,725
United States Treasury Notes [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		12,488
United States Treasury Notes [Member] \| Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		12,488
United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	45,922	71,940
United States Treasury Notes [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	45,922	71,940
United States Treasury Notes [Member] \| Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	45,922	71,940
Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	93,240	12,285
Agency Bonds [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	93,240	12,285
Agency Bonds [Member] \| Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 93,240	$ 12,285

Fair Value of Financial Assets - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	$ 0	$ 0	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0	0	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset transfers into level 3	0	0	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers out of level 3	$ 0	$ 0	$ 0	$ 0

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 4,011	$ 2,619
Accrued payroll and related expenses	1,567	2,526
Accrued professional fees	762	787
Deferred rent, current portion	601	71
Accrued property and equipment	396	713
Accrued other	254	211
Total	$ 7,591	$ 6,927

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		6 Months Ended
	Apr. 18, 2019	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense		$ 20.6
Period for recognition of unrecognized expense		2 years 4 months 24 days
Employees [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		1,610,566
Weighted average fair value of stock options granted		$ 5.94
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance		2,499,293
2019 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance	166,525	166,525
Annual Increase In Aggregate Number Of Shares Reserved For Future Issuance	1,000,000
Minimum percentage of shares issued and outstanding	1.00%

Stock-Based Awards - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 2,600	$ 3,051	$ 5,177	$ 4,476
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	1,268	1,699	2,524	2,590
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 1,332	$ 1,352	$ 2,653	$ 1,886

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				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Numerator:
Net loss	$ (17,708)	$ (14,834)	$ (13,660)	$ (11,161)	$ (32,542)	$ (24,821)	$ (57,500)
Accretion of redeemable convertible preferred stock to redemption value			(88)			(88)
Net loss attributable to common stockholders	$ (17,708)		$ (13,748)		$ (32,542)	$ (24,909)
Denominator:
Weighted average common shares outstanding, basic and diluted	36,662,562		4,391,363		35,051,371	3,434,175
Net loss per share attributable to common stockholders, basic and diluted	$ (0.48)		$ (3.13)		$ (0.93)	$ (7.25)

- 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	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	5,540,862	5,729,207
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	4,913,620	3,247,996
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	627,242	1,481,211
Option to purchase additional shares of common stock granted to the underwriters in connection with the IPO [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount		1,000,000

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended			6 Months Ended		12 Months Ended
	May 31, 2018 USD ($) ft²	Apr. 30, 2017 USD ($) shares	Nov. 30, 2016 USD ($) shares	Jun. 30, 2019 USD ($) shares	Dec. 31, 2018 USD ($) ft² Sublease shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Milestone shares	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Sublease commencement period							2018-06
Sublease expiration period							2028-02
Property sublease, description							In May 2018, the Company entered into a sublease for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease was amended effective March 2019 to increase the lease payments over the lease term and to increase the sublandlord-funded tenant improvements. The sublease is subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and expires in February 2028. The sublandlord has the right to terminate the sublease after five years.
Sublease termination period	5 years
Long-term deferred rent				$ 5,945,000	$ 1,246,000		$ 5,945,000
Deferred rent, current portion				601,000	$ 71,000		601,000
Rent expense				1,600,000		$ 800,000	3,100,000	$ 1,100,000
Research and development expense				$ 13,433,000		9,683,000	$ 23,970,000	17,532,000
Common stock issued \| shares				39,338,100	34,327,554		39,338,100
Redeemable convertible preferred stock issued \| shares				0	0		0
Prepaid Expenses and Other Current Assets [Member] \| Office and Lab Sublease [Member]
Other Commitments [Line Items]
Tenant improvement allowance				$ 5,500,000	$ 700,000		$ 5,500,000
Maximum [Member] \| Office and Lab Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease \| ft²	69,000
Maximum [Member] \| Prepaid Expenses and Other Current Assets [Member] \| Office and Lab Sublease [Member]
Other Commitments [Line Items]
Tenant improvement allowance				5,500,000			$ 5,500,000
Sub-Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease \| ft²					27,000
Rental payments receivable					$ 4,500,000
Lease agreement maturity period					2 years
Number of Sub-sublease \| Sublease					2
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] \| Maximum [Member]
Other Commitments [Line Items]
License agreement royalty percentage		20.00%
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Achievement of Clinical and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation		$ 177,000,000
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] \| 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
Collaboration Agreement [Member]
Other Commitments [Line Items]
Research and development expense						$ 100,000	$ 400,000	$ 800,000
Collaboration Agreement [Member] \| Maximum [Member]
Other Commitments [Line Items]
Research and development expense				100,000
Collaboration Agreement [Member] \| Development, Regulatory and Commercial Milestone [Member] \| Per Target [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation				$ 83,500,000			83,500,000
License Agreement With Harvard [Member]
Other Commitments [Line Items]
Research and development expense							$ 100,000
Payment of upfront fee			$ 100,000
Common stock issued \| shares			385,063
Fair value of shares as of issuance date										$ 300,000
License Agreement With Harvard [Member] \| Maintenance [Member] \| Annually Through 2019 [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation			$ 100,000
License Agreement With Harvard [Member] \| Maintenance [Member] \| Annually Thereafter [Member]
Other Commitments [Line Items]
Payment obligation			100,000
License Agreement With Harvard [Member] \| Achievement of Development and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation			7,400,000
License Agreement With Harvard [Member] \| Patents [Member]
Other Commitments [Line Items]
Reimbursed costs			$ 300,000
Letter of Credit [Member] \| Office and Lab Sublease [Member]
Other Commitments [Line Items]
Cash balance to secure letter of credit associated with sublease	$ 1,800,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Operating Leased Assets [Line Items]
Remainder of 2019 (six months)	$ 2,809
2020	5,782
2021	6,072
2022	6,375
2023	6,734
Thereafter	32,000
Operating leases future minimum payments due	$ 59,772

Related Parties - Additional Information (Detail) - Match Bio Therapies [Member] - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Administrative expenses related party				$ 400,000
Expenses paid to related party		$ 200,000	$ 400,000	400,000
Accounts Payable, Accrued Expenses and Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Amounts owed to related party	$ 0		0
Research and Development Expenses [Member]
Related Party Transaction [Line Items]
Research and development expense		0		$ 200,000
Maximum [Member]
Related Party Transaction [Line Items]
Administrative expenses related party	100,000	$ 1,000,000	100,000
Expenses paid to related party	100,000
Maximum [Member] \| Research and Development Expenses [Member]
Related Party Transaction [Line Items]
Research and development expense	$ 100,000		$ 100,000