Magenta Therapeutics Reports Second Quarter Financial Results and Recent Program Highlights Progress in addressing the FDA s recommendations regarding bioassay and dose escalations to clear the current clinical hold on t

Magenta Therapeutics Reports Second Quarter Financial Results

and Recent Program Highlights

Progress in addressing the FDA s recommendations regarding bioassay and dose escalations to clear the current clinical hold on

the company s Investigational New Drug Application to initiate MGTA-117 Phase 1/2 clinical trial

Enrollment complete for Phase 2 clinical trial evaluating MGTA-145 plus plerixafor in

patients with multiple myeloma; final clinical data expected in Q4 2021

Plans to initiate Phase 2 clinical trial of MGTA-145 plus plerixafor for mobilization and collection of stem cells in patients with sickle cell disease in Q4 2021

Ended Q2 2021 with approximately $207.8 million in cash, cash equivalents and marketable securities, and updates guidance that

cash reserves are expected to fund the company s operating plan into Q3 2023

Cambridge, MA August 5, 2021 Magenta

Therapeutics, Inc. (Nasdaq: MGTA), a clinical-stage biotechnology company developing novel medicines to bring the curative power of stem cell transplants to more patients, today reported financial results for the second quarter ended June 30,

2021, and recent program highlights.

The Magenta team has been hard at work prioritizing the advancement of our program portfolio, now with

multiple clinical trials underway, including the two Phase 2 clinical trials of MGTA-145. We plan to initiate a Phase 1/2 trial of MGTA-117 in patients with Acute

Myeloid Leukemia (AML) and Myelodysplastic Syndromes (MDS), assuming we are successful in resolving the clinical hold on our Investigational New Drug (IND) Application, and we also plan to initiate a Phase 2 clinical trial of MGTA-145 plus plerixafor for mobilization and collection of stem cells in patients with sickle cell disease, said Jason Gardner, D.Phil., President and Chief Executive Officer, Magenta Therapeutics. We

remain laser-focused on developing transformative mobilization and conditioning treatments for the many patients who stand to benefit.

Business Highlights:

MGTA-145 Stem Cell Mobilization and Collection Recent and Planned Activity:

MGTA-117 Targeted Conditioning Recent and

The company also announced today that Mike Bonney resigned from the company s Board of Directors and its committees,

effective August 3, 2021, to focus on his newly expanded role as Executive Chair of Alnylam Pharmaceuticals, Inc. The resignation was not the result of any disagreement with the company nor any of its affiliates on any matter relating to the

company s operations, policies or practices. In connection with Mr. Bonney s resignation, the size of the Board was reduced from 10 members to nine members and Alison Lawton, currently a member of the Board, was appointed Chair of the

Board, and Jeff Albers, also a current member of the Board, was appointed to the Board s audit committee.

I would like to thank Mike for his contribution to Magenta s success through the years,

commented Dr. Gardner. We are grateful for his support and wish him the very best in the future. We are also extremely pleased to announce the expansion of Alison s role as Chair, bringing us more than 30 years of experience in the

biopharmaceutical industry, as an executive and board member, as well as in a wide range of operational roles.

Cash Position: Cash, cash equivalents and marketable securities as of June 30, 2021 were $207.8 million, compared to

$148.8 million as of December 31, 2020. Magenta anticipates that its cash, cash equivalents and marketable securities will be sufficient to fund operations and capital expenditures into Q3 2023.

Research and Development Expenses: Research and development expenses were $11.1 million in Q2 2021, compared to $12.6 million in Q2 2020. The

decrease was driven primarily by the completion of the GMP manufacturing activities to support the IND Application for MGTA-117 and future clinical trials.

General and Administrative Expenses: General and administrative expenses were $6.5 million for Q2 2021, compared to $7.4 million for Q2 2020.

Net Loss: Net loss was $16.9 million for Q2 2021, compared to net loss of $19.1 million for Q2 2020.

About Magenta Therapeutics

Magenta Therapeutics is a

clinical-stage biotechnology company developing medicines to bring the curative power of stem cell transplants to more patients with blood cancers, genetic diseases and autoimmune diseases. Magenta is combining leadership in stem cell biology and

biotherapeutics development with clinical and regulatory expertise, a unique business model and broad networks in the stem cell transplant community to revolutionize immune reset for more patients.

Magenta is based in Cambridge, Massachusetts. For more information, please visit www.magentatx.com.

Follow Magenta on Twitter: @magentatx.

This press release may contain forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995 and

other federal securities laws, including express or implied statements regarding Magenta s future expectations, plans and prospects, including, without limitation, statements regarding expectations and plans for presenting pre-clinical and clinical data, the development of a biomarker assay, working with the FDA to resolve the clinical hold on the MGTA-117 Phase 1/2 clinical trial, projections

regarding future revenues and financing performance, long-term growth, cash, cash equivalents and marketable securities, the anticipated timing of clinical trials and regulatory filings, the initiation of clinical trials or the results of ongoing

and planned clinical trials, the development of product candidates and advancement of preclinical programs, the potential benefits of product candidates, the timing, progress and success of collaborations, as well as other statements containing the

words anticipate, believe, continue, could, endeavor, estimate, expect, anticipate, intend, may, might,

plan, potential, predict, project, seek, should, target, will or would and similar expressions that constitute forward-looking statements

under the Private Securities Litigation Reform Act of 1995. The express or implied forward-looking statements included in this press release are only predictions and are subject to a number of risks, uncertainties and assumptions, including, without

limitation: uncertainties inherent in clinical studies and in the availability and timing of data from ongoing clinical studies; the development of biomarker assays; whether interim results from a clinical trial will be predictive of the final

results of the trial; whether results from preclinical studies or earlier clinical studies will be predictive of the results of future trials; the expected timing of submissions for regulatory approval or review by governmental authorities;

discussions with governmental agencies such as the FDA; regulatory approvals to conduct trials or to market products; whether Magenta s cash resources will be sufficient to fund Magenta s foreseeable and unforeseeable operating expenses

and capital expenditure requirements; risks, uncertainties and assumptions regarding the impact of the continuing COVID-19 pandemic on Magenta s business, operations, strategy, goals and anticipated

timelines, Magenta s ongoing and planned preclinical activities, Magenta s ability to initiate, enroll, conduct or complete ongoing and planned clinical trials, Magenta s timelines for regulatory submissions and Magenta s

financial position; and other risks concerning Magenta s programs and operations are described in additional detail in its Annual Report on Form 10-K filed on March 3, 2021, as updated by

Magenta s most recent Quarterly Report on Form 10-Q, and its other filings made with the Securities and Exchange Commission from time to time. Although Magenta s

forward-looking statements reflect the good faith judgment of its management, these statements are based only on facts and factors currently known by Magenta. As a result, you are cautioned not

to rely on these forward-looking statements. Any forward-looking statement made in this press release speaks only as of the date on which it is made. Magenta undertakes no obligation to publicly update or revise any forward-looking statement,

whether as a result of new information, future developments or otherwise.

Magenta Therapeutics

Lyndsey Scull, Director, Corporate

Magenta Therapeutics, Inc.

STATEMENTS OF OPERATIONS

thousands, except share and per share data)