Magenta Therapeutics Reports Recent Business Highlights and Third Quarter Financial Results Magenta advanced its MGTA-145 mobilization program, with two Phase 2 trials to be initiated by end of 2020; the Company s MGTA-1

Magenta Therapeutics Reports Recent Business Highlights and Third Quarter Financial Results

Magenta advanced its MGTA-145 mobilization program, with two Phase 2 trials to be initiated

by end of 2020; the Company s MGTA-117 conditioning program is on track to file an IND and generate clinical data in 2021;

Magenta announced appointment of Lisa M. Olson, Ph.D., as Head of Research and Chief Scientific Officer

Magenta named co-recipient of a U19 grant from the National Institutes of Health as part of

an interdisciplinary effort to explore use of novel targeted conditioning agents with gene editing approaches to cure HIV

Ended quarter with $161.7 million in cash, cash equivalents and marketable securities

Cambridge, MA November 5, 2020 Magenta Therapeutics (Nasdaq: MGTA), a clinical-stage biotechnology company developing novel medicines to

bring the curative power of immune reset to more patients, today reported recent business highlights and financial results for the third quarter ended September 30, 2020.

Magenta showed steady progress throughout the third quarter, bringing aboard new scientific leadership in Lisa Olson to broaden our technical expertise

across research and discovery. We continued to progress both clinically and preclinically, showcasing findings at the European Society for Blood and Marrow Transplantation annual meeting, said Jason Gardner, D.Phil., President and Chief

Executive Officer, Magenta Therapeutics. We are driven by what s ahead, including presentations at the upcoming American Society of Hematology annual meeting, and into 2021, and the anticipation of clinical data in our conditioning and

mobilization programs.

Recent Program Updates:

In September 2020, Magenta announced it was named as co-recipient along with the

University of Southern California, University of Washington and Fred Hutchinson Cancer Research Center, Harvard

University and Massachusetts General Hospital, and the Ragon Institute of a multi-project, interdisciplinary U19 grant from the National Institutes of Health (NIH) to explore the use of

novel targeted conditioning agents with gene editing approaches to advance research in a cure for HIV.

Cash Position: Cash, cash equivalents and marketable securities as of September 30, 2020, were $161.7 million, compared to $145.7 million

as of December 31, 2019. Magenta anticipates that its cash, cash equivalents and marketable securities will be sufficient to fund operations and capital expenditures into the second half of 2022.

Research and Development Expenses: Research and development expenses were $11.8 million in the third quarter of 2020, compared to

$16.5 million in the third quarter of 2019. The decrease was driven primarily by lower clinical trial costs for our MGTA-145 Phase 1 clinical trials which were completed in the first quarter of 2020,

decreased preclinical costs for manufacturing related to our conditioning programs and lower manufacturing and clinical trial costs due to the discontinuance of enrollment in our Phase 2 trial of MGTA-456 in

inherited metabolic diseases in June 2020.

General and Administrative Expenses: General and administrative expenses were $6.6 million for the

third quarter of 2020, compared to $6.1 million for the third quarter of 2019. The increase was primarily due to an increase in personnel costs associated with the growth of the Company.

Net Loss: Net loss was $17.7 million for the third quarter of 2020, compared to net loss of $21.0 million for the third quarter of 2019.

About Magenta Therapeutics

Magenta Therapeutics is a

clinical-stage biotechnology company developing medicines to bring the curative power of immune system reset through stem cell transplant to more patients with autoimmune diseases, genetic diseases and blood cancers. 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 world to revolutionize immune reset for more patients.

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

Follow Magenta on Twitter: @magentatx.

Forward-Looking Statement

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, projections regarding future revenues and financing performance, our long-term growth,

cash, cash equivalents and marketable securities, the anticipated timing of our clinical trials and regulatory filings, the development of our product candidates and advancement of our preclinical programs, statements regarding Magenta s

participation in the collaboration involving the U19 grant from the NIH, including the timing, progress and success of such collaboration, as well as other statements containing the words anticipate, believe,

continue, could, endeavor, estimate, expect, 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; 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, including review under accelerated approval processes; orphan drug designation eligibility; 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, 2020, its Quarterly Reports 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

Jill Bertotti, W2O Group

Magenta Therapeutics, Inc.

STATEMENTS OF OPERATIONS

thousands, except share and per share data)