Magenta Therapeutics Reports Third Quarter Financial Results and Program Highlights Positive topline clinical data from all patients in an investigator-initiated Phase 2 clinical trial evaluating stem cell mobilization a

Magenta Therapeutics Reports Third Quarter Financial Results

and Program Highlights

Positive topline clinical data from all patients in an investigator-initiated Phase 2 clinical trial evaluating stem cell

mobilization and transplant engraftment using MGTA-145 plus plerixafor in patients with multiple myeloma will be presented as a poster at the 2021 American Society of Hematology (ASH) Annual Meeting on

MGTA-117 targeted conditioning program clinical trial start-up activities are ongoing after clearance of Investigational New Drug (IND) Application; Phase 1/2 trial expected to open in December 2021

Ended Q3 2021 with approximately $192.6 million in cash, cash equivalents and marketable securities, and the company

maintains guidance that cash reserves are expected to fund the operating plan into Q3 2023

Cambridge, MA November 4, 2021

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

quarter ended September 30, 2021, and recent program highlights.

We are pleased with our recent execution across the portfolio as we look to

continue to allocate our capital efficiently to our value-creating opportunities, said Jason Gardner, D.Phil., President and Chief Executive Officer, Magenta Therapeutics. In mid-September, we

cleared the IND process for the MGTA-117 targeted conditioning program and are now conducting the clinical trial start-up activities with the continued expectation of

opening the trial this year. The MGTA-145 stem cell mobilization program also continues to advance with a fully enrolled investigator-initiated trial in multiple myeloma patients, a Phase 2 clinical trial in

allogeneic transplant and the expected start of a Phase 2 clinical trial evaluating mobilization and collection of stem cells from patients with sickle cell disease.

Business Highlights:

In October 2021, Magenta welcomed Jeffrey Humphrey, M.D. to its Executive Team as Chief Medical Officer.

MGTA-145 Stem Cell Mobilization and Collection

Recent and Planned Activity:

Autologous Stem Cell Transplant: Multiple Myeloma

Magenta expects to open the Phase 2 clinical trial in December 2021. The trial is designed to evaluate the utility of MGTA-145 in combination with plerixafor, for the mobilization and collection of stem cells in patients with sickle cell disease where mobilization and collection is difficult and there is a clear unmet medical need.

MGTA-117 Targeted Conditioning

Recent and Planned Activity:

Cash Position: Cash, cash equivalents and marketable securities as of September 30, 2021, were $192.6 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 the third quarter of 2023.

Research and Development Expenses: Research and development expenses were $10.8 million in the third quarter of 2021, compared to

$11.8 million in the third quarter of 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

offset by an increase in personnel related costs.

General and Administrative Expenses: General and administrative expenses were $7.5 million

for the third quarter of 2021, compared to $6.6 million for the third quarter of 2020. The increase was primarily due to an increase in personnel related costs.

Net Loss: Net loss was $17.4 million for the third quarter of 2021, compared to net loss of $17.7 million for the third quarter of 2020.

About Magenta Therapeutics

Magenta Therapeutics is a

clinical-stage biotechnology company developing medicines designed 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 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.

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, the initiation of clinical trials or the results of ongoing and planned clinical

trials, the development of product candidates and advancement of preclinical programs, 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 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, 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; 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.

Bertotti, Real Chemistry (advisor to Magenta)

STATEMENTS OF OPERATIONS

thousands, except share and per share data)