Magenta Therapeutics Reports Recent Business Highlights and Second Quarter Financial Results Collaborations announced with AVROBIO and Beam Therapeutics to evaluate potential usage of MGTA-117 in conditioning patients in

Magenta Therapeutics Reports Recent Business Highlights and Second Quarter Financial Results

Collaborations announced with AVROBIO and Beam Therapeutics to evaluate potential usage of

MGTA-117 in conditioning patients in gene therapy and gene editing settings, respectively

MGTA-145 to move into Phase 2 trials in autologous and allogeneic transplant in

allogeneic study in newly announced collaboration with National Marrow Donor Program

Ended quarter with $176.5 million in cash, cash equivalents and marketable securities

Completed public offering of common stock in June 2020, raising gross proceeds of $69.0 million,

extending cash runway into the second half of 2022

Cambridge, MA August 6, 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 second quarter ended June 30, 2020.

Magenta had a very productive and impactful second quarter, executing three exciting collaborations that hold significant promise for patients. These

collaborations will facilitate further validation and advancement of our conditioning and mobilization programs, with the goal of allowing more patients to benefit from these treatments, said Jason Gardner, D.Phil., President and Chief

Executive Officer, Magenta. We believe we are well positioned as we head into the second half of 2020 and beyond to make progress across our portfolio.

Recent Business Highlights:

Magenta announced a non-exclusive collaboration with Beam Therapeutics to evaluate the potential utility of MGTA-117, Magenta s novel targeted antibody drug

conjugate (ADC) for conditioning of patients with sickle cell disease or beta-thalassemia receiving Beam s base editing therapies. Beam will be responsible for clinical trial costs related to development of Beam s base editors when

combined with MGTA-117, while Magenta will continue to be responsible for all other development costs of MGTA-117. Magenta will also continue to develop MGTA-117 in other diseases, including blood cancers and genetic diseases. Each company will retain all commercial rights to their respective technologies.

In May 2020, Magenta announced a collaboration with AVROBIO to evaluate the potential utility of MGTA-117, its novel targeted ADC, for conditioning patients with one or more AVROBIO investigational lentiviral gene therapies. The collaboration will combine Magenta s leadership in ADC-based conditioning with AVROBIO s expertise in lentiviral gene therapies and is expected to further the two companies shared mission to allow patients to live free from disease. Under the

collaboration, Magenta and AVROBIO will jointly evaluate MGTA-117 in conjunction with one or more of AVROBIO s investigational gene therapies. Magenta will retain all commercial rights to MGTA-117. AVROBIO will retain all commercial rights to its gene therapies and will be responsible for the clinical trial costs related to the evaluation of MGTA-117 with

AVROBIO s gene therapies.

In June 2020, Magenta announced a collaboration with the National Marrow Donor Program (NMDP)/Be The Match to evaluate the potential utility of MGTA-145 for

mobilizing and collecting hematopoietic stem cells (HSCs) from donors in a single day and then using them for allogeneic transplants in patients. The collaboration builds upon the existing partnership between the two organizations announced in May

2017. Under the collaboration, Magenta and NMDP/Be The Match will run a Phase 2 clinical trial of MGTA-145 to mobilize and collect HSCs from donors which will then be transplanted into patients with blood

cancers in need of a stem cell transplant. Magenta will retain all commercial rights to MGTA-145.

2020, Magenta announced that it completed a public offering of 8,625,000 shares of its common stock, including the exercise in full by the underwriters of their option to purchase an additional 1,125,000 shares, and raised gross proceeds of

$69.0 million. Magenta intends to use the proceeds from this offering to advance its clinical and earlier stage programs and for research and development, working capital and general corporate purposes. The Company anticipates

that its cash, cash equivalents and marketable securities, including the proceeds from this recent offering, will be sufficient to fund operations and capital expenditures into the second half of 2022.

In April 2020, Magenta announced the promotion of John Davis Jr., M.D., M.P.H., M.S. to Head of Research and Development, in addition to his current

role as Chief Medical Officer. Dr. Davis joined Magenta as Chief Medical Officer in 2018.

Recent Program Updates:

MGTA-456 Cell Therapy

Magenta has continued to take steps to help ensure the safety of employees and their families and to reduce the spread of

COVID-19 in the Cambridge community during the second quarter. While certain mitigation measures, including the stay-at-home

order, have been relaxed within the Cambridge and Greater Boston communities, Magenta has continued to enforce a work-from-home policy for all employees, other than those performing or supporting business-critical laboratory-based experiments, such

as certain members of the Company s laboratory and facilities staff. For those employees, Magenta implemented stringent safety measures designed to comply with applicable federal, state and local guidelines instituted in response to the COVID-19 pandemic and together with its internal, cross-function COVID-19 response team, continues to refine the protocols.

Cash Position: Cash,

cash equivalents and marketable securities as of June 30, 2020, were $176.5 million, compared to $145.7 million as of December 31, 2019. In addition, in June 2020 Magenta announced that it completed a public offering of common

stock and raised proceeds of $64.6 million, net of underwriting discounts and commissions and offering costs. 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 $12.6 million in the

second quarter of 2020, compared to $13.4 million in the second quarter of 2019. The decrease was driven primarily by lower clinical trial costs due to the completion of MGTA-145 Phase 1 clinical trial in

the first quarter of 2020, offset by investments in manufacturing related to our conditioning programs.

General and Administrative Expenses: General and administrative expenses were $7.4 million for

the second quarter of 2020, compared to $5.9 million for the second quarter of 2019. The increase was primarily due to an increase in personnel associated with the growth of the Company, in addition to an increase in patent related costs.

Net Loss: Net loss was $19.1 million for the second quarter of 2020, compared to net loss of $17.7 million for the second quarter of 2019.

About Magenta Therapeutics

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.

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 collaboration agreements with each of Beam Therapeutics, AVROBIO, and NMDP/Be The Match, including the

timing, progress and success of each collaboration, the anticipated cost allocation and other commercial terms under each collaboration agreement, Magenta s strategy and business plan, as well as the future development, manufacture and

commercialization between Magenta and its collaborators, 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:

Manisha Pai, Vice President, Communications & Investor Relations

Lyndsey Scull, Director, Corporate

Magenta Therapeutics, Inc.

STATEMENTS OF OPERATIONS

(In thousands, except share and per share data)