Acceleron to Host Conference Call and Webcast to Review Results of PULSAR Phase 2 Trial of Sotatercept Presented at American Thoracic Society 2020 Virtual Conference

Acceleron Pharma Inc. (Nasdaq: XLRN), a leading biopharmaceutical company in the discovery, development, and commercialization of TGF-beta superfamily therapeutics to treat serious and rare diseases, today announced it will host a conference call and webcast on Wednesday, June 24, 2020 at 4:30 p.m. EDT to review the topline results of the PULSAR Phase 2 trial of sotatercept in patients with pulmonary arterial hypertension (PAH). Data from the PULSAR trial will have been presented during a virtual session of the American Thoracic Society (ATS) 2020 earlier that day.

Guest Presenters:

• Marc Humbert¹, M.D., Ph.D., Professor of Medicine and Director of the French Pulmonary Hypertension Reference Center at the Université Paris-Saclay

• Vallerie McLaughlin¹, M.D., Professor of Medicine and Director of the Pulmonary Hypertension Program at the University of Michigan

The webcast will be accessible under "Events & Presentations" in the Investors/Media page of the company's website at www.acceleronpharma.com. Individuals can participate in the conference call by dialing 877-312-5848 (domestic) or 253-237-1155 (international) and referring to the "Acceleron ATS 2020 Conference Call."

A replay of the webcast will be available on the Acceleron website approximately two hours after the event.

ATS 2020 Presentation Information:

The PULSAR Phase 2 trial results will be presented by principal investigator David B. Badesch¹, M.D., of the University of Colorado, on Wednesday, June 24, 2020 from 3:18 p.m. to 3:43 p.m. EDT. The live presentation can be accessed via free registration at the ATS 2020 website.

^1. Drs. Humbert, McLaughlin, and Badesch are investigators in the PULSAR trial and are paid consultants to Acceleron.

About Sotatercept

Sotatercept is an investigational agent designed to be a selective ligand trap for members of the TGF-beta superfamily to rebalance BMPR-II signaling, which is a key molecular driver of PAH. In preclinical research published in Science Translational Medicine, sotatercept exhibited consistent effects across multiple components of disease, including suppressed proliferation of pulmonary arterial smooth muscle and microvascular endothelial cells, reduced pulmonary pressures, lessened right ventricular hypertrophy, improved right ventricular function, and attenuated vascular remodeling. Sotatercept, which is part of a licensing agreement with Bristol Myers Squibb, is also being evaluated in the exploratory, open-label SPECTRA Phase 2 trial in patients with PAH. For more information, please visit www.clinicaltrials.gov.

Sotatercept has been granted Breakthrough Therapy designation from the U.S. Food and Drug Administration and Priority Medicine (PRIME) designation from the European Medicines Agency in PAH. Sotatercept is an investigational therapy that is not approved for any use in any country.

About PAH

PAH is a rare and chronic, rapidly progressing disorder characterized by the constriction of small pulmonary arteries and elevated blood pressure in the pulmonary circulation. PAH results in significant strain on the heart, often leading to limited physical activity, heart failure, and reduced life expectancy. The 5-year survival rate for patients with PAH is approximately 57%. Available therapies generally act by promoting the dilation of pulmonary vessels without addressing the underlying cause of the disease. As a result, PAH often progresses rapidly for many patients despite standard of care treatment. A growing body of research has implicated imbalances in BMP and TGF-beta signaling as a primary driver of PAH in familial, idiopathic, and acquired forms of the disease.

About Acceleron

Acceleron is a biopharmaceutical company dedicated to the discovery, development, and commercialization of therapeutics to treat serious and rare diseases. Acceleron's leadership in the understanding of TGF-beta superfamily biology and protein engineering generates innovative compounds that engage the body's ability to regulate cellular growth and repair.

Acceleron focuses its commercialization, research, and development efforts in hematologic and pulmonary diseases. In hematology, Acceleron and its global collaboration partner, Bristol Myers Squibb, are co-promoting REBLOZYL^® (luspatercept-aamt), the first and only approved erythroid maturation agent, in the United States for the treatment of anemia in certain blood disorders. The Companies are also developing luspatercept for the treatment of anemia in patient populations of MDS, beta-thalassemia, and myelofibrosis. In pulmonary, Acceleron is developing sotatercept for the treatment of pulmonary arterial hypertension, having recently reported positive topline results of the Phase 2 PULSAR trial.

For more information, please visit www.acceleronpharma.com. Follow Acceleron on Social Media: @AcceleronPharma and LinkedIn.

Forward-Looking Statements

This press release contains forward-looking statements about Acceleron's strategy, future plans and prospects, including statements regarding the development of sotatercept in PAH, the timeline for clinical development and regulatory approval of sotatercept in PAH, the expected timing for reporting of data from ongoing clinical trials, and the potential of Acceleron's compounds as therapeutic drugs. The words "anticipate," "believe," "could," "estimate," "expect," "goal," "intend," "may," "plan," "possible," "potential," "project," "should," "target," "will," "would," and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words.

Actual results could differ materially from those included in the forward-looking statements due to various factors, risks and uncertainties, including, but not limited to, that preclinical testing of Acceleron's compounds and data from clinical trials may not be predictive of the results or success of ongoing or later clinical trials, that regulatory approval of Acceleron's compounds in one indication or country may not be predictive of approval in another indication or country, that the development of Acceleron's compounds will take longer and/or cost more than planned, that Acceleron will be unable to successfully complete the clinical development of Acceleron's compounds, that Acceleron may be delayed in initiating, enrolling or completing any clinical trials, that Acceleron's compounds will not receive regulatory approval or become commercially successful products, and that Breakthrough Therapy or PRIME designation may not expedite the development or review of sotatercept. These and other risks and uncertainties are identified under the heading "Risk Factors" included in Acceleron's most recent Annual Report on Form 10-K, Quarterly Report on Form 10-Q, and other filings that Acceleron has made and may make with the SEC in the future.

The forward-looking statements contained in this press release are based on management's current views, plans, estimates, assumptions, and projections with respect to future events, and Acceleron does not undertake and specifically disclaims any obligation to update any forward-looking statements.