Acceleron to Host Conference Call and Webcast to Review Clinical Trial Updates, Preclinical Presentations on Sotatercept in Pulmonary Arterial Hypertension (PAH) Presented at the American Thoracic Society (ATS) 2021 International 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, May 19, 2021 at 10:30 a.m. EDT to review updates from the PULSAR and SPECTRA Phase 2 clinical trials of sotatercept in patients with pulmonary arterial hypertension (PAH). Data from the PULSAR and SPECTRA trials will have been presented during clinical presentations at the American Thoracic Society 2021 International Conference (ATS 2021) earlier that day.

Guest Presenter:

• Aaron Waxman¹, M.D., Ph.D., Director of the Pulmonary Vascular Disease Program at Brigham and Women’s Hospital; Associate Professor of Medicine at Harvard Medical School

The webcast will be accessible under "Events & Presentations" in the Investors & Media page of the Company’s website at www.acceleronpharma.com. To participate in the conference call, please dial 833-494-1483 (domestic) or 236-714-2620 (international) and reference code #6565123.

An archived version of the webcast will be available for replay on the Company’s website for approximately one year.

1 Dr. Waxman is the principal investigator of the SPECTRA trial and a paid consultant to Acceleron.

About Sotatercept

Sotatercept is an investigational reverse-remodeling agent designed to be a selective ligand trap for members of the TGF-beta superfamily to rebalance signaling in the BMP pathway, which is a key molecular driver of PAH. In preclinical studies, sotatercept was shown to reverse the vascular remodeling that is a hallmark of PAH. The PULSAR Phase 2 trial evaluating sotatercept in combination with approved PAH-specific medicines in patients with PAH achieved its primary endpoint of improvement in pulmonary vascular resistance and its key secondary endpoint of improvement in 6-minute walk distance. Sotatercept was generally well tolerated in the trial. Adverse events observed in the study were generally consistent with previously published data on sotatercept in other diseases. Following the PULSAR results, sotatercept was granted Breakthrough Therapy designation from the FDA and Priority Medicines designation from the EMA in PAH. Sotatercept is also being evaluated in the SPECTRA Phase 2 exploratory trial.