PRESS RELEASE: Acceleron Appoints John Knopf to CEO Spot

Acceleron Appoints John Knopf to CEO Spot

CAMBRIDGE, MA - September 18, 2007 - Acceleron Pharma, Inc., a biopharmaceutical company developing novel therapeutics that modulate the growth of tissues including bone and muscle, today announced that John L. Knopf, Ph.D., has been named as the Chief Executive Officer of Acceleron. Dr. Knopf has also been elected to the Company's Board of Directors. Dr. Knopf, a founder of Acceleron, has served as the Executive Vice President of Corporate Development and recently, as President, before being unanimously elected by the Board as the Company's next CEO.

"John has been an instrumental leader at Acceleron since founding the Company. His keen insight into the scientific and clinical potential of Acceleron's R&D platform generates tremendous long-term value for the Company," said Richard Pops, a member of Acceleron's Board of Directors. "We are extremely pleased to leverage John's scientific acumen and business vision in his role as CEO."

"I am thrilled to lead Acceleron though its next stages of growth," said Dr. Knopf. "Acceleron is uniquely poised for continued success with the results of the ACE-011 phase 1 study being announced later today and efforts already underway to initiate phase 2 studies with ACE-011 later this year. We are also working diligently to bring ACE-031, our compound to increase muscle mass and strength, into the clinic in early 2008."

Prior to co-founding Acceleron, Dr. Knopf was formerly Site Head of the Wyeth Research facilities in Cambridge, MA and Vice President of Metabolic and Respiratory Disease until 2002. Dr. Knopf was a key scientist at Genetics Institute from 1982 to 1998, where he participated in the development of pioneering biopharmaceutical products and helped establish Genetics Institute as a premier biopharmaceutical company. He also established a structure-based small molecule discovery group at Genetics Institute. Dr. Knopf is the author of several key scientific manuscripts in the area of signal transduction and of a number of patents. He received his Ph.D. from SUNY Buffalo and his B.S. from SUNY Stonybrook.

About Acceleron Pharma

Acceleron is a privately held biopharmaceutical company committed to discover, develop, manufacture and commercialize novel biotherapeutics that modulate the growth of bone, muscle, fat and the vasculature to treat musculoskeletal, metabolic and cancer-related diseases. Acceleron's scientific approach takes advantage of its unique insight into the regenerative powers of two protein families: the Growth and Differentiation Factors (GDFs) and Bone Morphogenetic Proteins (BMPs). ACE-011, an anabolic bone agent, is the company's lead program, and is being developed to reverse bone loss in diseases such as cancer-related bone loss and osteoporosis. In addition, the company is advancing through preclinical development with product candidates that increase muscle mass, control angiogenesis and inhibit fat accumulation. Acceleron utilizes proven biotherapeutic technologies and capitalizes on the company's internal GMP manufacturing capability to rapidly and efficiently advance its therapeutic programs. The investors in Acceleron are Advanced Technology Ventures, Flagship Ventures, OrbiMed Advisors, Polaris Ventures, Sutter Hill Ventures and Venrock.

Contact: Steven Ertel, Vice President, Corporate Development, Acceleron Pharma, tel: 617-520-1334 and Paul Kidwell (Media) Suda Communications, tel: 617-296-3854.