Acceleron Pharma Inc. (NASDAQ:XLRN), a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases, today announced that it has commenced an underwritten public offering of $100 million of its common stock. All of the shares in the offering are to be sold by Acceleron. Citigroup and Leerink Partners LLC are acting as joint book-running managers for the offering. Piper Jaffray & Co. is acting as lead manager and JMP Securities is acting as co-manager for the offering. In connection with this offering, Acceleron intends to grant the underwriters an option to purchase up to an additional fifteen percent of the number of shares sold. The offering is subject to market and other conditions, and there can be no assurances as to whether or when the offering may be completed, or as to the actual size or terms of the offering. A registration statement on form S-1 relating to the public offering of the shares of common stock described above has been filed with the SEC, but has not yet become effective. These securities may not be sold, nor may offers to buy be accepted, prior to the time the registration statement becomes effective. This press release shall not constitute an offer to sell or a solicitation of an offer to buy, nor shall there be any sale of these securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful, prior to registration or qualification under the securities laws of any such state or jurisdiction. The offering will be made only by means of a prospectus. Copies of the preliminary prospectus relating to the offering may be obtained from Citigroup, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY, 11717, or by email at firstname.lastname@example.org, or by phone at 800-831-9146; or Leerink Partners LLC, Attention: Syndicate Department, One Federal Street, 37th Floor, Boston, MA, 02110, or by email at Syndicate@Leerink.com, or by phone at 800-808-7525. About Acceleron Acceleron is a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel protein therapeutics for cancer and rare diseases. The company is a leader in understanding the biology of the TGF-β protein superfamily, a large and diverse group of molecules that are key regulators in the growth and repair of tissues throughout the human body, and in targeting these pathways to develop important new medicines. Acceleron has built a highly productive R&D platform that has generated innovative clinical and preclinical protein therapeutic candidates with novel mechanisms of action. These protein therapeutic candidates have the potential to significantly improve clinical outcomes for patients with cancer and rare diseases.