Acceleron Announces Proposed Public Offering of Common Stock

Acceleron Pharma Inc. XLRN, a leading biopharmaceutical company in the discovery, development and commercialization of TGF-beta superfamily therapeutics to treat serious and rare diseases, today announced that it intends to offer and sell, subject to market and other conditions, $400 million of its common stock in an underwritten public offering. As part of this offering, Acceleron intends to grant the underwriters a 30-day option to purchase up to an additional fifteen percent (15%) of the shares of common stock offered in the public offering. All of the shares in the proposed offering are to be sold by Acceleron.

J.P. Morgan, SVB Leerink and Cowen are acting as joint book-running managers for the offering. Barclays, Credit Suisse, and Piper Sandler & Co. are also acting as joint book-running managers.

Acceleron intends to use the net proceeds from the offering to conduct clinical trials and associated activities in connection with its therapeutic candidates in its pulmonary programs; to prepare for the potential launch and commercialization of sotatercept; and the remainder for general corporate purposes, including potential future development programs, capital expenditures and working capital, as well as potential acquisitions of rights to additional programs from third parties.

The securities described above are being offered by Acceleron pursuant to its registration statement on Form S-3 filed with the Securities and Exchange Commission (the "SEC") on September 19, 2017, which became effective automatically when it was filed. The offering will be made only by means of a preliminary prospectus supplement and accompanying prospectus, copies of which may be obtained, when available, from J.P. Morgan Securities LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY 11717, by telephone at (866) 803-9204; or by email at Prospectus-eq_fi@jpmchase.com; SVB Leerink LLC, by mail at One Federal Street, 37th Floor, Boston, MA 02110, Attention: Syndicate Department, by telephone at (800) 808-7525, ext. 6218, or by email at syndicate@svbleerink.com; and Cowen and Company, LLC c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY, 11717, Attn: Prospectus Department, by email at PostSaleManualRequests@broadridge.com or by telephone at (833) 297-2926.

The final terms of the offering will be disclosed in a final prospectus supplement to be filed with the SEC.

This press release does not constitute an offer to sell or a solicitation of an offer to buy the securities in the offering, nor shall there be any sale of these securities in any jurisdiction in which an offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of such jurisdiction.

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.

Cautionary Note on Forward-Looking Statements

This press release contains forward-looking statements about Acceleron's strategy, future plans and prospects, including statements regarding the use of proceeds from the sale of common stock and the development and commercialization of Acceleron's compounds, the timeline for clinical development and regulatory approval of Acceleron's compounds, and the potential of Reblozyl^® (luspatercept-aamt) as a therapeutic drug. The words "anticipate," "believe," "could," "estimate," "expect," "goal," "intend," "may," "plan," "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, risks and uncertainties associated with the consummation of the proposed offering, changes in general economic or regulatory conditions, and the clinical development and commercialization of Acceleron's compounds. 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. We caution investors not to place considerable reliance on the forward-looking statements contained in this press release. The forward-looking statements in this press release speak only as of the date of this document, and we undertake no obligation to update or revise any of the statements. Our business is subject to substantial risks and uncertainties, including those referenced above. Investors, potential investors, and others should give careful consideration to these risks and uncertainties.

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