Acceleron Announces Closing of Public Offering of Common Stock and Exercise of Underwriters' Option to Purchase Additional Shares

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 has issued an aggregate of 5,594,593 shares of common stock at a price of $92.50 per share at the closing of the Company's previously-announced public offering. The aggregate net proceeds to the Company of the public offering, including pursuant to the full exercise of the underwriters' option to purchase additional shares and after deducting underwriting discounts and commissions and estimated offering expenses, were approximately $492.5 million.

J.P. Morgan, SVB Leerink and Cowen acted as joint book-runners for the offering. Barclays, Credit Suisse, and Piper Sandler & Co. also acted as joint book-running managers. H.C. Wainwright & Co. acted as lead manager.

The shares were offered by Acceleron pursuant to an effective shelf registration statement on Form S-3 that was previously filed with the Securities and Exchange Commission (the "SEC"). A preliminary prospectus supplement relating to and describing the terms of the offering was filed with the SEC on June 30, 2020. The final prospectus supplement relating to the offering was filed with the SEC on July 2, 2020. Copies of the final prospectus supplement and the accompanying prospectus can be obtained at the SEC's website http://www.sec.gov or 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.

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, having recently reported positive topline results of the Phase 2 PULSAR trial.

Cautionary Note on Forward-Looking Statements

Certain statements contained in this press release are forward-looking statements that involve a number of risks and uncertainties that could cause actual results to differ materially from those in the forward-looking statements, including statements about Acceleron's strategy, future plans and prospects. 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. 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.

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