Acceleron Announces Pricing of Public Offering of Common Stock

Acceleron Pharma Inc. (NASDAQ:XLRN), a clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel therapeutic candidates that regulate cellular growth and repair, today announced that it has priced an underwritten public offering of 3,750,000 shares of common stock at a price to the public of $40.00 per share for gross proceeds of $150 million. In connection with this offering, Acceleron has granted the underwriters a 30-day option to purchase up to an additional 562,500 shares of common stock.

Morgan Stanley, Leerink Partners LLC and UBS Investment Bank are acting as joint book-running managers for the offering. JMP Securities is acting as the lead manager and FBR is acting as co-manager for the offering. The offering is expected to close on or about January 11, 2016, subject to the satisfaction of customary closing conditions.

Acceleron expects to receive net proceeds from the offering, after deducting underwriting discounts and commissions and other offering expenses payable by the company, of approximately $140.4 million. Acceleron intends to use the net proceeds to conduct clinical trials and associated activities in connection with ACE-083, Acceleron’s clinical stage therapeutic candidate designed to promote muscle growth and function in specific, targeted muscles, and Acceleron’s other potential therapeutic candidates, including antibodies and proteins from Acceleron’s IntelliTrapTM platform, to further expand Acceleron’s research and development efforts, and for general and administrative expenses, potential future development programs, capital expenditures and working capital and other general corporate purposes.

The shares are being 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 January 4, 2016. The final prospectus supplement relating to the offering will be filed with the SEC. When available, copies of the final prospectus supplement and the accompanying prospectus can be obtained at the SEC’s website http://www.sec.gov or from Morgan Stanley, Attention: Prospectus Department, 180 Varick Street, 2nd Floor, New York, NY 10014; Leerink Partners LLC, Attention: Syndicate Department, One Federal Street, 37th Floor, Boston, MA, 02110, or by e-mail at Syndicate@Leerink.com, or by phone at 800-808-7525, ext. 6142; or UBS Securities LLC, 1285 Avenue of the Americas, New York, New York 10019, Attention: Prospectus Department, or by phone at 1-888-827-7275.

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 clinical stage biopharmaceutical company focused on the discovery, development and commercialization of novel therapeutic candidates that regulate cellular growth and repair. The company is a leader in understanding the biology of the Transforming Growth Factor-Beta (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 therapeutic candidates with novel mechanisms of action. These therapeutic candidates have the potential to significantly improve clinical outcomes for patients with cancer and rare diseases.

Cautionary Note on Forward-Looking Statements

Certain statements contained in this release, including those relating to the use of proceeds from the sale of common stock and the expected closing of the offering, 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. These risks and uncertainties include, but are not limited to: our ability to successfully complete the offering; the volatility of our stock price may prevent shares purchased in the offering from being resold at or above the price paid for them; purchasers of shares in the offering will suffer immediate and substantial dilution; we may use the proceeds from the offering in ways that may not enhance our operating results or the market price of our common stock; and other risk factors identified from time to time in the reports we file with the SEC, including our Annual Report on Form 10-K, Quarterly Reports on Form 10-Q, and Current Reports on Form 8-K, which are available at www.sec.gov. 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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