Achillion Pharmaceuticals, Inc. Announces Closing of Public Offering and Exercise of Overallotment Option

NEW HAVEN, Conn., June 27, 2011 (GLOBE NEWSWIRE) -- Achillion Pharmaceuticals, Inc. (Nasdaq:ACHN - News) announced today that the underwriters exercised their option to purchase an additional 1,440,000 shares from Achillion in its previously announced underwritten public offering of common stock at a price to the public of $5.90 per share. The offering of 11,040,000 shares of common stock, including the shares purchased upon exercise of the overallotment option, closed today. The net proceeds to Achillion from the sale of the shares of common stock, after deducting underwriting discounts and commissions and estimated offering expenses, are approximately $60.9 million. BofA Merrill Lynch and Leerink Swann LLC acted as joint book-running managers for the offering.

The shares were sold pursuant to Achillion’s existing shelf registration statement, base prospectus, and a prospectus supplement as filed with the Securities and Exchange Commission (SEC) and available on the SEC’s website at http://www.sec.gov. Copies of the prospectus supplement and the base prospectus relating to this offering may be obtained from BofA Merrill Lynch, 4 World Financial Center, New York, NY 10080, Attn: Prospectus Department or email dg.prospectus_requests@baml.com or from Leerink Swann LLC, Attention: Syndicate Department, One Federal Street, 37th Floor, Boston, MA 02110, telephone number (800) 808-7525, Ext. 4814.

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

About Achillion Pharmaceuticals, Inc.

Achillion is an innovative pharmaceutical company dedicated to bringing important new treatments to patients with infectious disease.

Forward-Looking Statements

This press release includes “forward-looking statements” made under the provisions of the Private Securities Litigation Reform Act of 1995. Forward-looking statements include statements, other than statements of historical fact. In some cases, words such as “will,” “expect” or other comparable words identify forward-looking statements. Actual results, performance or experience may differ materially from those expressed or implied by any forward-looking statement as a result of various important factors, including without limitation risks and uncertainties relating to Achillion’s business and financial condition, general market conditions and the satisfaction of customary closing conditions associated with the proposed public offering. Risks and uncertainties that Achillion faces are described in greater detail under the heading “Risk Factors” in its Form 8-K filed on June 20, 2011 and in other filings that it makes with the SEC. As a result of the risks and uncertainties, the results or events indicated by any forward-looking statement may not occur. Achillion cautions you not to place undue reliance on any forward-looking statement.

In addition, any forward-looking statement in this press release represents Achillion’s views only as of the date of this press release and should not be relied upon as representing its views as of any subsequent date. Achillion disclaims any obligation to update any forward-looking statement, except as required by applicable law.

Contact:

Corporate Contact:Glenn SchulmanAchillion Pharmaceuticals, Inc.Tel. (203) 752-5510gschulman@achillion.com Investor

Contact:Mary Kay FentonAchillion Pharmaceuticals, Inc. Tel. (203) 624-7000mfenton@achillion.com Media

Contact:Christin Culotta MillerOgilvy PRTel. (646) 229-5178christin.miller@ogilvypr.com

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