Achillion Pharmaceuticals, Inc. (Nasdaq:ACHN) has entered into a definitive agreement with funds managed by QVT Financial LP for the sale of 6,367,853 shares of its common stock.

The investors have agreed to purchase the shares of common stock at a price of $6.57 per share, resulting in gross proceeds to Achillion, before deducting estimated offering expenses, of approximately $41.8 million. The sale and issuance of the shares is expected to close on or about September 4, 2012, subject to customary closing conditions.

The securities described above are being offered directly to the purchasers by Achillion pursuant to a registration statement previously filed with and subsequently declared effective by the Securities and Exchange Commission (the "SEC"). This press release is not an offer to sell any of the securities described herein, and this press release is not an offer to buy these securities in any state where the offer or sale is not permitted. Copies of the prospectus supplement and accompanying base prospectus relating to this offering may be obtained by visiting EDGAR on the SEC's website at www.sec.gov.

About Achillion Pharmaceuticals
Achillion is an innovative pharmaceutical company dedicated to bringing important new treatments to patients with infectious disease. Achillion's proven discovery and development teams have advanced multiple product candidates with novel mechanisms of action. Achillion is focused on solutions for the most challenging problems in infectious disease including HCV and resistant bacterial infections. For more information on Achillion Pharmaceuticals, please visit www.achillion.com or call 1-203-624-7000.