Achillion Pharmaceuticals Announces Closing of Public Offering

Achillion Pharmaceuticals Announces Closing of Public Offering

Underwriters Exercise Option to Purchase Additional Shares

NEW HAVEN, Conn., Feb. 27, 2013 (GLOBE NEWSWIRE) -- Achillion Pharmaceuticals,
Inc. (Nasdaq:ACHN) announced today that the underwriters exercised their
option to purchase an additional 1,894,410 shares from Achillion in its
previously announced underwritten public offering of common stock at a price
to the public of $8.40 per share. The offering of 16,894,410 shares of common
stock, including the shares purchased upon exercise of the option to purchase
additional shares, 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 $133.5 million.
Citigroup and Leerink Swann LLC acted as joint book-running managers, and JMP
Securities LLC, Piper Jaffray & Co. and Wells Fargo Securities, LLC acted as
co-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).Copies of the prospectus supplement
and accompanying prospectus relating to this offering may be obtained from
Citigroup, c/o Broadridge Financial Solutions, 1155 Long Island Avenue,
Edgewood, NY 11717, via telephone at 1-800-831-9146 or email at
batprospectusdept@citi.com or from Leerink Swann LLC, Attention: Syndicate
Department, One Federal Street, 37th Floor, Boston, MA 02110, via telephone
number (800) 808-7525, Ext. 4814, or email at syndicate@leerink.com.

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, the
satisfaction of customary closing conditions associated with the public
offering, and the impact of general economic, industry or political conditions
in the United States or internationally. Risks and uncertainties that
Achillion faces are described in greater detail under the heading "Risk
Factors" in its 2012 Annual Report on Form 10-K and its subsequent SEC
filings, as well as 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: Company Contact:
         Glenn Schulman
         Achillion Pharmaceuticals, Inc.
         Tel. (203) 624-7000
         gschulman@achillion.com
        
         Investors:
         Mary Kay Fenton
         Achillion Pharmaceuticals, Inc.
         Tel. (203) 624-7000
         mfenton@achillion.com
        
         Media:
         Christin Culotta Miller
         Ogilvy PR
         Tel. (212) 880-5264
         christin.miller@ogilvy.com
        
         Investors:
         Seth Lewis
         The Trout Group, LLC
         Tel. (646) 378-2952
         slewis@troutgroup.com