Idenix Pharmaceuticals Announces Proposed Public Offering of Common Stock

Idenix Pharmaceuticals, Inc. (Nasdaq:IDIX), a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral diseases, today announced that it has commenced an underwritten registered public offering of $150 million of its common stock. All of these shares are being sold by Idenix.

J.P. Morgan Securities LLC is acting as sole book-running manager and Leerink Swann LLC is acting as lead manager for the proposed offering. Idenix intends to grant the underwriters a 30-day option to purchase up to an additional 15 percent of the amount sold to cover over-allotments, if any. The offering is subject to market conditions, and there can be no assurance as to whether or when the offering may be completed, or as to the final size or terms of the offering.

Idenix intends to use the net proceeds from this offering for general corporate purposes, which may include: research and development expenditures relating to combination phase IIb clinical trials of IDX184 with IDX719, proof-of-concept and phase IIa clinical trials with IDX19368 in combination with ribavirin and proof-of-concept clinical trials of one or more nucleoside/nucleotide drug candidates; working capital; capital expenditures; and potential acquisitions of new businesses, technologies or products that Idenix believes complements or expands its business.

The shares will be issued pursuant to an effective shelf registration statement on Form S-3 filed today with the Securities and Exchange Commission. Idenix will file with the Securities and Exchange Commission a preliminary prospectus supplement with respect to the offering.

This announcement shall not constitute an offer to sell or the solicitation of an offer to buy any securities of Idenix, nor shall there be any offer or sale of securities in any state or jurisdiction in which an offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction. Copies of the preliminary prospectus supplement and accompanying prospectus may be obtained from J.P. Morgan Securities LLC via Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY 11717 or by calling toll-free [phone].

About Idenix

Idenix Pharmaceuticals, Inc., headquartered in Cambridge, Massachusetts, is a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral diseases. Idenix's current focus is on the treatment of infections caused by the hepatitis C virus.

CONTACT: Idenix Pharmaceuticals Contacts:
         Kelly Barry [phone] (media)
         Daniella Beckman [phone] (investors)