Idenix Pharmaceuticals Appoints Jacques Dumas, Ph.D., as Chief Scientific Officer

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 appointed Jacques Dumas, Ph.D., as its Executive Vice President and Chief Scientific Officer. Dr. Dumas is trained as a medicinal chemist and brings over 20 years of experience in pharmaceutical discovery and development, supporting a variety of programs in infectious diseases and oncology. He is a co-inventor of two marketed drugs in the field of oncology, Nexavar® and Stivarga®. His appointment will be effective January 20, 2014.

"Dr. Dumas brings a good complement of deep chemistry expertise and overall business strategy to the Idenix team," said Ron Renaud, President and Chief Executive Officer of Idenix. "His knowledge and leadership will be important as we continue to conduct our nucleotide prodrug discovery work in HCV as well as explore potential therapeutic areas for which we can apply our chemistry expertise. Additionally, as a native of France and a resident of the Boston area, Jacques is an ideal fit to direct our talented research teams in both Cambridge and Montpellier."

Dr. Dumas served as Vice President and Head of Strategy, Infection Innovative Medicines at AstraZeneca plc since December 2010. In this position, he was responsible for disease area strategy and external collaborations on behalf of AstraZeneca's small-molecule infectious diseases unit. Prior to that role and between February 2007 and December 2010, Dr. Dumas held the successive positions of Associate Director, Medicinal Chemistry and Emerging Products Director at AstraZeneca. Prior to joining AstraZeneca, Dr. Dumas held several positions at Bayer HealthCare Pharmaceuticals, Inc., a subsidiary of Bayer AG, from 1995 to 2007. Dr. Dumas received his Ph.D. in Organic Chemistry from Paris 6 University in 1991.

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 patients with hepatitis C virus (HCV) infection. For further information about Idenix, please refer to www.idenix.com.

Nexavar® and Stivarga® are registered trademarks of Bayer HealthCare Pharmaceuticals, Inc. and Bayer, respectively.

CONTACT: Idenix Pharmaceuticals Contact:
         Teri Dahlman, [phone]