ABOUT IDX19368IDX19368 is an unpartnered liver-targeted nucleotide prodrug of 2'-methyl guanosine and was selected from a prodrug series of approximately 350 compounds. It has shown high triphosphate levels in vivo, with a low potential for drug-drug interactions. In the third quarter of 2012, Idenix submitted an investigational new drug (IND) application for IDX19368 with plans to initiate clinical trials in the third quarter of 2012. However, these clinical trials will not commence until the FDA removes the clinical hold. ABOUT IDX184 IDX184 is an unpartnered, novel, liver-targeted nucleotide prodrug of 2'-methyl guanosine, which includes Idenix's proprietary liver-targeting technology. This technology enables the delivery of nucleoside monophosphate to the liver, leading to the formation of high levels of nucleoside triphosphate, potentially maximizing drug efficacy and limiting systemic side effects with low, once-daily dosing. In July 2012, an independent data safety monitoring board reviewed the safety data for this study and confirmed that the side effect profile of IDX184 combined with PegIFN/RBV is consistent with that of PegIFN/RBV alone. The FDA placed IDX184 on partial clinical hold as reported on August 16, 2012. In previous clinical trials as well as the ongoing phase IIb clinical trial of IDX184 in combination with pegylated interferon and ribavirin (PegIFN/RBV), there has been no evidence to date of cardiotoxicity in patients dosed with IDX184 with PegIFN/RBV beyond that seen with PegIFN/RBV alone. There are currently no patients receiving IDX184 worldwide. 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 infection. For further information about Idenix, please refer to www.idenix.com . FORWARD-LOOKING STATEMENTS This press release contains "forward-looking statements" for purposes of the safe harbor provisions of The Private Securities Litigation Reform Act of 1995, including but not limited to the statements regarding the Company's development of drug candidates and its future business performance. For this purpose, any statements contained herein that are not statements of historical fact may be deemed forward-looking statements. Without limiting the foregoing, the words "expect," "plans," "anticipates," "intends," "will," and similar expressions are also intended to identify forward-looking statements, as are expressed or implied statements with respect to the Company's potential pipeline candidates, including any expressed or implied statements regarding the Company's ability to continue clinical development of IDX184 and IDX19368, the efficacy and safety of IDX184 and IDX19368; the likelihood and success of any future clinical trial involving IDX184 and IDX19368 and the successful development of these compounds. Actual results may differ materially from those indicated by such forward-looking statements as a result of risks and uncertainties, including but not limited to the following: there can be no guarantees that the Company will advance any clinical product candidate or other component of its potential pipeline to the clinic, to the regulatory process or to commercialization; management's expectations could be affected by unexpected regulatory actions or delays; uncertainties relating to, or unsuccessful results of, clinical trials, including additional data relating to the ongoing clinical trials evaluating its product candidates and clinical trials of compounds under development by other companies; changes in the Company's business plan or objectives; competition in general; and the Company's ability to obtain, maintain and enforce patent and other intellectual property protection for its product candidates and its discoveries. Such forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause actual results to be materially different from any future results, performance or achievements expressed or implied by such statements. These and other risks which may impact management's expectations are described in greater detail under the heading "Risk Factors" in the Company's annual report on Form 10-K for the year ended December 31, 2011 and the quarterly report on Form 10-Q for the quarter ended June 30, 2012, each as filed with the Securities and Exchange Commission (SEC) and in any subsequent periodic or current report that the Company files with the SEC.