Idenix Pharmaceuticals Announces Decision by the United States Patent and Trademark Office Patent Trial and Appeal Board in the

Idenix Pharmaceuticals Announces Decision by the United States Patent and
Trademark Office Patent Trial and Appeal Board in the First Phase of the
Interference

CAMBRIDGE, Mass., March 22, 2013 (GLOBE NEWSWIRE) -- 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 the United States Patent and Trademark Office Patent Trial and
Appeal Board (USPTO) issued a decision in the first phase of the ongoing
interference concerning one of the Company's patent applications (U.S. Patent
Application 12/131,868) and an issued patent ( U.S. Patent 7,429,572) owned by
Gilead Pharmasset LLC ("Gilead") that covers certain 2'-methyl- 2'-fluoro
nucleoside compounds useful in the treatment of the hepatitis C virus.

An interference is an adversarial proceeding declared by the USPTO when a
party has a U.S. patent application that covers the same invention as another
patent application or issued patent to determine priority of invention in the
United States.An interference proceeding is divided into two stages. The
first phase determines the application filing dates each party will have
benefit of for the interfering subject matter.The party with the benefit of
the earliest application filing date is deemed the 'senior party' and the
party with the later date is deemed the 'junior party'.The second phase
determines who was first to invent.The party who is deemed first to invent
prevails in the interference proceeding.

Today the USPTO issued a decision whereby Idenix was determined to have a
later application filing date than Gilead. Therefore Idenix was determined to
be the 'junior party' and Gilead the 'senior party' in the interference.The
second phase of the interference is expected to commence in the second quarter
of 2013 and, as noted above, will determine which party was first to invent.
The decision that Idenix is the junior party does not determine which party
will be deemed first to invent and ultimately prevail in the interference.

Idenix does not believe that the patent application at issue in the
interference is relevant to any of the compounds currently under development,
including IDX719, its NS5A inhibitor which is expected to begin phase II
studies in the first half of 2013, or its uridine nucleoside forwhich the
Company expects to file an investigational new drug application in the first
half of 2013.

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.

CONFERENCE CALL AND WEBCAST INFORMATION

Idenix will hold a conference call on Monday, March 25, 2013 at 8:00 a.m. ET.
To access the call please dial (877) 640-9809 U.S./Canada or (914) 495-8528
International and enter passcode 28471980 or to listen to a live webcast go to
"Events & Presentations" in the Idenix Investor Center at www.idenix.com. A
replay of the call will also be available from 11:00 a.m. ET on March 25, 2013
until April 1, 2013, at 11:59 p.m. ET. To access the replay, please dial (855)
859-2056 U.S./Canada or (404) 537-3406 International and enter passcode
28471980. An archived webcast will also be available for one week after the
call on the Idenix website.

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
future business and financial 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," "will," "believe," and similar expressions are also intended to
identify forward-looking statements, as are expressed or implied statements
with respect to future actions taken by the Company, including any expressed
or implied statements regarding future litigation or adverse proceedings.
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: 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, 2012 as filed with the Securities and Exchange
Commission (SEC) and in any subsequent periodic or current report that the
Company files with the SEC.

All forward-looking statements reflect the Company's estimates only as of the
date of this release (unless another date is indicated) and should not be
relied upon as reflecting the Company's views, expectations or beliefs at any
date subsequent to the date of this release. While Idenix may elect to update
these forward-looking statements at some point in the future, it specifically
disclaims any obligation to do so, even if the Company's estimates change.

CONTACT: Idenix Pharmaceuticals Contacts:
         Kelly Barry (617) 995-9033 (media)
         Teri Dahlman (617) 995-9807 (investors)
 
Press spacebar to pause and continue. Press esc to stop.