Isis Acquires Antisense Patent and Technology Estate from Gilead Sciences

Carlsbad, CA and Foster City, CA -- December 18, 1998

Isis Pharmaceuticals (Nasdaq: ISIP) announced today that it has purchased from Gilead Sciences, Inc. (Nasdaq: GILD) the holdings of its antisense patent estate. This acquisition includes patents and patent applications covering a broad proprietary suite of antisense chemistry and antisense drug delivery systems.

Under the agreement, Isis will purchase from Gilead the patent rights to all of its patents involving antisense chemistry and applications for $6 million, payable in four installments over the next three years. This acquisition augments Isisí substantial patent estate. Isis currently has over 100 issued patents and over 30 U.S. patent applications allowed.

"This acquisition is consistent with Isis' strategy to build on its leadership position in antisense technology," said B. Lynne Parshall, Executive Vice President and Chief Financial Officer of Isis. ìIsis is committed to consolidating and supplementing our dominant proprietary patent position in antisense with the hope that this technology will fulfill its promise as a revolutionary drug discovery tool."

Founded in 1987, Gilead Sciences was one of the pioneers in the field of antisense, also known as genetic code blockers. In 1990, Gilead entered into a collaboration with Glaxo Wellcome, then Glaxo Inc., for the research and development of genetic code blockers. This collaboration resulted in a number of important scientific and proprietary discoveries that have advanced the field of antisense, including identification of compounds with increased cell permeation and potency. In June 1998, the collaboration was terminated with each company retaining rights to the technology jointly developed as part of the collaboration and to scientific advancements each company made on its own. Gilead's rights in the technology that were part of the Gilead-Glaxo collaboration are included in the patent estate being purchased by Isis.

"We are pleased that Gilead can realize the value of our code blocker research program and that Isis will be able to utilize and expand upon our antisense discoveries," said Norbert Bischofberger, Ph.D., Senior Vice President, Research of Gilead Sciences.

Gilead Sciences is an independent biopharmaceutical company that seeks to provide accelerated solutions for patients and the people who care for them. The Company discovers, develops and commercializes proprietary therapeutics for important viral diseases, including a currently marketed product for the treatment of CMV retinitis, a sight-threatening viral infection in patients with AIDS. In addition, the Company is developing products to treat diseases caused by HIV, hepatitis B virus and influenza virus. Gilead common stock is traded on The Nasdaq Stock Market under the symbol GILD.

Isis Pharmaceuticals, based in northern San Diego County, is engaged in the discovery and development of novel human therapeutic compounds. Isis has six compounds in human clinical trials: Vitravene (fomivirsen), to treat CMV-induced retinitis in AIDS patients, has been approved for marketing by the FDA and is currently undergoing regulatory review in Europe; ISIS 2302, an inhibitor of ICAM-1, is in a pivotal quality trial for Crohn's disease, Phase II clinical trials for renal transplant rejection, rheumatoid arthritis and ulcerative colitis, and is being explored as a topical administration for psoriasis and an aerosol administration for asthma; ISIS 3521 is in Phase II trials as a treatment for cancer; ISIS 5132 is in Phase II clinical trials as a treatment for cancer; ISIS 2503 is in Phase I trials as a treatment for cancer; and ISIS 13312 is in Phase I/II clinical trials for the treatment of CMV retinitis in AIDS patients. The company also has several additional compounds in preclinical development. Isisí broad medicinal chemistry and biology research programs support efforts in both antisense and small molecule drug discovery.

This press release contains forward-looking statements concerning the agreement between Isis and Gilead. Such statements are subject to certain risks and uncertainties, particularly those inherent in the process of discovering, developing and commercializing drugs that are safe and effective for use as human therapeutics. Actual results could differ materially from those projected in this release. As a result, the reader is cautioned not to rely on these forward-looking statements. These and other risks concerning proprietary positions are described in additional detail in Isis' Annual Report on Form 10-K for the year ended December 31, 1997 which is on file with the U.S. Securities Exchange Commission, copies of which are available from the company.

Vitravene is a trademark of Novartis AG.

CONTACT:Jane Green, Senior DirectorInvestor RelationsIsis Pharmaceuticals760-603-3880www.isip.com

Mark PerrySenior Vice President,Chief Financial Officer and General CounselGilead Sciences, Inc.650-573-4772