Gilead Begins Phase I Study of GS7904L, an Investigational Anticancer Drug

Foster City, CA -- November 28, 2001

Gilead Sciences, Inc. (Nasdaq: GILD) today announced that it has initiated the first Phase I clinical trial of GS7904L, an investigational liposomal thymidylate synthase inhibitor for the potential treatment of cancer, in Germany. Additional Phase I trials of GS7904L are planned to begin in other countries in the coming months.

Study Design
This Phase I trial is designed to evaluate the safety, tolerability and toxicity of GS7904L in up to 40 patients at two sites. The study will enroll patients with a variety of solid tumors refractory to standard therapy or for whom no curative therapy exists. In this first Phase I trial, the dose of GS7904L will be escalated in successive cohorts of patients to determine the maximum tolerated and recommended Phase II doses.

OSI Pharmaceuticals to Acquire Gilead Oncology Assets
On Monday, November 26, 2001, Gilead and OSI Pharmaceuticals (Nasdaq: OSIP) announced the signing of an agreement for OSI to acquire Gilead's pipeline of clinical candidates in oncology and Gilead's Boulder, Colorado operations, including clinical research and drug development personnel and infrastructure. In consideration of these assets, OSI will pay to Gilead $130 million in cash and $40 million in shares of OSI common stock upon the closing of the transaction. OSI will also pay to Gilead up to an additional $30 million in either cash or a combination of cash and OSI common stock upon the achievement of certain milestones. The transaction is expected to close by year-end.

Under the terms of the transaction, OSI will receive exclusive worldwide development and commercialization rights to Gilead's three investigational oncology agents, including GS7904L. The two additional product candidates are NX211 (liposomal lurtotecan), a proprietary liposomal formulation of the active topoisomerase I inhibitor lurtotecan, currently in Phase II clinical trials for the potential treatment of a variety of solid tumors; and GS7836, a novel nucleoside analogue, which is in Phase I clinical trials and has demonstrated activity in a variety of refractory solid tumor xenograft models.

"Gilead's oncology group has advanced a second anticancer compound from preclinical testing to Phase I trials in just under one year," said John C. Martin, Ph.D., President and Chief Executive Officer, Gilead Sciences. "This continued momentum highlights the oncology team's effectiveness in moving compounds along and further underscores the experience and ability that they will bring to OSI."

GS7904L
Gilead licensed the compound GW1843 from GlaxoWellcome and Glaxo Group Limited (now GlaxoSmithKline) in December 2000 and reformulated the product into a liposome (now re-named GS7904L) with the goal of enhancing its antitumor activity. Both GW1843 and its liposomal formulation GS7904L have demonstrated activity in human tumor cells in vitro and antitumor activity in human tumor xenografts in mice. In the xenograft studies, the liposomal formulation showed superior efficacy. Under the terms of the agreement with GlaxoWellcome, Gilead acquired exclusive, worldwide development and commercialization rights to the compound.

GS7904L is a thymidylate synthase inhibitor and belongs to the same class of agents as the anticancer drug 5-fluorouracil (5-FU). 5-FU is one of the most prescribed chemotherapeutic drugs in the world and is used for the treatment of patients with colon, breast, head and neck, stomach and pancreatic cancers. GS7904L has demonstrated potent activity in a variety of preclinical cancer models including colorectal, ileocecal and ovarian cancers.

Gilead Sciences
Gilead Sciences, Inc., headquartered in Foster City, CA, is an independent biopharmaceutical company that seeks to provide accelerated solutions for patients and the people who care for them. Gilead discovers, develops, manufactures and commercializes proprietary therapeutics for challenging infectious diseases (viral, fungal and bacterial infections). Gilead maintains research, development, manufacturing or sales and marketing facilities in the United States, Europe and Australia.

This press release includes forward-looking statements, within the meaning of the Private Securities Litigation Reform Act of 1995, that are subject to risks, uncertainties and other factors that could cause actual results to differ materially from those referred to in the forward-looking statements, particularly the risks associated with safety, efficacy and development timelines for an investigational compound, and the risk that the Phase I studies of GS7904L will not demonstrate a dose that is suitable for Phase II clinical trials. Actual results could differ materially from those projected in this release. In addition, these risks include the risk to Gilead that the conditions to closing the sale of the oncology assets will not be satisfied. The reader is cautioned not to rely on these forward-looking statements. Certain of these risks are described in detail in the Gilead Annual Report on Form 10-K for the year ended December 31, 2000 and in Gilead's Quarterly Reports on Form 10-Q, all of which are on file with the U.S. Securities and Exchange Commission.