EXHIBIT 99.1


                OSI PHARMACEUTICALS AND GILEAD SCIENCES ANNOUNCE
                         CLOSING OF ONCOLOGY ASSETS DEAL


         MELVILLE, N.Y. & FOSTER CITY, Calif.-- (BUSINESS WIRE) -- Dec. 21, 2001
- -- OSI Pharmaceuticals (Nasdaq: OSIP) and Gilead Sciences (Nasdaq: GILD)
announced today that the two companies have completed the sale of Gilead's
oncology assets to OSI.

         Through this transaction, OSI has acquired Gilead's pipeline of
clinical stage oncology products and related intellectual property, as well as
Gilead's Boulder, Colorado operations, including clinical research and drug
development personnel, infrastructure and facilities.

         In consideration for the assets, Gilead received from OSI $130 million
in cash and 924,984 shares of OSI common stock. Additionally, OSI will 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 related to the
development of NX211, the most advanced of Gilead's oncology product candidates.

         Through the acquisition, OSI accelerates its development and
commercialization capabilities by augmenting and balancing its pipeline of
gene-targeted small molecule therapeutics with three next generation cytotoxic
drug candidates in clinical trials. For Gilead, the transaction allows the
company to increase its focus on its core expertise in infectious diseases.

         "The addition of this outstanding oncology development team and strong
clinical programs takes us one step further toward our goal of becoming leaders
in oncology. These assets complement our existing strengths, help balance our
pipeline and allow us to maintain our commitment to across-the-board excellence
in our discovery and development capabilities," said Colin Goddard, Ph.D.,
Chairman and Chief Executive Officer of OSI Pharmaceuticals.

         "This transaction delivers clear benefits for both companies. At
Gilead, we will continue to strengthen our core area of expertise in infectious
diseases, while OSI will solidify its competitive business in oncology," said
John C. Martin, Ph.D., President and Chief Executive Officer of Gilead Sciences.

GILEAD SCIENCES

         Gilead Sciences, Inc., headquartered in Foster City, CA, USA, 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.

OSI PHARMACEUTICALS

         OSI Pharmaceuticals is a leading biotechnology company primarily
focused on the discovery, development and commercialization of innovative
products for the treatment of cancer. OSI has built a pipeline of discovery
programs and drug candidates addressing major, unmet medical needs in cancer and
selected opportunities including diabetes, arising from the Company's extensive
drug discovery research programs that represent significant commercial
opportunities. OSI's most advanced drug candidate, Tarceva(TM), a small molecule
inhibitor of the EGFR gene, is currently in Phase III clinical trials for lung
and pancreatic cancers.

         This press release contains 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. The risk specific to Gilead is that the product development
milestones related to NX211 will not occur and that Gilead will therefore not be
entitled to future milestone payments. The reader is cautioned not to rely on
these forward-looking statements. Product development 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. All forward-looking statements
are based on information currently available to Gilead and Gilead assumes no
obligation to update any such forward-looking statements. There are also risks
specific to OSI, which include, among others, preclinical development,
completion of clinical trials, the FDA review process and other governmental
regulation, pharmaceutical collaborators' ability to successfully develop and
commercialize drug candidates, competition from other pharmaceutical companies,
product pricing and third party reimbursement, and other factors described in
OSI Pharmaceuticals' filings with the Securities and Exchange Commission. All
forward-looking statements are based on information currently available to OSI
and OSI assumes no obligation to update any such forward-looking statements.

TARCEVA IS A TRADEMARK OF OSI PHARMACEUTICALS.






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