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Exhibit 99.1

CERTIFICATION

        Pursuant to Section 906 of the Public Company Accounting Reform and Investor Protection Act of 2002 (18 U.S.C. § 1350, as adopted), John C. Martin, the Chief Executive Officer of Gilead Sciences, Inc. (the "Company"), and John F. Milligan, the Chief Financial Officer of the Company, each hereby certifies that, to the best of his knowledge:

1.    The Company's Annual Report on Form 10-K for the period ended December 31, 2002, to which this Certification is attached as Exhibit 99.1 (the "Annual Report"), fully complies with the requirements of Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934, as amended; and

2.    The information contained in the Annual Report fairly presents, in all material respects, the financial condition and results of operations of the Company for the periods covered by the Annual Report.

Dated: March 11, 2003

/s/  JOHN C. MARTIN John C. Martin
Chief Executive Officer /s/  JOHN F. MILLIGAN John F. Milligan
Chief Financial Officer

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Exhibit 99.1

CERTIFICATION