South San Francisco, California
August 5, 2003

Exelixis, Inc. (Nasdaq: EXEL) announces the extension of its mechanism of action research collaboration with Dow AgroSciences LLC. The one-year extension will enable the two companies to build on their substantial accomplishments in elucidating the mechanism of action of important herbicidal compounds based on the identification of their gene targets and development of specific target screening assays.

These novel insights are designed to enable Dow AgroSciences to accelerate the development of new products with enhanced selectivity and potency and greater effectiveness as crop protection agents. Financial terms of the collaboration are not being disclosed. "We are very pleased with the progress in identifying new molecular target sites based on our chemistry through the collaborative effort with Exelixis," added Bill Kleschick, Ph.D., global leader of discovery research, Dow AgroSciences. "Extending the collaboration will enable Dow AgroSciences to realize additional value."

"Our partnership with Dow AgroSciences has been extremely productive and collaborative, resulting in significant advances in understanding the mechanism of action of active herbicidal compounds and progressing toward the discovery and development of environmentally safe and commercially valuable crop protection products," said D. Ry Wagner, Ph.D., vice president of research, Exelixis Plant Sciences, a subsidiary of Exelixis, Inc.

"We believe that by working together with Dow AgroSciences, one the leading providers of agricultural solutions, these insights into herbicide action can be translated into viable product opportunities and make significant contributions to improving crop quality and crop yield worldwide."

In July 2000, Exelixis and Dow AgroSciences established a mechanism of action collaboration in which Exelixis identifies targets for specific fungicide and herbicide compounds that are active, yet have unknown molecular targets. Leveraging its proprietary discovery platform and expertise in
comparative genetics and functional genomics, Exelixis Plant Sciences identifies targets for proprietary Dow AgroSciences herbicidal compounds using model plant species. Exelixis receives milestones and royalties for products developed from this collaboration.

Exelixis, Inc. is a leading genomics-based drug discovery company dedicated to the discovery and development of novel therapeutics. The company is leveraging its fully integrated gene-to-drug platform to fuel the growth of its proprietary drug pipeline. Exelixis has established broad corporate alliances with major pharmaceutical and biotechnology companies, including SmithKlineBeecham Corporation and Bristol-Myers Squibb Company. The company has also established agricultural research collaborations with Bayer CropScience Dow AgroSciences and Renessen LLC. Other partners include
Merck & Co., Inc., Schering-Plough Research Institute, Inc., Cytokinetics, Inc., Elan Pharmaceuticals, Inc. and Scios Inc. For more information, please visit the company's web site at www.exelixis.com.

Exelixis and the Exelixis logo are registered U.S. trademarks.