Contacts:

Sharon Tetlow diaDexus, Inc. (650) 246-6400

Carolyn Bumgardner Wang WeissComm Partners (415) 362-5018 x123 carolyn@weisscommpartners.com

MEDAREX AND DIADEXUS EXPAND COLLABORATION FOR DEVELOPMENT OF FULLY HUMAN ANTIBODY THERAPEUTICS FOR THE TREATMENT OF CANCER

diaDexus Also Exclusively Licenses Novel Ovarian Cancer Target to Medarex for Antibody Uses

Princeton, NJ and South San Francisco, CA - September 16, 2004 - Medarex, Inc. (Nasdaq: MEDX) and diaDexus, Inc. today announced an expansion of their existing collaboration for the research and development of fully human antibody therapeutics against novel cancer targets provided by diaDexus.

Under the terms of this expansion, diaDexus and Medarex expect to develop antibodies against three novel cancer targets outside of the lung cancer field, which is the focus of the original collaboration between Medarex and diaDexus announced in January 2003.

The two companies expect to share the cost of development and commercialization as well as potential profits resulting from any products developed through this collaboration.

Separately, diaDexus also granted Medarex an exclusive right to develop and commercialize therapeutic antibodies to a novel target for use in ovarian cancer for which diaDexus expects to receive an upfront payment and potential milestone payments and royalties on future sales of products developed and commercialized by Medarex.

Medarex plans to use its UltiMAb Human Antibody Development System® to generate antibodies to the targets provided by diaDexus.

"We are pleased to extend this collaboration with Medarex, a leading player in the field of antibody therapeutics," said Patrick Plewman, President and Chief Executive Officer of diaDexus. "We believe this collaboration enables us to cost-effectively advance a subset of our promising therapeutic targets into the clinic and complements our development of high-value, early-detection diagnostic tests."

"We are impressed with diaDexus' ability to identify potential important new targets as well as their ability to move products into development," said Donald L. Drakeman, Ph.D., President and Chief Executive Officer of Medarex. "We look forward to continuing our collaboration with diaDexus and moving these potential products forward."

About diaDexus
diaDexus, Inc., a privately held biotechnology company, based in South San Francisco, California, is focused on the discovery, development and commercialization of novel, patent-protected diagnostic and therapeutic products with high clinical value. The PLAC® test (Lp-PLA2), developed by diaDexus, Inc., is a blood test cleared by the U.S. Food and Drug Administration to aid in the prediction of an individual's risk for a coronary event, in conjunction with clinical evaluation and patient risk assessment. The PLAC test is currently available through laboratories nationwide. For more information about the PLAC test visit www.plactest.com. More information about the company may be found at www.diaDexus.com.

About Medarex
Medarex is a biopharmaceutical company focused on the discovery and development of fully human antibody-based therapeutics to treat life-threatening and debilitating diseases, including cancer, inflammation, autoimmune and infectious diseases. Medarex applies its UltiMAb™ technology and product development and clinical manufacturing experience to generate support and potentially commercialize a broad range of fully human antibody products for itself and its partners. Nineteen of these therapeutic products derived from Medarex technology are currently in human clinical testing, with the most advanced presently in a Phase III clinical trial. Medarex is committed to building value by developing a diverse pipeline of antibody products to address the world's unmet healthcare needs. For information about Medarex, visit its website at www.medarex.com.

For Medarex: Except for the historical information presented herein, matters discussed herein may constitute forward-looking statements that are subject to certain risks and uncertainties that could cause actual results to differ materially from any future results, performance or achievements expressed or implied by such statements. Statements that are not historical facts, including statements preceded by, followed by, or that include the words "potential"; "believe"; "anticipate"; "intend"; "plan"; "expect"; "estimate"; "could"; "may"; or similar statements are forward-looking statements. Medarex disclaims, however, any intent or obligation to update these forward-looking statements. Risks and uncertainties include risks associated with product discovery and development, uncertainties related to the outcome of clinical trials, slower than expected rates of patient recruitment, unforeseen safety issues resulting from the administration of antibody products in patients, uncertainties related to product manufacturing as well as risks detailed from time to time in Medarex's public disclosure filings with the U.S. Securities and Exchange Commission (SEC), including its Annual Report on Form 10-K for the fiscal year ended December 31, 2003 and subsequent Quarterly Reports on Form 10-Q. There can be no assurance that such development efforts will succeed, that such products will receive required regulatory clearance or that, even if such regulatory clearance were received, such products would ultimately achieve commercial success. Copies of Medarex's public disclosure filings are available from its investor relations department.

PLAC® is a trademark of diaDexus, Inc. All rights are reserved.

Medarex®, the Medarex logo and UltiMAb Human Antibody Development System® are registered trademarks of Medarex, Inc. UltiMAbTM is a trademark of Medarex, Inc. All rights are reserved.