South San Francisco, CA - November 15, 2004

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL - News) today announced that it has entered into a broad collaboration agreement with Merck & Co., Inc. (NYSE: MRK - News) to investigate ubiquitin ligases, a new class of drug target, to find treatments for cancer and potentially other diseases.

Under the terms of the agreement, Rigel will receive an initial cash payment and funding for Rigel research scientists for two and a half years. Rigel is also eligible to receive milestone payments for preclinical and clinical events. Merck is responsible for worldwide development and commercialization of any resulting compounds and will pay Rigel royalties on future product sales, if any. The collaboration is based on a number of new targets designated by Merck and do not include Rigel's current ligase targets. In addition, Rigel may nominate its own targets for potential inclusion in the collaboration. Additional terms were not disclosed.

"Merck is working to build a broad discovery and development franchise in cancer therapeutics," said Stephen H. Friend, M.D., Ph.D., senior vice president for molecular profiling and basic cancer research at Merck Research Laboratories. "We believe that ubiquitin ligases may represent a promising and novel area of research for new targeted cancer drugs. We hope this partnership will enable us to combine Rigel's capabilities in ubiquitin ligases with Merck's molecular profiling technology and downstream clinical infrastructure to explore a promising new class of drug targets."

Rigel is a leader in investigating and characterizing the ubiquitin ligase system for the discovery and development of potential new therapeutics. The company has initiated one of the industry's broadest efforts, working on the development of numerous ligase targets. Rigel was one of the first companies to discover potent and highly selective small molecule inhibitors of ubiquitin ligases. Some of these inhibitors have shown positive activity in animal models of disease and are in preclinical development at Rigel.

"Ubiquitin ligases are important targets for potential treatments for oncology, inflammatory and viral diseases, underscored by this year's Nobel prize in chemistry awarded to scientists for their work in this area," said Donald G. Payan, M.D., executive vice president and chief scientific officer of Rigel. "This collaboration will allow us to expand our efforts in this area and bring to bear the tremendous resources of Merck across the spectrum of discovery and development activities on this new target class. We believe this may be the largest dedicated commitment to ligases by a pharmaceutical or biotech enterprise to date."

About Ubiquitin Ligases
Ubiquitin ligases are enzymes that regulate protein degradation within the cell. Breakdown of proteins, in turn, affects many important cellular functions, including cell division. Targeting ligases represents a novel approach to treating diseases where normal cellular processes are out of balance. Because unchecked cell division is the hallmark of cancer, researchers believe that this part of the cell machinery represents a particularly compelling target for cancer therapies. Ubiquitin ligase targets are numerous and modular. This provides the potential for intervening in a highly specific fashion in a disease, potentially improving efficacy and minimizing side-effects.

About Rigel (www.rigel.com)
Rigel's mission is to become a source of novel, small-molecule drugs to address large, unmet medical needs. The company has initiated four development programs: asthma/allergy, hepatitis C, rheumatoid arthritis and oncology. Rigel has begun clinical testing of its first two product candidates, R112 for allergic rhinitis and R803 for hepatitis C, and expects to begin clinical trials of R406 for the treatment of rheumatoid arthritis by the end of 2004, which is expected to be followed by clinical trials for product candidates in oncology and asthma.

This press release contains "forward-looking" statements, including statements related to Rigel's plans to pursue clinical development of product candidates and the timing thereof and the potential efficacy of product candidates. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. Words such as "will," "plans," "intends," "expects" and similar expressions are intended to identify these forward-looking statements. There are a number of important factors that could cause Rigel's results to differ materially from those indicated by these forward-looking statements, including risks that if conflicts arise between us and our corporate collaborators or scientific advisors, the other party may act in its self-interest and not in the interest of our stockholders and if any of our corporate collaborators were to breach or terminate its agreement with us or otherwise fail to conduct the collaborative activities successfully and in a timely manner, the preclinical or clinical development or commercialization of the affected product candidates or research programs could be delayed or terminated, as well as other risks detailed from time to time in Rigel's SEC reports, including its Quarterly Report on Form 10-Q for the quarter ended September 30, 2004. Rigel does not undertake any obligation to update forward-looking statements.

SOURCE Rigel Pharmaceuticals, Inc.
-0- 11/15/2004
/CONTACT: Raul Rodriguez of Rigel Pharmaceuticals, Inc., +1-650-624-1302;
or Melinda Bagatelos or Ayanna Anderson, +1-415-512-0770, or
rigel@schwartz-pr.com, both of Schwartz Communications, for Rigel
Pharmaceuticals, Inc./
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/Web site: http://www.rigel.com /
(RIGL MRK)

CO: Rigel Pharmaceuticals, Inc.; Merck & Co., Inc.
ST: California
IN: MTC HEA BIO
SU: CON SVY

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