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Rigel Receives Two Milestone Payments from Daiichi

Identifies and Selects Two Potent, Selective Compounds that Inhibit the Collaboration Ligase Target

South San Francisco, CA - May 05, 2005

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) announced that it has received two milestone payments from Daiichi Pharmaceuticals Co., Ltd., as part of their collaboration to identify a new type of oncology drug. The two compounds are potent and selective inhibitors of a specific ligase target that is the basis of the collaboration. Moving forward, Daiichi will work to advance these novel compounds into preclinical development.

The research collaboration, which began in August 2002 aims to identify specific small molecule drug candidates against a ligase target that controls cancer cell proliferation through protein degradation. Throughout the past three years, the partnership has met several milestones and Daiichi's selection of two small molecule compounds marks the successful completion of a key step towards moving these compounds into development.

“Rigel in collaboration with Daiichi is aggressively moving forward and delivering results,” said James M. Gower, Chairman and CEO, Rigel Pharmaceuticals. “These potent and selective compounds support our belief that ubiquitin ligases may represent a promising and novel area of research for new targeted cancer drugs. We look forward to working with Daiichi as the company leverages its considerable resources to move these compounds into animal models of oncology.”

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. In addition to the Daiichi collaboration, Rigel last year established a collaboration with Merck & Co. investigating a number of Merck ligase targets in oncology.

About Ubiquitin Ligases
Ubiquitin ligases are enzymes that regulate protein degradation within the cell. The 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. We have four research and development programs investigating treatments for asthma/allergy, rheumatoid arthritis, oncology and hepatitis C. Our strategy is to initiate clinical trials with at least one new product candidate annually and to pursue partnerships with pharmaceutical and biotechnology companies for late-stage clinical development and commercialization of those product candidates.

This press release contains "forward-looking" statements, including statements related to Rigel's collaborations and 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 "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 associated with the timing and success of clinical trials and the commercialization of product candidates, as well as other risks detailed from time to time in Rigel's SEC reports, including its Annual Report on Form 10-K for the year ended December 31,2004. Rigel does not undertake any obligation to update forward-looking statements.

Rigel Contact:
Rigel Pharmaceuticals
Raul Rodriguez
650-624-1302

Media Contact:
Schwartz Communications
Melinda Bagatelos or
Ayanna Anderson
415-512-0770
rigel@schwartz-pr.com