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Rigel Granted Key Patents in Ubiquitin Ligase Field

Potential New Targets Against Cancer and Other Diseases

South San Francisco, CA - June 09, 2004

Rigel Pharmaceuticals, Inc. (Nasdaq: RIGL) today announced that it has been granted United States Patent Nos. 6,737,244 and 6,740,495, relating to its program in ubiquitin ligases. These fundamental patents cover methods for identifying and/or measuring E3 ubiquitin ligase activity and screening for agents that modulate their activity, including small molecule or biological modulators.

Rigel believes that it is a leader in characterizing and exploiting the ubiquitin ligase system for the discovery of potentially new therapeutics. It has one of the industry's broadest efforts, working on numerous ligase targets. Rigel was one of the first companies to discover potent and highly selective small molecule inhibitors of ubiquitin ligase targets. Some of these inhibitors have shown good activity in animal models of disease and are in pre-clinical development.

"Rigel's breadth and experience in the field of ubiquitin ligases, now buttressed by these key patents, give Rigel a significant competitive advantage in this nascent field," said Donald G. Payan, M.D., Rigel's Executive Vice President and Chief Scientific Officer, "Ubiquitin ligases play an important role as targets for potential treatments for oncology, inflammatory and viral diseases as well as other important areas."

Rigel has entered into ubiquitin ligase collaborations with key industry partners. In 2002, Rigel began working with Daiichi Pharmaceuticals Co. Ltd. on a specific ubiquitin ligase target that controls cancer cell proliferation. Additionally, in late 2003, Johnson & Johnson accepted a ubiquitin ligase target in oncology as part of its collaboration with Rigel.

About Ubiquitin Ligases
Ubiquitin ligases play a key role in protein localization and protein turnover within the cell. Modulation of these targets presents a novel approach to treating diseases where the normal cell processes are out of balance, such as in cancer where the cell cycling is abnormal. Ubiquitin ligase cancer targets play a role in the regulation of stability, localization, and activity of key proteins such as oncoproteins. 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 three development programs: asthma/allergy, hepatitis C and rheumatoid arthritis. 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, to be followed by clinical trials for drug candidates in asthma and oncology.

This press release contains "forward-looking" statements, including statements related to the potential value of Rigel's ubiquitin ligase program and 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 "plans," "intends," "expects," "believes" 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, 2003 and its Quarterly Report on Form 10-Q for the quarter ended March 31, 2004. Rigel does not undertake any obligation to update forward-looking statements.