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Rigel Ubiquitin Ligase Program Identifies Molecules with Anti-HIV Activity

Rigel Program to be Presented at Keystone Scientific Meeting

SOUTH SAN FRANCISCO, Calif. - March 27, 2007

Rigel today announced that Yonchu Jenkins, Ph.D., Associate Director of Virology, will be presenting on the company’s efforts to develop a small molecule therapeutic to treat patients with the Human Immunodeficiency Virus (HIV) infection at the Keystone Symposium in British Columbia on March 27th and 29th. Dr. Jenkins will present both a poster and an oral presentation entitled, the Identification of Novel Antiretroviral Compounds by Targeting HIV Vif Function.

Dr. Jenkins and her colleagues at Rigel are investigating ways to inhibit Vif, a specific viral protein produced by HIV that is essential for its replication. Inside HIV-infected cells, the invading protein, Vif, neutralizes the potent anti-viral activity of a host human protein called Apobec3G. By recruiting Apobec3G into an ubiquitin ligase complex, Vif enables the subsequent degradation of Apobec3G, thus eradicating it from HIV-infected cells. When Vif is absent, Apobec3G’s potent anti-viral activity renders HIV virus particles non-infectious. Rigel is exploring a number of small molecule compounds aimed at modulating the intracellular ubiquitin ligase signaling pathways utilized by Vif so that Apobec3G will remain active in the presence of HIV and impede the virus from replicating.

“Our approach focuses on restoring the body’s natural ability to fight the virus by means of a ubiquitin ligase target. By focusing on a host target, rather than a viral target we may create a new class of anti-retroviral therapeutics,” said Donald G. Payan, M.D., Executive Vice President and Chief Scientific Officer of Rigel. “Rigel is uniquely positioned to pursue this target given our extensive experience in the ubiquitin ligase field and in the virology area.” he added.

For further information on Rigel’s HIV program, Dr. Jenkins poster will be available for viewing at www.rigel.com after 11:00am PT today.

Rigel’s Rich Ligase Drug Discovery Program
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 in working on the development of numerous ligase targets in oncology, inflammation, virology and metabolism. Rigel was one of the first companies to discover potent and highly selective small molecule inhibitors of ubiquitin ligases. Rigel has a broad-based platform in the enzymology and biology of ubiquitin ligases, which includes a large number of biochemical and cell-based assays to identify the specificity and selectivity of ligase targets. Rigel has an extensive patent portfolio covering the ligase area.

About Rigel (www.rigel.com)
Rigel is a clinical-stage drug development company that discovers and develops novel, small-molecule drugs for the treatment of inflammatory diseases, cancer and viral diseases. Our goal is to file one new investigational new drug (IND) application in a significant indication each year. We have achieved this goal since 2002. Our pioneering research focuses on intracellular signaling pathways and related targets that are critical to disease mechanisms. Rigel’s productivity has resulted in strategic collaborations with large pharmaceutical partners to develop and market our product candidates. We have product development programs in inflammatory/autoimmune diseases such as rheumatoid arthritis, thrombocytopenia and asthma, as well as in cancer.

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 "plans," "intends," “promising,” "expects," “anticipates” 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 Form 10-K for the year ended December 31, 2006. Rigel does not undertake any obligation to update forward-looking statements.

Contact: Raul Rodriguez
Phone: 650.624.1302
Email: invrel@rigel.com

Media Contact: Susan C. Rogers, Alchemy Consulting, Inc.
Phone: 650.430.3777
Email: susan@alchemyemail.com