XOMA (US) LLC Suspends HepeX-B(TM) Work Following Cubist Pharmaceuticals, Inc. Decision

BERKELEY, Calif.--(BUSINESS WIRE)--July 28, 2006--XOMA Ltd. (Nasdaq:XOMA - News) today announced that, in light of Cubist Pharmaceuticals, Inc.'s (Nasdaq:CBST - News) recently announced decision to cease investment in its HepeX-B(TM) product because of stringent FDA requirements for regulatory approval, XOMA has placed its production process development work for Cubist on hold and has issued a notice of contract termination to Cubist. Cubist and XOMA have begun discussions to consider other development options for HepeX-B.

In September of 2005, XOMA and Cubist signed a letter agreement under which XOMA’s extensive antibody development and manufacturing capabilities were to be used to develop new processes to manufacture HepeX-B. HepeX-B is a combination of two fully human monoclonal antibodies that target the hepatitis B virus (HBV) surface. The product, which has been granted Orphan Drug Status in both the U.S. and the European Union, has been under evaluation for the prevention of HBV re-infection in liver transplant patients.

About XOMA

XOMA is a leader in the discovery, development and manufacture of therapeutic antibodies, with a therapeutic focus that includes cancer and immune diseases. XOMA has royalty interests in RAPTIVA® (efalizumab), a monoclonal antibody product marketed worldwide (by Genentech, Inc. and Serono, SA) to treat moderate-to-severe plaque psoriasis, and LUCENTIS(TM) (ranibizumab injection), a monoclonal antibody product marketed worldwide (by Genentech and Novartis AG) to treat neovascular (wet) age-related macular degeneration.

The company has built a premier antibody discovery and development platform that includes access to seven of the leading commercially available antibody phage display libraries and XOMA’s proprietary Human Engineering(TM) and bacterial cell expression (BCE) technologies. More than 45 companies have signed BCE licenses. XOMA’s development collaborators include Lexicon Genetics, Inc., Novartis, and Schering-Plough Corp. With a fully integrated product development infrastructure, XOMA’s product development capabilities extend from preclinical sciences to product launch. For more information, please visit the company’s website at www.xoma.com.

Certain statements contained herein concerning current collaborations and product development or that otherwise relate to future periods are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. These statements are based on assumptions that may not prove accurate. Actual results could differ materially from those anticipated due to certain risks inherent in the biotechnology industry and for companies engaged in the development of new products in a regulated market. These risks, including those related to the results of discovery research and preclinical testing; the timing or results of pending and future clinical trials (including the design and progress of clinical trials; safety and efficacy of the products being tested; action, inaction or delay by the FDA, European or other regulators or their advisory bodies; and analysis or interpretation by, or submission to, these entities or others of scientific data); uncertainties regarding the status of biotechnology patents; uncertainties as to the cost of protecting intellectual property; changes in the status of the existing collaborative and licensing relationships; the ability of collaborators, licensees and other third parties to meet their obligations; market demand for products; scale up and marketing capabilities; competition; international operations; share price volatility; XOMA’s financing needs and opportunities and risks associated with XOMA’s status as a Bermuda company, are described in more detail in XOMA’s most recent annual report on Form 10-K and in other SEC filings. Consider such risks carefully in considering XOMA’s prospects.

Contact: XOMA Ltd. Paul Goodson, 510-204-7270 www.xoma.com

Source: XOMA Ltd.

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