Chiron and Enanta Announce Agreement for Development and Commercialization of HCV Therapeutics; Enanta Granted Worldwide License to Chiron's Portfolio of Hepatitis C Protease Inhibitors

"Chiron's pioneering science in cloning, identifying andsequencing the hepatitis C virus laid the groundwork for the battleagainst HCV infection. The company has demonstrated its commitment toaddressing the toll from this terrible disease through its broadlicensing policy and work in vaccines and blood screening," saidKenneth W. Bair, Ph.D., senior vice president, head ofBioPharmaceuticals research at Chiron. "Our agreement with Enantaallows us to fulfill our commitment to the development of HCVtherapeutics. Chiron has collaborated with Enanta in the field of HCVprotease research in the past, and we are pleased to license ourportfolio to a company that has demonstrated the potential to makegreat strides in the research and development of HCV proteasetherapeutics."

"Through this agreement with Chiron, a leader in the field of HCVresearch, we will accelerate the advancement of novel therapeutics forHCV and strengthen our growing pipeline of novel anti-infective drugsthat address significant unmet medical needs worldwide," stated JayLuly, president and CEO of Enanta Pharmaceuticals.

"Hepatitis C continues to be an important and key focus area forEnanta, and this new licensing agreement with Chiron strengthens ourpresence in HCV and leverages our unique chemistry expertise in thefield," stated Yujiro Hata, vice president of business development atEnanta Pharmaceuticals.

Under the terms of the agreement, Enanta will have exclusiverights to develop and commercialize Chiron's portfolio of HCV proteaseinhibitors. As part of the agreement, Enanta will make pre-commercialpayments upon the achievement of development milestones and payroyalties on commercial sales. Chiron will retain certainco-development and commercialization opt-in rights worldwide,excluding Asia.

About Hepatitis C Virus

In 1987, Chiron scientists Michael Houghton, Ph.D.; Qui-Lim Choo,Ph.D.; and George Kuo, Ph.D., cloned and first identified HCV as thecause of transfusion-related non-A, non-B hepatitis. This breakthroughmarked the first time a virus was cloned before it had been grown intissue culture or otherwise isolated. The Chiron scientists receivedthe prestigious Lasker Award in recognition of this discovery. Sincethe initial work, Chiron has been granted more than 100 HCV-relatedpatents in over 20 countries, including patents directed to hepatitisC polypeptides encoded throughout the genomes of HCV. Suchpolypeptides can be used in a variety of medical applications,including blood screening, clinical diagnosis, vaccines and astherapeutic targets for drug screening. More than 15 companies havebeen granted nonexclusive licenses to Chiron's HCV technology for drugscreening and diagnostic purposes, including Bristol-Myers Squibb,GlaxoSmithKline, Japan Tobacco Inc. and Gilead.

About Chiron

Chiron delivers innovative and valuable products to protect humanhealth by advancing pioneering science across the landscape ofbiotechnology. The company works to deliver on the limitless promiseof science and make a positive difference in people's lives. For moreinformation, please visit www.chiron.com.

About Enanta

Enanta Pharmaceuticals is a research and development company thatuses its novel chemistry approach and drug discovery capabilities tocreate best in class small molecule drugs in the anti-infective field.The heart of Enanta is its commitment to innovative chemistry thatsurpasses traditional medicinal chemistry approaches. The Company'ssuccessful integration of chemistry with biology has created a newclass of macrolide and ketolide antibiotics that overcome bacterialresistance. Additionally, Enanta has discovered antiviral agentstargeted against the Hepatitis C virus (HCV). Enanta is a privatelyheld company with offices in Watertown, MA. More information about thecompany can be found at www.enanta.com.

Chiron Forward-Looking Statements

This news release contains forward-looking statements, includingstatements regarding collaboration and licensing revenues andstrategies relating to licensing activities that involve risks anduncertainties and are subject to change. Forward-looking statementsoften address our expected future performance and often contain wordssuch as "expects," "anticipates," "intends," "plans," "believes,""seeks" or "will." A discussion of the company's operations andfinancial condition, including factors that may affect its businessand future prospects that could cause actual results and developmentsto differ materially from those expressed or implied byforward-looking statements, is contained in documents the company hasfiled with the SEC, including the Form 10-K for the year endedDecember 31, 2004, and the Form 10-Q for the quarter ended March 31,2005, and will be contained in all subsequent periodic filings madewith the SEC. These documents identify important factors that couldcause the company's actual performance to differ from currentexpectations, including, among others, the outcome of clinical trials,regulatory review and approvals, manufacturing capabilities,intellectual property protections and defenses, stock-price andinterest-rate volatility, and marketing effectiveness. In particular,there can be no assurance that Chiron will increase sales of existingproducts, successfully develop and receive approval to market newproducts, or achieve market acceptance for such new products. Therecan be no assurance that Chiron's out-licensing activities willgenerate significant revenue, nor that its in-licensing activitieswill fully protect it from claims of infringement by third parties. Inaddition, the company may engage in business opportunities, thesuccessful completion of which is subject to certain risks, includingapproval by Novartis AG, regulatory approvals, and the integration ofoperations.

Chiron does not undertake an obligation to update theforward-looking information the company is giving today.