( Kosan's epothilone program is partnere...)Kosan Presents Data on Lead Hsp90 Inhibitor, Tanespimycin, Showing,Promising Antitumor Activity in Multiple Myeloma at ASCO,Health

For additional information on Kosan Biosciences, please visit the company's website at http://www.kosan.com.

This press release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995 (the "Act"). Such forward-looking statements include but are not limited to the further development and potential safety, efficacy, commercialization, and other characteristics of tanespimycin, and Kosan's development plans with respect to tanespimycin, including but not limited to Kosan's plans and timing for the initiation of the TIME registration program. Any statements contained in this press release that are not statements of historical fact may be deemed to be forward-looking statements. There are a number of important factors that could cause the results of Kosan to differ materially from those indicated by these forward-looking statements, including, among others, risks related to the development of tanespimycin, including the risk that studies may not demonstrate safety and efficacy sufficient to initiate clinical trials, continue clinical development, obtain the requisite regulatory approvals or to result in a marketable product; and other risks detailed from time to time in the Company's SEC reports, including its Quarterly Report on Form 10-Q for the quarter ended March 31, 2007 and other periodic filings with the SEC. Kosan does not undertake any obligation to update forward-looking statements.

Velcade(R) (bortezomib) is a registered trademark of Millennium Pharmaceuticals, Inc.

Herceptin(R) (trastuzumab) is a registered trademark of Genentech, Inc.

Revlimid (R) (lenalidomide) is a registered trademark of Celgene Corporation

CONTACT: Jane Green, VP, Corporate Communications of Kosan
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