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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549



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FORM 10-Q



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(Mark one)

|x | Quarterly report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. For the quarterly period ended June 30, 2007
|- | -------------------------------------------------------------------------------------------------------------------------------------

or



| ̈ | Transition report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. For the transition period from
 to .
|- | ------------------------------------------------------------------------------------------------------------------------------

Commission File Number:

000-31633



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Kosan Biosciences Incorporated

(Exact name of registrant as specified in its charter)



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|                                                               |  |                                     
|-------------------------------------------------------------- |  | ------------------------------------
|Delaware                                                       |  | 94-3217016                          
|(State or other jurisdiction of incorporation or organization) |  | (I.R.S. Employer Identification No.)

3832 Bay Center Place, Hayward, California 94545

(address of principal executive offices)

(510) 732-8400

(Registrant's telephone number, including area code)



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Indicate by check mark whether the registrant:
(1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and
(2) has been subject to filing requirements for the past 90 days.    Yes  x    No ̈

Indicate by check mark whether the registrant is a large accelerated filer, an
accelerated filer, or a non-accelerated filer. See definition of accelerated filer and large accelerated filer in Rule 12b-2 of the Exchange Act). (Check one):

Large accelerated filer ̈Accelerated filer  x    Non-accelerated filer ̈

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes ̈No  x

Indicate the number of shares outstanding of each of the issuer's classes of common stock, as of the latest practicable date: Common Stock, $.001 par value; 42,526,428 shares outstanding at July 31, 2007.



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KOSAN BIOSCIENCES INCORPORATED

Form 10-Q

Quarter Ended June 30, 2007

INDEX



|               |  |                                                                                                          |  |     
|-------------- |  | -------------------------------------------------------------------------------------------------------- |  | ----
|               |  |                                                                                                          |  | Page
|PART I.        |  | FINANCIAL INFORMATION                                                                                    |  |     
|               |  |                                                                                                         
|Item 1:        |  | Condensed Financial Statements and Notes:                                                                |  |     
|               |  |                                                                                                         
|               |  | Condensed Balance Sheets as of June 30, 2007 (unaudited) and December 31, 2006                           |  | 3   
|               |  |                                                                                                         
|               |  | Condensed Statements of Operations (unaudited) for the three and six months ended June 30, 2007 and 2006 |  | 4   
|               |  |                                                                                                         
|               |  | Condensed Statements of Cash Flows (unaudited) for the six months ended June 30, 2007 and 2006           |  | 5   
|               |  |                                                                                                         
|               |  | Notes to Condensed Financial Statements (unaudited)                                                      |  | 6   
|               |  |                                                                                                         
|Item 2:        |  | Managements Discussion and Analysis of Financial Condition and Results of Operations                    |  | 14  
|               |  |                                                                                                         
|Item 3:        |  | Quantitative and Qualitative Disclosures About Market Risk                                               |  | 22  
|               |  |                                                                                                         
|Item 4:        |  | Controls and Procedures                                                                                  |  | 23  
|               |  |                                                                                                         
|PART II.       |  | OTHER INFORMATION                                                                                        |  |     
|               |  |                                                                                                         
|Item 1A:       |  | Risk Factors                                                                                             |  | 24  
|               |  |                                                                                                         
|Item 4:        |  | Submission of Matters to a Vote of Security Holders                                                      |  | 42  
|               |  |                                                                                                         
|Item 6:        |  | Exhibits                                                                                                 |  | 43  
|               | 
|SIGNATURES     |  | 44                                                                                                      
|               | 
|CERTIFICATIONS |  |                                                                                                         



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##### Table of Contents

PART I. FINANCIAL INFORMATION



|Item 1: | Financial Statements
|------- | --------------------

KOSAN BIOSCIENCES INCORPORATED

CONDENSED BALANCE SHEETS

(in thousands)



|                                              |             |               |          |   |                      |   |          |  
|--------------------------------------------- | ----------- | ------------- | -------- | - | -------------------- | - | -------- | -
|                                              |             | June 30, 2007 |          |   | December 31,2006 (1) |  
|                                              | (unaudited) |               |          |   |                     
|Assets                                        |            
|Current assets:                               |             |               |          |   |                      |   |          |  
|Cash and cash equivalents                     |             | $             | 39,012   |   |                      | $ | 35,655   |  
|Short-term investments                        |             |               | 52,440   |   |                      |   | 27,483   |  
|Accounts receivable                           |             |               | 1,964    |   |                      |   | 1,152    |  
|Prepaid and other current assets              |             |               | 987      |   |                      |   | 907      |  
|                                              |             |               |          |   |                      |   |          |  
|Total current assets                          |             |               | 94,403   |   |                      |   | 65,197   |  
|                                              |             |              
|Restricted cash                               |             |               | 949      |   |                      |   | 949      |  
|Property and equipment, net                   |             |               | 4,119    |   |                      |   | 4,801    |  
|Other assets                                  |             |               | 240      |   |                      |   | 240      |  
|                                              |             |               |          |   |                      |   |          |  
|Total assets                                  |             | $             | 99,711   |   |                      | $ | 71,187   |  
|                                              |             |               |          |   |                      |   |          |  
|Liabilities and Stockholders Equity          |            
|Current liabilities:                          |             |               |          |   |                      |   |          |  
|Accounts payable                              |             | $             | 1,896    |   |                      | $ | 1,323    |  
|Accrued liabilities                           |             |               | 9,323    |   |                      |   | 6,331    |  
|Current portion of deferred revenue           |             |               | 1,428    |   |                      |   | 13,992   |  
|Current portion of equipment loans            |             |               | 1,091    |   |                      |   | 1,289    |  
|                                              |             |               |          |   |                      |   |          |  
|Total current liabilities                     |             |               | 13,738   |   |                      |   | 22,935   |  
|                                              |             |              
|Deferred revenue, less current portion        |             |               | 6,425    |   |                      |   | 5,599    |  
|Equipment loans, less current portion         |             |               | 986      |   |                      |   | 899      |  
|                                              |             |              
|Stockholders equity:                         |             |               |          |   |                      |   |          |  
|Common stock                                  |             |               | 43       |   |                      |   | 35       |  
|Additional paid-in capital                    |             |               | 246,882  |   |                      |   | 202,016  |  
|Accumulated other comprehensive (loss) income |             |               | (13      | ) |                      |   | 9        |  
|Accumulated deficit                           |             |               | (168,350 | ) |                      |   | (160,306 | )
|                                              |             |               |          |   |                      |   |          |  
|Total stockholders equity                    |             | $             | 78,562   |   |                      |   | 41,754   |  
|                                              |             |               |          |   |                      |   |          |  
|Total liabilities and stockholders equity    |             | $             | 99,711   |   |                      | $ | 71,187   |  
|                                              |             |               |          |   |                      |   |          |  

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|(1) | The balance sheet data at December 31, 2006 has been derived from the audited financial statements at that date.
|--- | ----------------------------------------------------------------------------------------------------------------

See accompanying notes.



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KOSAN BIOSCIENCES INCORPORATED

CONDENSED STATEMENTS OF OPERATIONS

(unaudited)

(in thousands, except
per share data)



|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---- | -------------------------- | ------- | ---- | ------------------------ | - | ------ | - |  | ---- | ------- | - |  | - | ------- | -
|                                                                                                                                                                                                                                            |      | Three Months EndedJune 30, |         |      | Six Months EndedJune 30, |  
|                                                                                                                                                                                                                                            | 2007 |                            |         | 2006 |                          |   | 2007   |   |  | 2006 |        
|Revenues:                                                                                                                                                                                                                                   |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Contract revenue                                                                                                                                                                                                                            |      | $                          | 2,206   |      |                          | $ | 2,586  |   |  | $    | 15,124  |   |  | $ | 5,318   |  
|Grant revenue                                                                                                                                                                                                                               |      |                            |        |      |                          |   | 80     |   |  |      |        |   |  |   | 309     |  
|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Total revenues                                                                                                                                                                                                                              |      |                            | 2,206   |      |                          |   | 2,666  |   |  |      | 15,124  |   |  |   | 5,627   |  
|                                                                                                                                                                                                                                            |      |                            |         |     
|Operating expenses:                                                                                                                                                                                                                         |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Research and development (including charges for stock-based compensation of $433 and $232 in the three months ended June 30, 2007 and
2006, respectively, and $1,000 and $574 in the six months ended June 30, 2007 and 2006, respectively) |      |                            | 12,092  |      |                          |   | 8,300  |   |  |      | 21,082  |   |  |   | 19,650  |  
|General and administrative (including charges for stock-based compensation of $366 and $206 in the three months ended June 30, 2007 and
2006, respectively, and $851 and $399 in the six months ended June 30, 2007 and 2006, respectively) |      |                            | 2,052   |      |                          |   | 1,542  |   |  |      | 4,203   |   |  |   | 4,101   |  
|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Total operating expenses                                                                                                                                                                                                                    |      |                            | 14,144  |      |                          |   | 9,842  |   |  |      | 25,285  |   |  |   | 23,751  |  
|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Loss from operations                                                                                                                                                                                                                        |      |                            | (11,938 | )    |                          |   | (7,176 | ) |  |      | (10,161 | ) |  |   | (18,124 | )
|Other income, net                                                                                                                                                                                                                           |      |                            | 1,150   |      |                          |   | 387    |   |  |      | 2,117   |   |  |   | 754     |  
|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Net loss                                                                                                                                                                                                                                    |      | $                          | (10,788 | )    |                          | $ | (6,789 | ) |  | $    | (8,044  | ) |  | $ | (17,370 | )
|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Basic and diluted net loss per common share                                                                                                                                                                                                 |      | $                          | (0.25   | )    |                          | $ | (0.20  | ) |  | $    | (0.20   | ) |  | $ | (0.54   | )
|                                                                                                                                                                                                                                            |      |                            |         |      |                          |   |        |   |  |      |         |   |  |   |         |  
|Shares used in computing basic and diluted net loss per common share                                                                                                                                                                        |      |                            | 42,525  |      |                          |   | 34,386 |   |  |      | 40,813  |   |  |   | 31,880  |  

See accompanying notes.



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KOSAN BIOSCIENCES INCORPORATED

CONDENSED STATEMENTS OF CASH FLOWS

(unaudited)

(in thousands)



|                                                                           |      |                          |         |      |  |   |         |  
|-------------------------------------------------------------------------- | ---- | ------------------------ | ------- | ---- |  | - | ------- | -
|                                                                           |      | Six Months EndedJune 30, |        
|                                                                           | 2007 |                          |         | 2006 | 
|Operating activities                                                       |      |                          |         |      |  |   |         |  
|Net loss                                                                   |      | $                        | (8,044  | )    |  | $ | (17,370 | )
|Adjustment to reconcile net loss to net cash used in operating activities: |      |                          |         |      |  |   |         |  
|Depreciation and amortization                                              |      |                          | 1,119   |      |  |   | 1,190   |  
|Amortization of investment premiums and discounts                          |      |                          | (321    | )    |  |   | (43     | )
|Stock-based compensation                                                   |      |                          | 1,851   |      |  |   | 973     |  
|Changes in assets and liabilities:                                         |      |                          |         |      |  |   |         |  
|Accounts receivable                                                        |      |                          | (812    | )    |  |   | 1,478   |  
|Prepaid and other current assets                                           |      |                          | (80     | )    |  |   | 195     |  
|Other assets                                                               |      |                          |        |      |  |   | 59      |  
|Accounts payable and accrued liabilities                                   |      |                          | 3,565   |      |  |   | (1,778  | )
|Deferred revenue                                                           |      |                          | (11,738 | )    |  |   | (1,639  | )
|                                                                           |      |                          |         |      |  |   |         |  
|Net cash used in operating activities                                      |      |                          | (14,460 | )    |  |   | (16,935 | )
|                                                                           |      |                          |         |      |  |   |         |  
|Investing activities                                                       |      |                          |         |      |  |   |         |  
|Acquisition of property and equipment                                      |      |                          | (437    | )    |  |   | (233    | )
|Purchase of investments                                                    |      |                          | (46,708 | )    |  |   | (26,534 | )
|Proceeds from maturity of investments                                      |      |                          | 22,050  |      |  |   | 27,900  |  
|                                                                           |      |                          |         |      |  |   |         |  
|Net cash (used in) provided by investing activities                        |      |                          | (25,095 | )    |  |   | 1,133   |  
|                                                                           |      |                          |         |      |  |   |         |  
|Financing activities                                                       |      |                          |         |      |  |   |         |  
|Proceeds from issuance of common stock                                     |      |                          | 43,023  |      |  |   | 24,391  |  
|Proceeds from equipment loans                                              |      |                          | 678     |      |  |   | 270     |  
|Principal payments under equipment loans                                   |      |                          | (789    | )    |  |   | (1,060  | )
|                                                                           |      |                          |         |      |  |   |         |  
|Net cash provided by financing activities                                  |      |                          | 42,912  |      |  |   | 23,601  |  
|                                                                           |      |                          |         |      |  |   |         |  
|Net increase in cash and cash equivalents                                  |      |                          | 3,357   |      |  |   | 7,799   |  
|Cash and cash equivalents at beginning of period                           |      |                          | 35,655  |      |  |   | 18,750  |  
|                                                                           |      |                          |         |      |  |   |         |  
|Cash and cash equivalents at end of period                                 |      | $                        | 39,012  |      |  | $ | 26,549  |  
|                                                                           |      |                          |         |      |  |   |         |  

See accompanying notes.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS

(unaudited)



|1. | Organization and Summary of Significant Accounting Policies
|-- | -----------------------------------------------------------

Overview

Kosan Biosciences Incorporated (the Company or Kosan) was incorporated under the laws of
the State of California on January 6, 1995 and commenced operations in 1996. In July 2000, the Company was reincorporated under the laws of the State of Delaware.

Kosan is a cancer therapeutics company focused on advancing two new classes of anticancer agents
through clinical development: heat shock protein 90, or Hsp90, inhibitors and epothilones. Hsp90 inhibitors have a novel mechanism of action targeting multiple pathways involved in cancer cell growth and survival. The Companys proprietary
formulations of tanespimycin (KOS-953), are in Phase 1 and 2 clinical trials, primarily for multiple myeloma in combination with Velcade® (bortezomib), for HER2-positive metastatic breast
cancer in combination with Herceptin® (trastuzumab) and as monotherapy for melanoma. In addition, intravenous and oral formulations of a second-generation Hsp90 inhibitor, alvespimycin HCl
(KOS-1022), are being evaluated in Phase 1 clinical trials.

Kosan is also developing KOS-1584 in Phase 1 clinical trials in patients with
solid tumors. Epothilones inhibit cell division with a mechanism of action similar to taxanes, one of the most successful classes of anti-tumor agents. The epothilone program is partnered with Hoffmann-La Roche, Inc. and F. Hoffmann-La Roche Ltd.
(collectively, Roche) through a global development and commercialization agreement.

Kosan also has a motilin receptor agonist
program for the stimulation of gastrointestinal movement, or GI motility. In December 2006, the Company established a worldwide exclusive license agreement with Pfizer Inc. for its motilin agonist program, including KOS-2187 and related compounds.
Pfizer is responsible for all development, regulatory and commercial activities related to the motilin agonist program and has initiated Phase 1 clinical testing of KOS-2187 as a potential treatment of gastroesophageal reflux disease
(GERD).

Kosan also has next-generation Hsp90 inhibitor, next-generation epothilone and nuclear export inhibitor programs for
cancer that are undergoing preclinical evaluation. These programs are based on the use of Kosans technology to improve the structure of known polyketides and the efficiency of large-scale production. Kosan has funded its operations primarily
through equity financing, contract payments under its collaboration agreements, equipment financing arrangements and government grants.

Basis of
Presentation

The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles
generally accepted in the United States for interim financial information. The information as of June 30, 2007, and for the three and six months ended June 30, 2007 and 2006, reflects all adjustments (including normal recurring
adjustments) that the management of the Company believes are necessary for a fair presentation of the results for the periods presented. Results for any interim period are not necessarily indicative of results for any future interim period or for
the entire year. The accompanying unaudited condensed financial statements should be read in conjunction with the financial statements and notes thereto included in the Companys Annual Report on Form 10-K for the year ended December 31,
2006 filed with the Securities and Exchange Commission on March 16, 2007.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|1. | Organization and Summary of Significant Accounting Policies (continued)
|-- | -----------------------------------------------------------------------



Use of Estimates

The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities
and the disclosure of contingent assets and liabilities as of the date of the financial statements and the accompanying notes. Estimates and assumptions are reviewed periodically, and the effects of revisions are reflected in the financial
statements in the period they are determined. Actual results could differ from those estimates.

Cash Equivalents and Investments

The Company considers all highly liquid investments with a maturity from date of purchase of three months or less to be cash equivalents. The Company
limits its concentration of risk by diversifying its investments among a variety of issuers. All investment securities are classified as available-for-sale and are recorded at fair value based on quoted market prices, with unrealized gains and
losses excluded from earnings and reported in other comprehensive income (loss). Purchase premiums and discounts are recognized in interest income using the interest method over the terms of the securities. Realized gains and losses and declines in
the fair value that are deemed to be other-than-temporary are reflected in earnings. The cost of securities sold is based on the specific identification method.

The Company recognizes an impairment charge when the decline in the estimated fair value of a marketable security below the amortized cost is determined to be other-than-temporary. The Company considers various
factors in determining whether to recognize an impairment charge, including the duration of time during which the fair value has been less than its amortized cost, any adverse changes in the investees financial condition and associated
downgrades to credit ratings and the Companys intent and ability to hold the marketable security for a period of time sufficient to allow for any anticipated recovery in market value. For the six months ended June 30, 2007 and 2006, the
Company did not recognize an impairment charge related to its investment securities.

Restricted Cash

The Company held a restricted investment consisting of a certificate of deposit of approximately $949,000 at June 30, 2007 and December 31,
2006. This investment is carried at fair value and is restricted as to withdrawal under a letter of credit agreement related to a facility lease.

Property and Equipment

Property and equipment are stated at cost, net of accumulated depreciation and amortization.
Property and equipment are depreciated on a straight-line basis over the estimated useful lives of three to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the lease term.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|1. | Organization and Summary of Significant Accounting Policies (continued)
|-- | -----------------------------------------------------------------------



Revenue Recognition

The Company generates revenue under collaborative agreements with pharmaceutical companies and in prior years under research grants from the National Institutes of Health. The arrangements may include up-front
non-refundable fees, reimbursement for personnel and supply costs, milestone payments for the achievement of defined collaboration objectives and royalties on potential sales of commercialized products. The Company recognizes revenue under these
arrangements when (i) persuasive evidence of an arrangement exists; (ii) delivery of the services, supplies or technology license has occurred; (iii) the price is fixed or determinable; and (iv) collectability is reasonably
assured.

The Company recognizes license and other up-front fees pursuant to research and development collaboration agreements over the
Companys estimated period of continuing involvement with research and development of the respective agreement. These estimates are reviewed on a periodic basis and updated if the underlying assumptions are modified. Any changes in these
estimates will result in either an acceleration or further deferral of the related revenue recognition. Payments related to substantive performance milestones that are at risk at the initiation of an agreement are recognized upon successful
completion of a performance milestone event.

Contract revenues related to research and development efforts are recognized as revenue as
the related services are performed or delivered in accordance with contract terms. Such payments generally are made based on the number of full-time equivalent researchers assigned to the project and the related research and development expenses
incurred or as other deliverables under the contracts are fulfilled. Revenues related to government grants are recognized at the time a grant is awarded and as related research expenses are incurred. Any amounts received in advance of performance
are recorded as deferred revenue until earned.

Research and Development

Research and development consists of costs incurred for Company-sponsored and collaborative research and development activities. These costs consist
primarily of salaries and other personnel-related expenses, including associated stock-based compensation, facility-related expenses, licensing-related expenses, depreciation of facilities and equipment, lab consumables, services performed by
clinical research organizations and research institutions, contract manufacturers and other outside service providers. Expenses related to clinical trials and drug manufacturing generally are accrued based on estimates of work performed or the level
of patient enrollment and activities according to the planned protocols or agreements. The Company monitors planned protocols, work performed, patient enrollment levels and related activities to the extent possible and adjusts estimates accordingly.
Research and development expenses under government grant awards and collaborative agreements approximated the revenue recognized, excluding milestone and up-front fees received under such arrangements.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|1. | Organization and Summary of Significant Accounting Policies (continued)
|-- | -----------------------------------------------------------------------



Research and Development (continued)

The Companys research and development expenses do not reflect the costs incurred by the Companys partners, Roche, the National Cancer
Institute (the NCI) or Pfizer, associated with the clinical trials they are conducting in connection with the Companys epothilone, Hsp90 inhibitor and motilin receptor agonist programs, respectively.

Stock-Based Compensation

Effective January 1,
2006, the Company adopted the fair value recognition provisions of Statement of Financial Accounting Standards (SFAS) No. 123 (revised 2004) or SFAS 123R, Share-Based Payment, using the modified prospective
transition method. SFAS 123R requires that the compensation cost relating to share-based payment transactions, including stock options and employee stock purchase plans, be recognized in the financial statements. Under this transition method,
stock-based compensation expense is recognized for all stock-based compensation awards granted prior to, but not yet vested as of January 1, 2006, based on the grant date fair value estimated in accordance with the original provision of SFAS
No. 123, Accounting for Stock-Based Compensation. Stock-based compensation expense for all stock-based compensation awards granted after December 31, 2005 is based on the grant-date fair value estimated in accordance with the
provisions of SFAS 123R. The Company recognizes these compensation costs net of an expected forfeiture rate on a graded-vesting basis over the requisite service period of the award, which is generally the option vesting term of one or four years.
Prior to the adoption of SFAS 123R, the Company accounted for common stock options granted to employees using the intrinsic value method as prescribed by Accounting Principles Board (APB) No. 25 Accounting for Stock Issued to
Employees and related interpretations, and thus, recognized compensation expense for options granted with exercise prices less than the fair value of the Company's common stock on the date of the grant. Other stock-based compensation expense
for options granted to non-employees has been determined in accordance with SFAS 123 and Emerging Issues Task Force (EITF) 96-18 Accounting for Equity Instruments That Are Issued to Other Than Employees for Acquiring, or in
Conjunction with Selling, Goods or Services as the fair value of the consideration received or the fair value of the equity instruments issued, whichever is more reliably measured. The measurement of other stock-based compensation to
non-employees is subject to periodic adjustment as the underlying awards vest. As such, changes to these measurements could be substantial should the Company experience significant changes in its stock price.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|1. | Organization and Summary of Significant Accounting Policies (continued)
|-- | -----------------------------------------------------------------------



Income Taxes

Effective January 1, 2007, the Company adopted FASB Interpretation No. (FIN) 48 Accounting for Uncertainty in Income Taxes. FIN 48 prescribes the minimum recognition threshold a tax
position is required to meet before being recognized in the financial statements. FIN 48 also requires additional disclosure of the beginning and ending unrecognized tax benefits and details regarding the uncertainties that may cause the
unrecognized benefits to increase or decrease within a twelve month period. The Company did not recognize any material adjustment in its liability for unrecognized income tax benefits upon the adoption of FIN 48, including any amounts for interest
and penalties. The Company had approximately $5.3 million of unrecognized tax benefits related to research and development credits at the date of adoption. The Company recognizes interest and penalties related to income taxes in income tax expense.
The Company is subject to income tax examinations for Federal and State income taxes from 2003 and 2002, respectively, forward. The Company does not anticipate that total unrecognized tax benefits will significantly change within the following
twelve month period.

Net Loss per Share

Basic and diluted net loss per common share has been computed using the weighted-average number of shares of common stock outstanding during the period. The Company has excluded all outstanding stock options and warrants from the
calculation of diluted net loss per common share because all such securities are antidilutive for all applicable periods presented. Diluted net loss per share is not presented separately as the Company is in a net loss position and including
potentially dilutive securities in the loss per share computation would be antidilutive.

Recent Accounting Pronouncements

In September 2006, the Financial Accounting Standards Board (FASB) issued SFAS No. 157, Fair Value Measurements (SFAS
157). SFAS 157 defines fair value, establishes a framework for measuring fair value in generally accepted accounting principles, and expands disclosures about fair value measurements. SFAS 157 applies under other accounting pronouncements that
require or permit fair value measurements. Accordingly, SFAS 157 does not require any new fair value measurements. SFAS 157 is effective beginning January 1, 2008. The Company is currently evaluating the impact of SFAS 157 on its financial
statements.

In June 2007, the Emerging Issues Task Force of the Financial Accounting Standards Board reached a consensus on EITF
No. 07-03, Accounting for Nonrefundable Advance Payments for Goods or Services to Be Used in Future Research and Development Activities. Under EITF 07-03, nonrefundable advance payments for goods or services to be received in the
future for use in research and development activities should be deferred and capitalized. Such amounts should be expensed as the related goods are delivered or services are performed. If the Companys expectations change such that it does not
expect the goods to be delivered or services to be rendered, the capitalized advance payment should be charged to expense. EITF 07-03 is effective for new contracts entered into beginning January 1, 2008. The Company is currently evaluating the
impact of EITF 07-03 on its financial statements.



10



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|2. | Research and Development Agreements
|-- | -----------------------------------

Roche

In September 2002, the Company entered into a research and development collaboration agreement (the Roche Agreement) with Roche. Under the
terms of the Roche Agreement, Roche has worldwide exclusive rights to market and sell KOS-1584 and any other epothilones developed under the collaboration in the field of oncology, and the Company has the right to co-promote in the United States any
epothilone products developed under the collaboration. The Roche Agreement provides for the Company to receive payments for the reimbursement of agreed upon research and development expenditures, funding of a back-up research program that led to the
selection of KOS-1584 as a clinical candidate, achievement of clinical, regulatory and commercial milestone payments and royalties on sales of collaboration products. In addition, the Company is entitled to increase its royalties on any sales of
collaboration products through a buy-in at a later stage of clinical development and by co-promotion in the United States of products resulting from the collaboration. Effective July 1, 2004, the Company entered into an amendment to the Roche
Agreement that provided for the reimbursement of costs that exceeded the previously stipulated amounts set forth in the Roche Agreement. In March 2006, the Company entered into a letter agreement with Roche, replacing a particular at-risk milestone
payment obligation in the Roche Agreement with an obligation by Roche to pay the Company $2.0 million for certain patent expenses.

The
Company recognized revenue related to the Roche Agreement of approximately $2.2 million and $2.6 million for the three months ended June 30, 2007 and 2006, respectively, and approximately $4.4 million and $5.3 million for the six months ended
June 30, 2007 and 2006, respectively. Such amounts, excluding the ratable portion of up-front fees and milestone payments, approximated research and development expenses incurred under the Roche Agreement. Included in contract revenues was
approximately $0.3 million and $0.8 million for the three months ended June 30, 2007 and 2006, respectively, and $1.0 million and $1.6 million for the six months ended June 30, 2007 and 2006, respectively, related to the ratable portion of
the $25.0 million up-front fee that is being recognized over the estimated clinical development period for product candidates in clinical trials. In the first quarter of 2007, upon the decision to advance KOS-1584 into Phase 2 clinical trials, the
Company determined that the estimated clinical development period extended from 2009 to 2012, resulting in a further deferral of the unrecognized portion of the up-front fee.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|2. | Research and Development Agreements (continued)
|-- | -----------------------------------------------



Pfizer

In December 2006, the Company entered into an exclusive license agreement (the Pfizer Agreement) with Pfizer Inc. under which the Company granted to Pfizer a worldwide exclusive license to its motilin agonist program. Under the
terms of the Pfizer Agreement, Pfizer and the Company agreed to collaborate on filing of regulatory documents for the initiation of a Phase 1 clinical trial of the Companys clinical candidate, KOS-2187. Pfizer will be responsible for all
development, regulatory and commercial activities related to the motilin agonist program. The Company received an up-front fee of $12.5 million in December 2006 and is eligible to receive milestone payments for the successful development and
commercialization of licensed compounds, including milestone payments for achieving certain sales amounts, as well as royalties on worldwide sales. For the three and six months ended June 30, 2007, the Company recognized revenue of
approximately $10.7 million related to the up-front fee.

License Agreements

The Company has collaborative and license agreements with several academic, government and medical institutions. Included in research and development
expenses were fees incurred in connection with these agreements of approximately $10,000 and $38,000 for the three months ended June 30, 2007 and 2006, respectively, and $102,000 and $115,000 for the six months ended June 30, 2007 and
2006, respectively.



|3. | Comprehensive Income
|-- | --------------------

For the three and six months
ended June 30, 2007 and 2006, comprehensive loss was as follows (in thousands):



|                                                        |      |                            |         |      |                          |   |        |   |  |      |        |   |  |   |         |  
|------------------------------------------------------- | ---- | -------------------------- | ------- | ---- | ------------------------ | - | ------ | - |  | ---- | ------ | - |  | - | ------- | -
|                                                        |      | Three Months EndedJune 30, |         |      | Six Months EndedJune 30, |  
|                                                        | 2007 |                            |         | 2006 |                          |   | 2007   |   |  | 2006 |       
|Net loss                                                |      | $                          | (10,788 | )    |                          | $ | (6,789 | ) |  | $    | (8,044 | ) |  | $ | (17,370 | )
|Unrealized gain (loss) on available-for-sale securities |      |                            | (24     | )    |                          |   | 4      |   |  |      | (22    | ) |  |   | 91      |  
|                                                        |      |                            |         |      |                          |   |        |   |  |      |        |   |  |   |         |  
|Comprehensive loss                                      |      | $                          | (10,812 | )    |                          | $ | (6,785 | ) |  | $    | (8,066 | ) |  | $ | (17,279 | )
|                                                        |      |                            |         |      |                          |   |        |   |  |      |        |   |  |   |         |  



|4. | Equipment Financing
|-- | -------------------

The Company finances certain
equipment and facility improvements under debt obligations with terms of 48 months.In April 2004, the Company entered into a $3.5 million equipment line of credit agreement, all of which had been fully utilized as of March 31, 2007. The
interest rates of each of the loans are fixed at the time of the draw down, with the interest rates ranging from 6.31% to 9.09%. Obligations under the loans are secured by the assets financed under the loans.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)



|5. | Accrued Liabilities
|-- | -------------------

Accrued liabilities consisted
of the following (in thousands):



|                                 |  |              |       |                  |   |      
|-------------------------------- |  | ------------ | ----- | ---------------- | - | -----
|                                 |  | June 30,2007 |       | December 31,2006
|Research and development-related |  | $            | 6,677 |                  | $ | 3,538
|Compensation-related             |  |              | 1,323 |                  |   | 1,564
|Professional services            |  |              | 704   |                  |   | 486  
|Facilities-related               |  |              | 560   |                  |   | 508  
|Other                            |  |              | 59    |                  |   | 235  
|                                 |  |              |       |                  |   |      
|                                 |  | $            | 9,323 |                  | $ | 6,331
|                                 |  |              |       |                  |   |      



|6. | Stockholders Equity
|-- | --------------------

In February 2007, the
Company completed a registered direct public offering of 7,000,000 shares of common stock at a public offering price of $6.50 per share. The Company received approximately $42.3 million in net proceeds after placement agent fees and other offering
costs.



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##### Table of Contents

|Item 2: | Management's Discussion and Analysis of Financial Condition and Results of Operations
|------- | -------------------------------------------------------------------------------------

Forward-Looking Statements

The following discussion
of our financial condition and results of operations contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and within the meaning of Section 21E of the Securities Exchange Act of
1934, as amended, which are subject to the safe harbor created by those sections. These forward-looking statements include, but are not limited to, statements about:



| |  |  | our strategy, including our plans with respect to presenting clinical data and initiating clinical trials;
| | - |  | ----------------------------------------------------------------------------------------------------------



| |  |  | our research and development programs, including clinical testing;
| | - |  | ------------------------------------------------------------------



| |  |  | sufficiency of our cash resources;
| | - |  | ----------------------------------



| |  |  | revenues from partnering arrangements;
| | - |  | --------------------------------------



| |  |  | our research and development and other expenses; and
| | - |  | ----------------------------------------------------



| |  |  | our operations and legal risks.
| | - |  | -------------------------------

In
some cases, forward-looking statements can be identified by words such as expect, anticipate, intend, believe, hope, assume, estimate, plan,
will and other similar words and expressions. In addition, any statements that refer to expectations, projections or other characterizations of future events or circumstances are forward-looking statements. Discussions containing these
forward-looking statements may be found throughout this Form 10-Q, including the section entitled Part I, Item 2 Managements Discussion and Analysis of Financial Condition and Results of Operations. These forward-looking
statements involve risks and uncertainties, including the risks discussed below in Part II, Item 1A Risk Factors that could cause our actual results to differ materially from those in the forward-looking statements. We undertake no
obligation to publicly release any revisions to the forward-looking statements or reflect events or circumstances after the date of this document. The risks discussed below in Part II, Item 1A Risk Factors and elsewhere in this
report should be considered in evaluating our prospects and future financial performance.

The name Kosan Biosciences Incorporated, our
logo and all other Kosan names are our trademarks. All other trademarks or brand names appearing in this Quarterly Report are the property of their respective holders.



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##### Table of Contents

Overview

We are a cancer therapeutics company focused on advancing two new classes of anticancer agents through clinical development: Hsp90 inhibitors and epothilones. The following is the status of our product candidates.

Hsp90 Inhibitors

Tanespimycin

Our proprietary formulations of tanespimycin are in Phase 1 and 2 clinical trials, primarily for multiple myeloma in combination with Velcade, for
HER2-positive metastatic breast cancer in combination with Herceptin and as monotherapy for melanoma. In June, at the 2007 annual meeting of the American Society of Clinical Oncology (ASCO), we presented updated data from a Phase 1b
clinical trial of tanespimycin in multiple myeloma in combination with Velcade demonstrating antitumor activity and manageable toxicity. Our Tanespimycin in Myeloma Evaluation, or TIME, clinical program includes TIME-1, a Phase 3 pivotal
study of tanespimycin in combination with Velcade in first-relapse patients with multiple myeloma, which we expect to initiate in the fourth quarter of 2007 or the first quarter of 2008, and TIME-2, a Phase 2/3 study of tanespimycin in combination
with Velcade in patients with relapsed-refractory multiple myeloma, which we opened for enrollment of patients in the third quarter of 2007. At the ASCO annual meeting, we also presented initial data from a Phase 2 clinical trial of tanespimycin in
metastatic melanoma demonstrating antitumor activity and manageable toxicity. We anticipate presenting updated data on tanespimycin in combination with Herceptin in HER2-positive metastatic breast cancer later in 2007.

Alvespimycin

Intravenous and oral formulations of a
second-generation Hsp90 inhibitor, alvespimycin HCl, are currently in Phase 1 clinical trials. In June, at the ASCO annual meeting, we presented data from a Phase 1b clinical trial of intravenous alvespimycin in combination with Herceptin in
patients with solid tumors, primarily HER2-positive metastatic breast cancer and ovarian cancer, demonstrating antitumor activity and manageable toxicity. We anticipate presenting updated data from this trial later in 2007. We intend to initiate a
Phase 2 clinical trial of alvespimycin as monotherapy in patients with HER2-positive metastatic breast cancer later in 2007. We anticipate initiating a Phase 2/3 clinical trial investigating combination with Herceptin in patients with HER2-positive
metastatic breast cancer. Our oral Phase 1 trial is ongoing on two schedules, daily and every other day dosing, and we plan to evaluate additional doses and schedules.

Epothilones

KOS-1584 is our epothilone anticancer clinical candidate that is being evaluated
in dose-escalating Phase 1 clinical trials in patients with solid tumors. In June, at the 2007 ASCO annual meeting, we presented data from one of our KOS-1584 Phase 1 clinical trials, utilizing different dosing schedules, demonstrating antitumor
activity and manageable toxicity in patients with solid tumors. We expect to initiate Phase 2 clinical trials with KOS-1584 later in 2007.

Our epothilone program is partnered with Roche through a global development and commercialization agreement, which requires Roche to fund all of the current and anticipated KOS-1584 clinical trials.

Other Programs

We also have a motilin
receptor agonist program for the stimulation of gastrointestinal movement, or GI motility. In December 2006, we established a worldwide exclusive license agreement with Pfizer for



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##### Table of Contents

our motilin agonist program, including KOS-2187 and related compounds. Pfizer is responsible for all development, regulatory and commercial activities
related to the motilin agonist program. Pfizer has initiated Phase 1 clinical testing of KOS-2187 as a potential treatment for gastroesophageal reflux disease (GERD).

We also have next-generation Hsp90 inhibitor, next-generation epothilone and nuclear export inhibitor programs for cancer that are undergoing preclinical
evaluation. These programs are also based on the use of our technology to improve the structure of known polyketides and the efficiency of large-scale production.

We have incurred significant losses since our inception. As of June 30, 2007, our accumulated deficit was $168.4 million. We expect to incur additional operating losses over the next several years as we continue
to advance our product candidates into and through clinical trials.

We believe that our existing cash and investments and anticipated cash
flow from our existing collaboration with Roche will be sufficient to support our current operating plan into the first half of 2009. We expect that additional financing will be required in order to fund our operations.

Critical Accounting Policies

Our discussion and
analysis of our financial condition and results of operations are based upon our financial statements, which have been prepared in accordance with U.S. generally accepted accounting principles. The preparation of these financial statements requires
us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and the disclosure of contingent assets and liabilities as of the date of the financial statements. Estimates and assumptions are
reviewed periodically and the effects of revisions are reflected in the financial statements in the period they are determined. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable under
the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results could differ from these estimates. The basis of our
current estimates or assumptions has not significantly changed since we filed our Annual Report on Form 10-K for the year ended December 31, 2006 with the Securities and Exchange Commission on March 16, 2007, except as noted below.

Clinical Trial and Drug Manufacturing Accruals

Research and development expenditures are expensed as incurred. Our expenses related to clinical trials and drug manufacturing are based on estimates of the services received or efforts expended pursuant to contracts with multiple
institutions, clinical research organizations, contract manufacturers and other service providers that conduct and manage clinical trial activities or drug manufacturing on our behalf. The financial terms of these agreements are established prior to
the initiation of the related services, thus establishing the basis of our estimates. However, these terms may be subject to amendment due to changes in the scope and length of the related clinical trial or work performed. Expenses related to
clinical trials and drug manufacturing generally are accrued based on estimates of work performed or the level of patient enrollment and activities according to the planned protocols or agreements. We monitor planned protocols, work performed,
patient enrollment levels and related activities to the extent possible and adjust our estimates accordingly. All estimates may differ significantly from the actual amounts subsequently invoiced. No adjustments for material changes in estimates have
been recognized in any period presented.



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##### Table of Contents

Income Taxes

Effective January 1, 2007, we adopted FASB Interpretation No. (FIN) 48 Accounting for Uncertainty in Income Taxes. FIN 48 prescribes the minimum recognition threshold a tax position is required to meet before
being recognized in the financial statements. FIN 48 also requires additional disclosure of the beginning and ending unrecognized tax benefits and details regarding the uncertainties that may cause the unrecognized benefits to increase or decrease
within a twelve month period. We did not recognize any material adjustment in our liability for unrecognized income tax benefits upon the adoption of FIN 48, including any amounts for interest and penalties. We had approximately $5.3 million of
unrecognized tax benefits related to research and development credits at the date of adoption. We recognize interest and penalties related to income taxes in income tax expense. We are subject to income tax examinations for Federal and State income
taxes from 2003 and 2002, respectively, forward. We do not anticipate that total unrecognized tax benefits will significantly change within the following twelve month period.

Results of Operations

Revenues

Revenues were approximately $2.2 million and $15.1 million for the three and six months ended June 30, 2007, respectively, compared to approximately
$2.7 million and $5.6 million for the same periods in 2006. Revenues in 2007 consisted primarily of contract revenues recognized under our agreements with Roche and Pfizer. Revenue in 2006 consisted primarily of contract revenue under our agreement
with Roche.



|                                       |      |                            |       |          |   |       |                         |       |          |  |   |        |  |   |       |  |       |  
|-------------------------------------- | ---- | -------------------------- | ----- | -------- | - | ----- | ----------------------- | ----- | -------- |  | - | ------ |  | - | ----- |  | ----- | -
|(In thousands, except for percentages) |      | Three Months EndedJune 30, |       | % Change |   |       | Six Month EndedJune 30, |       | % Change | 
|                                       | 2007 |                            | 2006  |          |   | 2007  |                         | 2006  |         
|Contract revenue                       |      | $                          | 2,206 |          | $ | 2,586 |                         | - 15  | %        |  | $ | 15,124 |  | $ | 5,318 |  | 184   | %
|Grant revenue                          |      |                            |      |          |   | 80    |                         | - 100 | %        |  |   |       |  |   | 309   |  | - 100 | %
|                                       |      |                            |       |          |   |       |                         |       |          |  |   |        |  |   |       |  |       |  
|Total revenues                         |      | $                          | 2,206 |          | $ | 2,666 |                         | - 17  | %        |  | $ | 15,124 |  | $ | 5,627 |  | 169   | %
|                                       |      |                            |       |          |   |       |                         |       |          |  |   |        |  |   |       |  |       |  

Total revenues decreased by approximately 17%, or $0.5 million, for the three months ended
June 30, 2007 compared to the same period in 2006. The decrease for the three month period is due to lower contract revenue from Roche, primarily from the extension of the amortization of our up-front fee. Total revenues increased by
approximately 169%, or $9.5 million, for the six months ended June 30, 2007 compared to the same period last year. The increase for the six month period was primarily due to amortization of the remaining $10.7 million up-front fee from Pfizer
recognized in 2007, partially offset by lower contract revenue from Roche as previously discussed. As of March 31, 2007, we had fully amortized the Pfizer up-front fee and, as a result, we do not expect to recognize any further contract revenue
related to the amortization of this fee. In February 2007, Roche and we jointly made a decision to advance KOS-1584 into later stage clinical trials. In the first quarter of 2007, we determined that the estimated clinical development period
resulting from this decision would be extended from 2009 to 2012. We expect that our contract revenues from Roche will increase over the next several quarters as Phase 2 clinical trials are initiated with KOS-1584. If we do not maintain our
agreement with Roche, our revenues will significantly decrease unless we enter into additional collaborations that provide substantial new revenues.



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##### Table of Contents

Research and Development Expenses

Our research and development expenses were approximately $12.1 million and $21.1 million for the three and six months ended June 30, 2007,
respectively, compared to $8.3 million and $19.7 million for the same periods in 2006. Our research and development activities consist primarily of salaries and other personnel-related expenses, clinical trial-related services performed by clinical
research organizations and research organizations and research institutions and other outside service providers, licensing-related expenses, lab consumables, services performed by clinical research organizations and research institutions, contract
manufacturers and other outside service providers. We group these activities into two major categories: research and preclinical and clinical development. We are unable to estimate the nature, timing or costs to complete our
research and development projects, or when material net cash inflows to us could be expected to commence, if ever, due to the numerous risks and uncertainties associated with developing pharmaceutical products. These risks and uncertainties include
those discussed in this report in Part II, Item 1A Risk Factors.



|                                       |      |                            |      |          |  |      |                          |      |          |  |        |                          |     |   |  |        
|-------------------------------------- | ---- | -------------------------- | ---- | -------- |  | ---- | ------------------------ | ---- | -------- |  | ------ | ------------------------ | --- | - |  | -------
|(In thousands, except for percentages) |      | Three Months EndedJune 30, |      | % Change |  |      | Six Months EndedJune 30, |      | % Change |  |        | Inception - June 30,2007
|                                       | 2007 |                            | 2006 |          |  | 2007 |                          | 2006 |          | 
|Clinical development                   |      |                            |      |          |  |      |                          |      |          |  |        |                          |     |   |  |        
|Epothilones                            |      | 2,332                      |      | 1,910    |  | 22   | %                        |      | 4,100    |  | 4,337  |                          | -5  | % |  | 61,268 
|Hsp90 inhibitors                       |      | 6,710                      |      | 3,653    |  | 84   | %                        |      | 10,946   |  | 7,742  |                          | 41  | % |  | 42,700 
|                                       |      |                            |      |          |  |      |                          |      |          |  |        |                          |     |   |  |        
|Total clinical development             |      | 9,042                      |      | 5,563    |  | 63   | %                        |      | 15,046   |  | 12,079 |                          | 25  | % |  | 103,968
|Research and preclinical (1)           |      | 3,050                      |      | 2,737    |  | 11   | %                        |      | 6,036    |  | 7,571  |                          | -20 | % |  | 157,169
|                                       |      |                            |      |          |  |      |                          |      |          |  |        |                          |     |   |  |        
|Total research and development         |      | 12,092                     |      | 8,300    |  | 46   | %                        |      | 21,082   |  | 19,650 |                          | 7   | % |  | 261,137
|                                       |      |                            |      |          |  |      |                          |      |          |  |        |                          |     |   |  |        

---

|(1) | Research and preclinical constitutes internal research and development costs for our early stage programs in the areas of cancer, gastrointestinal motility and
technology development. Expenses for the three months ended June 30, 2007 and 2006 included allocated personnel-related expenses of approximately $1.1 million and $1.0 million, allocated facility-related expenses of approximately $0.7 million
and $1.0 million and allocated lab consumables of $0.1 million and $0.1 million, respectively. Expenses for the six months ended June 30, 2007 and 2006 included allocated personnel-related expenses of approximately $2.5 million and $3.3
million, allocated facility-related expenses of approximately $1.6 million and $2.2 million and allocated lab consumables of $0.3 million and $0.3 million, respectively. Expenses for the period from inception through June 30, 2007 included
allocated personnel-related expenses of approximately $77.1 million, allocated facility-related expenses of approximately $38.9 million and allocated lab consumables of $10.6 million.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

The increase of 46%, or approximately $3.8 million, and 7%, or approximately $1.4 million, in research and development expenses for the three and six
months ended June 30, 2007 compared to the same periods in 2006 were the result of the following:



| |  |  | approximately $3.5 million and $3.0 million in increased clinical costs for the three and six months ended June 30, 2007, respectively, primarily due to the
increased clinical costs in the Hsp90 inhibitor and epothilone programs, including costs associated with the planned initiation of the clinical trials in our TIME clinical programs and KOS-1584 phase 2 clinical trials, including the manufacturing of
clinical materials to support our Hsp90 program.
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | approximately $0.3 million increase in preclinical and research costs for the three month period in 2007, primarily related to efforts to support our next
generation epothilone.
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | Approximately $1.5 million decrease in preclinical and research costs for the six month period in 2007, primarily related to reorganization related expenses and a
reduction in investment in certain early-stage research programs in 2006.
| | - |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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We opened our TIME clinical program studying tanespimycin in combination with Velcade in multiple myeloma
in the third quarter of 2007 and intend to initiate a Phase 2 clinical trial of alvespimycin as monotherapy in patients with HER2-positive metastatic breast cancer later in 2007. We also intend to initiate Phase 2 clinical trials of KOS-1584 later
in 2007. The preparation for, and initiation of, these clinical trials will result in an increase in our research and development expenses over the next several quarters.

Roche is funding all of the current and planned KOS-1584 clinical trial costs. Pfizer is funding all of the current KOS-2187 clinical trial costs. We are responsible, at our cost, to supply tanespimycin and
alvespimycin for clinical trials sponsored by the NCI under the CRADAs, and the NCI is responsible for substantially all of the remaining costs of these trials through Phase 2. In addition, we are sponsoring other clinical trials of tanespimycin and
alvespimycin at our sole expense. Our research and development expenses do not reflect the costs incurred by our partners, Roche, the NCI or Pfizer, associated with the clinical trials they are conducting in connection with our epothilone, Hsp90
inhibitor and motilin receptor agonist programs, respectively.

General and Administrative Expenses



|                                       |      |                            |       |          |   |       |                          |      |          |  |   |       |  |   |       |  |   |  
|-------------------------------------- | ---- | -------------------------- | ----- | -------- | - | ----- | ------------------------ | ---- | -------- |  | - | ----- |  | - | ----- |  | - | -
|(In thousands, except for percentages) |      | Three Months EndedJune 30, |       | % Change |   |       | Six Months EndedJune 30, |      | % Change | 
|                                       | 2007 |                            | 2006  |          |   | 2007  |                          | 2006 |         
|General and administrative             |      | $                          | 2,052 |          | $ | 1,542 |                          | 33   | %        |  | $ | 4,203 |  | $ | 4,101 |  | 2 | %
|                                       |      |                            |       |          |   |       |                          |      |          |  |   |       |  |   |       |  |   |  

Our general and administrative expenses were approximately $2.1 million and $4.2 million for the
three and six months ended June 30, 2007, respectively, compared to $1.5 million and $4.1 million for the same periods in 2006. For the three months ended June 30, 2007, general and administrative expenses increased by 33%, or
approximately $0.5 million compared to the same period in 2006. For the six months ended June 30, 2007, general and administrative expenses increased by 2%, or approximately $0.1 million, compared to the same period in 2006. The increase is
primarily due to increased personnel costs to support the growth in our research and development activities. We expect our general and administrative expenses to moderately increase in support of our research and development activities as we
continue to advance our product candidates through clinical trials.

Other Income, Net



|                                       |      |                            |       |      |          |   |      |                          |  |      |          |  |   |       |   |  |   |       |   |  |      |  
|-------------------------------------- | ---- | -------------------------- | ----- | ---- | -------- | - | ---- | ------------------------ |  | ---- | -------- |  | - | ----- | - |  | - | ----- | - |  | ---- | -
|(In thousands, except for percentages) |      | Three Months EndedJune 30, |       |      | % Change |   |      | Six Months EndedJune 30, |  |      | % Change | 
|                                       | 2007 |                            |       | 2006 |          |   |      | 2007                     |  |      | 2006     |  |  
|Interest income                        |      | $                          | 1,212 |      |          | $ | 711  |                          |  | 70   | %        |  | $ | 2,238 |   |  | $ | 1,145 |   |  | 95   | %
|Interest expense                       |      |                            | (62   | )    |          |   | (324 | )                        |  | - 81 | %        |  |   | (121  | ) |  |   | (391  | ) |  | - 69 | %
|                                       |      |                            |       |      |          |   |      |                          |  |      |          |  |   |       |   |  |   |       |   |  |      |  
|Other income, net                      |      | $                          | 1,150 |      |          | $ | 387  |                          |  | 197  | %        |  | $ | 2,117 |   |  | $ | 754   |   |  | 181  | %
|                                       |      |                            |       |      |          |   |      |                          |  |      |          |  |   |       |   |  |   |       |   |  |      |  

Interest income increased to approximately $1.2 million and $2.2 million for the three and six
months ended June 30, 2007 from approximately $0.7 million and $1.1 million for the same periods in 2006. The increase resulted from higher average investment balances in 2007 compared to 2006 and higher returns in the current interest rate
environment.



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##### Table of Contents

Interest expense decreased to approximately $0.1 million for the three and six months ended June 30,
2007 from approximately $0.3 million and $0.4 million for the same periods in 2006. The decrease is due to expiration of our Silicon Valley Bank line of credit facility in May 2006.



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##### Table of Contents

Liquidity and Capital Resources

Since inception, we have financed our operations primarily through sales of our equity securities, contract payments received under our collaboration and license agreements and government grant awards, interest income
and equipment financing arrangements. As of June 30, 2007, we had received approximately $221.2 million from the sales of equity securities, approximately $128.3 million from contract payments received under our collaboration and license
agreements and government grant awards, approximately $20.6 million from interest income and approximately $14.9 million from equipment financing arrangements since inception. As of June 30, 2007, we had approximately $92.4 million in cash,
cash equivalents, restricted cash and investments, compared to approximately $64.1 million as of December 31, 2006. Our funds are currently invested in government agency and corporate obligations.

Cash used in operating activities was approximately $14.5 million for the six months ended June 30, 2007, compared to approximately $16.9 million
for the same period last year. Our net loss of approximately $8.0 million for the six months ended June 30, 2007 included non-cash revenue of $10.7 million related to the amortization of our up-front payment received from Pfizer in December
2006, and non-cash charges of approximately $2.6 million related to stock-based compensation, depreciation and amortization of investment premiums and discounts. Cash used in operating activities for the same period in 2006 was primarily used to
fund the net loss of $17.4 million, partially offset by non-cash charges of approximately $2.1 million related to stock-based compensation, depreciation and amortization of investment premiums and discounts.

Cash used in investing activities, excluding changes in our investments, was approximately $0.4 million for the six months ended June 30, 2007,
compared to approximately $0.2 million for the same period in 2006, due to higher capital spending for the six months ended June 30, 2007 as compared to the same period in 2006.

Cash provided by financing activities was approximately $42.9 million for the six months ended June 30, 2007, compared to approximately $23.6
million for the same period in 2006. Financing activities for the six months ended June 30, 2007 included cash inflows of approximately $0.7 million of equipment debt financing and approximately $43.0 million in proceeds from the sale of our
common stock related to our February 2007 registered direct public offering of 7,000,000 shares, stock option exercises and stock purchases made under our 2000 Employee Stock Purchase Plan, offset by cash outflows of approximately $0.8 million of
scheduled payments on new and existing debt. Financing activities for the six months ended June 30, 2006 included cash inflows of approximately $0.3 million of equipment debt financing and approximately $24.4 million in proceeds from the sale
of our common stock related to our April 2006 underwritten offering of 5,100,000 shares of common stock, stock option exercises and stock purchases made under our 2000 Employee Stock Purchase Plan, offset by approximately $1.1 million of scheduled
payments on new and existing debt.

We believe that our existing cash and investment securities and anticipated cash flow from our existing
collaboration with Roche will be sufficient to support our current operating plan into the first half of 2009. We have based this estimate on assumptions that may prove to be wrong. Our future capital uses and requirements depend on numerous
forward-looking factors, including those set forth in Part II, Item 1A Risk Factors.

In addition, we review from time to
time potential opportunities to expand our technologies or add to our portfolio of drug candidates. In the future, we may need further capital in order to acquire or invest in technologies, products or businesses.

We expect that additional financing will be required to fund operations. We expect to finance future cash needs through the sale of equity securities,
debt financings, additional collaboration or licensing arrangements or any combination of the foregoing or other arrangements. We may, pursuant to two shelf



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registration statements on Form S-3, sell approximately $26.0 million of our common stock, plus an additional approximately $5.2 million that we could sell
under immediately effective related registration statements.

We filed a resale registration statement covering the resale of 6,879,868
shares issuable pursuant to the committed equity financing facility (CEFF) with Kingsbridge and 285,000 shares underlying the warrant at an exercise price of $4.94 issued to Kingsbridge in connection with our CEFF. We have the
availability to sell under the CEFF up to $47.0 million of common stock in the future, subject to certain conditions.

We have no current
commitments to offer and sell any securities that may be offered or sold pursuant to the registration statements described above. Additional financing or collaboration and licensing arrangements may not be available when needed or, if available, may
not be on terms favorable to us or our stockholders. Insufficient funds may require us to delay, scale back or eliminate some or all of our research or development programs, to lose rights under existing licenses or to relinquish greater or all
rights to product candidates at an earlier stage of development or on less favorable terms than we would otherwise choose. Insufficient funds may preclude us from meeting the conditions required for the extension of credit and may adversely affect
our ability to operate as a going concern. If additional funds are obtained by issuing equity securities, substantial dilution to existing stockholders may result. In addition, see Part II, Item 1A Risk Factors.

Contractual Obligations

As of June 30, 2007,
our obligations and commitments to make future payments under contracts such as debt and lease agreements were as follows:



|                                |                |                       |                 |  |           |       |           |   |              |  |   |       |  |   |    
|------------------------------- | -------------- | --------------------- | --------------- |  | --------- | ----- | --------- | - | ------------ |  | - | ----- |  | - | ---
|                                |                | Payment Due by Period
|                                | Total          |                       | Less than1 Year |  | 1-3 Years |       | 4-5 Years |   | After 5Years
|                                | (in thousands)
|Equipment financing obligations |                | $                     | 2,295           |  | $         | 1,222 |           | $ | 958          |  | $ | 115   |  | $ |   
|Operating leases                |                |                       | 6,805           |  |           | 1,479 |           |   | 2,177        |  |   | 2,332 |  |   | 817
|                                |                |                       |                 |  |           |       |           |   |              |  |   |       |  |   |    
|Total contractual obligations   |                | $                     | 9,100           |  | $         | 2,701 |           | $ | 3,135        |  | $ | 2,447 |  | $ | 817
|                                |                |                       |                 |  |           |       |           |   |              |  |   |       |  |   |    



|Item 3: | Quantitative and Qualitative Disclosures About Market Risk
|------- | ----------------------------------------------------------

Interest Rate Risk

The primary objective of our investment activities is to preserve principal while
at the same time maximize the income we receive from our investments without significantly increasing risk. Some of the securities that we invest in may have market risk. This means that a change in prevailing interest rates may cause the principal
amount of the investments to fluctuate. To minimize this risk in the future, we intend to maintain our portfolio of cash equivalents and investments in a variety of securities, including commercial paper, money market funds, government and
non-government debt securities and investment-grade corporate obligations. Although changes in interest rates may affect the fair value of our portfolio and cause unrealized gains and losses, such gains and losses would not be realized unless the
investments were sold prior to maturity. Through our money managers, we maintain risk management control systems to monitor interest rate risk. The risk management control systems use analytical techniques, including sensitivity analysis. If market
interest rates were to increase by 100 basis points, or 1%, at June 30, 2007 and December 31, 2006 rates, the fair value of our portfolio on that date would decline by approximately $0.3 million and $0.1 million, respectively.



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We did not hold any derivative instruments as of June 30, 2007, and we have never held such
instruments in the past. In addition, we had outstanding debt, consisting of borrowings under equipment financings, of approximately $2.1 million as of June 30, 2007, with a range of annual interest rates from 6.31% to 9.09%.



|Item 4: | Controls and Procedures
|------- | -----------------------

Evaluation of Disclosure Controls and Procedures. Based on their evaluation as of June 30, 2007, our chief executive officer and chief financial officer have concluded that the Companys disclosure controls and procedures
(as defined in Rules 13a-15(e) and 15d-15(e) under the Securities Exchange Act of 1934, as amended (the Exchange Act)) were effective.

Changes in Internal Controls over Financial Reporting.There were no changes in our internal controls over financial reporting (as defined in Rule 13a-15(f) under the Exchange Act) during the quarter ended June 30, 2007 that
have materially affected, or are reasonably likely to materially affect, our internal control over financial reporting.

Limitations on
the Effectiveness of Controls. The Companys management, including our chief executive officer and chief financial officer, does not expect that our disclosure controls and procedures or our internal controls will prevent all error and all
fraud. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Because of the inherent limitations in all control systems, no evaluation of
controls can provide absolute assurance that all control issues and instances of fraud, if any, within the Company have been detected.



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PART II. OTHER INFORMATION



|Item 1A: | Risk Factors
|-------- | ------------

You should
carefully consider the risks described below, together with all of the other information included in this report, in considering our business and prospects. The risks and uncertainties described below are not the only ones facing us. Additional
risks and uncertainties not presently known to us or that we currently deem immaterial also may impair our business operations. The occurrence of any of the following risks could harm our business, financial condition or results of operations.

We have marked with an asterisk (*) those risk factors below that reflect substantive changes from the risk factors included in
our Annual Report on Form 10-K filed with the Securities and Exchange Commission on March 16, 2007.

We have a history of net losses and may
never become profitable.*

We commenced operations in 1996 and are still developing our product candidates. We have not commercialized
any products, and we have incurred significant losses to date. We have a history of net losses and as of June 30, 2007 we had an accumulated deficit of approximately $168.4 million. To date, our revenues have been primarily from partnering
arrangements and government grant awards. Our expenses have consisted principally of costs incurred in research and development and from general and administrative costs associated with our operations. We expect our expenses to increase and to
continue to incur operating losses for at least the next several years as we continue our research and development efforts for our product candidates and research programs. The amount of time necessary to successfully commercialize any of our
product candidates is long and uncertain, and successful commercialization may not occur at all. As a result, we may never become profitable.

We expect
that additional financing will be required, and an inability to obtain the capital necessary to fund our operations on acceptable terms or at all would threaten the continued operation of our business.*

We expect that additional financing will be required to fund operations. We do not know whether additional financing will be available when needed, or
that, if available, we will obtain financing on favorable terms. We have consumed substantial amounts of cash to date and expect to incur significant operating expenditures over the next several years as we continue to advance our product candidates
into and through clinical trials.

We may raise additional financing through public or private equity offerings, debt financings,
additional collaboration or licensing arrangements, government grant awards or any combination of the foregoing or other arrangements. To the extent we raise additional capital by issuing equity securities, our stockholders may experience dilution.
Debt financing, if available, may subject us to restrictive covenants and significant interest costs. To the extent that we raise additional funds through collaboration and licensing arrangements, it may be necessary to relinquish some rights to our
technologies, product candidates or marketing territories.

If we are unable to raise sufficient funds when needed, we may be required to
delay, scale back or eliminate some or all of our research or development programs; lose rights under existing licenses; or relinquish greater or all rights to product candidates at an earlier stage of development or on less favorable terms than we
would otherwise choose. Insufficient funds may adversely affect our ability to operate as a going concern. See Part I, Item 2 Managements Discussion and Analysis of Financial Condition and Result of Operations.



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We believe that our existing cash and investment securities and our anticipated cash flow from our
existing collaboration with Roche will be sufficient to support our current operating plan into the first half of 2009. We may choose to obtain additional financing from time to time. We have based this estimate on assumptions that may prove to be
wrong. Our future capital uses and requirements depend on numerous forward-looking factors, including the following:



| |  |  | our ability to establish any new partnering arrangements, our rights and obligations under any new partnering arrangements and our ability to generate revenues
under any new partnering arrangements;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | the extent to which clinical and other development activities are funded or conducted by our current partners, Roche, Pfizer and the NCI;
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------



| |  |  | our ability to sell shares of our common stock or other securities, including under our CEFF with Kingsbridge;
| | - |  | --------------------------------------------------------------------------------------------------------------



| |  |  | the progress, success and costs of preclinical testing and clinical trials of our product candidates;
| | - |  | -----------------------------------------------------------------------------------------------------



| |  |  | any acceleration or expansion of our clinical development plans;
| | - |  | ----------------------------------------------------------------



| |  |  | our ability to maintain or extend our existing partnering arrangements with Roche, Pfizer and the NCI;
| | - |  | ------------------------------------------------------------------------------------------------------



| |  |  | the progress, number and costs of our research programs;
| | - |  | --------------------------------------------------------



| |  |  | the costs and timing of obtaining, enforcing and defending patent and other intellectual property rights;
| | - |  | ---------------------------------------------------------------------------------------------------------



| |  |  | any need to obtain licenses to additional patents or other intellectual property in order to use, import, manufacture, market or sell our product candidates;
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | any need to expand our manufacturing capabilities; and
| | - |  | ------------------------------------------------------



| |  |  | expenses associated with any possible future litigation.
| | - |  | --------------------------------------------------------

Our committed equity financing facility with Kingsbridge may not be available to us if we elect to make a draw down, may require us to make additional blackout or other payments to Kingsbridge and may result in dilution to
our stockholders.*

In July 2006, we entered into a CEFF with Kingsbridge. The CEFF entitles us to sell and obligates Kingsbridge to
purchase, from time to time through September 25, 2009, shares of our common stock for cash consideration up to an aggregate of $47.0 million as of June 30, 2007, subject to certain conditions and restrictions. Kingsbridge will not be
obligated to purchase shares under the CEFF unless certain conditions are met, which include a minimum price for our common stock; the accuracy of representations and warranties made to Kingsbridge; compliance with laws; and the effectiveness of a
registration statement registering for resale the shares of common stock to be issued in connection with the CEFF. In addition, among other termination rights, Kingsbridge is permitted to terminate the CEFF by providing written notice to us within
10 trading days after it obtains actual knowledge that an event has occurred resulting in a material and adverse effect on our business, operations, properties or financial condition. If we are unable to access funds through the CEFF, or if
Kingsbridge terminates the CEFF, we may be unable to access capital on favorable terms, or at all.

We are entitled, in certain
circumstances, to deliver a blackout notice to Kingsbridge to suspend the use of the resale registration statement and prohibit Kingsbridge from selling shares under the resale registration statement for a certain period of time. If we deliver a
blackout notice in certain circumstances, or if the resale registration statement is not effective in circumstances not permitted by the agreement, then we must make a payment to Kingsbridge, or issue Kingsbridge additional shares in lieu of this
payment, calculated on the basis of the number of shares purchased by Kingsbridge in the most recent draw down and held by Kingsbridge immediately prior to the blackout period and the change in the market price of our common stock during the period
in which the use of the registration statement is suspended. If the trading price of our common stock declines during a suspension of the resale registration statement, the blackout or other payment could be significant.



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Should we sell shares to Kingsbridge under the CEFF, or issue shares in lieu of a blackout payment, it
will have a dilutive effect on the holdings of our current stockholders and may result in downward pressure on the price of our common stock. If we draw down under the CEFF, we will issue shares to Kingsbridge at a discount of up to 10% from the
volume weighted average price of our common stock. If we draw down amounts under the CEFF when our share price is decreasing, we will need to issue more shares to raise the same amount than if our stock price was higher. Issuances in the face of a
declining share price will have an even greater dilutive effect than if our share price were stable or increasing and may further decrease our share price.

If we are unable to recruit and retain skilled employees and consultants, we may not be able to successfully operate our business.*

Retaining our current management and other employees and recruiting qualified personnel to perform future research, manufacturing, development and other work will be critical to our success. None of our employees has employment commitments
for any fixed period of time and could leave our employment at will. Competition is intense among biotechnology, pharmaceutical and healthcare companies, universities and non-profit research institutions for experienced scientists and other
personnel, and we may not be able to retain or recruit sufficient skilled personnel on acceptable terms to allow us to develop our product candidates and research programs, which would likely have an adverse effect on our business.

If our current partnering arrangements are unsuccessful or if conflicts develop with our partners, our research and development efforts could be delayed, curtailed or
terminated, our revenues could significantly decrease and our operations may be adversely affected.

We have a collaboration agreement
with Roche in the field of epothilones and a license agreement with Pfizer for our motilin agonist program. We also have collaborations with, or have licenses to technology and compounds from, several research groups, including Sloan-Kettering in
the field of epothilones, the NCI in the field of geldanamycin analogs and Stanford University in the field of polyketide technology. The agreements permit our collaborators or licensors to terminate the agreements under certain circumstances. Our
collaboration agreements with the NCI are currently set to expire in October 2007. We may not be able to maintain or extend our collaborations or license agreements on acceptable terms, if at all. If we do not maintain, extend or replace our
collaboration with Roche or our license to Pfizer, the research and development efforts for our product candidates could be delayed, our revenues would significantly decrease and our operations could be adversely affected. If we are unable to
maintain our research collaborations, including our collaboration agreements with the NCI, or if our in-license agreements are terminated, our research and development efforts could be delayed, curtailed or terminated or we could lose our rights to
use the licensed technology and compounds.

We control neither the amount nor timing of resources that our partners devote to our programs
or potential products, nor the scope, content and timing of the efforts that they conduct or permit. As a result, we do not know if our partners will dedicate sufficient resources or if the development or commercialization efforts by our corporate
partners will be successful. We also do not know if the development or commercialization efforts by our partners will be the same as those we would choose to devote if we solely controlled the development and commercialization of our programs and
product candidates. In particular, in our collaboration with Roche, we do not control the amount and timing of resources that Roche devotes to the epothilone program, and we do not control the scope, content and timing of the preclinical studies,
clinical trials and other development efforts that Roche conducts or permits under the program, including for KOS-1584. In our collaboration with the NCI, we do not control the selection, conduct, timing and resources provided to clinical trials of
tanespimycin or alvespimycin



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sponsored by the NCI. For these reasons, we may choose to undertake product development efforts that are within the scope of our collaborations at our own
expense. Under our license agreement with Pfizer, Pfizer controls all development, regulatory and commercial activities related to our motilin agonist program. We also do not know whether our current partners or future partners, if any, might pursue
alternative technologies or develop alternative products either on their own or in collaboration with others, including our competitors, as a means for developing treatments for the diseases targeted by the partnering arrangements with us. In
addition, business combinations or significant changes in a partners business strategy may adversely affect its willingness or ability to continue the partnering arrangement with us.

Failure by our partners to develop or commercialize a compound or product for which they have rights from us could materially harm our business,
financial condition and results of operations. For example, if Roche does not successfully develop and commercialize a product from our epothilone program, we may not receive any future milestone payments and will not receive any royalties under our
collaboration with Roche.

If our partners fail to conduct research, development or commercialization activities successfully and in a
timely manner or if they or our licensors breach or terminate their agreements with us, the development or commercialization of the affected product candidates, technology or research program could be delayed or terminated. If any of our existing
partnering arrangements are terminated, we may be required to seek new partners or to undertake product development and commercialization at our own expense. This may limit the number of product candidates we will be able to develop and
commercialize, significantly increase our capital requirements and reduce the likelihood of successful product introduction. Disputes might also arise with partners or licensors concerning rights to particular compounds or technologies. If we are
unable to resolve these disputes in our favor, we could lose our rights to use those compounds or technologies.

If we fail to enter into new partnering
arrangements in the future, our business and operations would be negatively impacted.

Our strategy depends upon the formation and
sustainability of multiple collaborative arrangements and license agreements with third parties. We expect to rely on these arrangements for not only financial resources, but also for expertise that we expect to need in the future relating to
clinical trials, manufacturing, sales and marketing, and for license and technology rights. Although we have established collaborative arrangements and various license agreements, we do not know if we will be able to establish additional
arrangements on favorable terms, or whether current or any future partnering arrangements will ultimately be successful. There have been, and may continue to be, a significant number of business combinations among pharmaceutical companies that have
resulted, and may continue to result, in a reduced number of potential future partners, which may limit our ability to find partners who will work with us in developing and commercializing our product candidates. If we do not enter into new
collaborative agreements, in particular, a collaborative arrangement for the further development of our Hsp90 inhibitor program, we may be required to curtail, suspend, delay or terminate research and development programs, including planned clinical
trials for our product candidates, and therefore our ability to generate revenues from these programs will be adversely affected. Our ability to start new research and development programs may also be materially harmed.

Our potential products are in development, and substantial additional effort and expense will be necessary for further development.

Our product candidates are in various stages of research and development. We may not be able to develop products that prove to be safe and effective, meet
applicable regulatory standards, are capable of being manufactured at reasonable costs or can be marketed successfully. All of the potential products that we are currently developing will require significant development and investment, including
extensive



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clinical testing, before we can submit any application for regulatory approval. For example, we expect to incur significant expenses in connection with our
TIME clinical trial program for tanespimycin in combination with Velcade for multiple myeloma. These expenses will be even higher to the extent we are required to pay for patients medical or other expenses, including, for example, the cost of
Velcade for these studies.

Our products must satisfy rigorous standards of safety and efficacy before they can be approved by the FDA and
international regulatory authorities for commercial use. We will need to conduct significant additional research and clinical trials before we can determine if our products are sufficiently safe and effective to file with the FDA and other
regulatory agencies for product approval. Clinical trials are expensive and time-consuming, and therefore, significant amounts of money will need to be spent testing our products.

In addition, significant time and investment will be required to try to develop manufacturing processes for our products so that they are economical to
manufacture on a commercial scale and satisfactory to the FDA and other governmental authorities.

The progress and results of our preclinical and
clinical testing are highly uncertain.*

We must provide the FDA and foreign regulatory authorities with clinical data that demonstrate
the safety and efficacy of our products before they can be approved for commercial sale. As a result, commercialization of our product candidates depends upon successful completion of preclinical and clinical trials. Preclinical testing and clinical
development are long, expensive and uncertain processes. It may take us a number of years to complete our testing, and failure can occur at any stage of testing. For example, in October 2005, we discontinued a Phase 2 clinical study of our
first-generation epothilone, KOS-862, in hormone-refractory prostate cancer because the study did not meet the primary objective of tumor marker response in the first stage of a two stage clinical trial, and we previously discontinued clinical
studies of KOS-862 in patients with non-small cell lung cancer and colorectal cancer. We could experience similar failures in other current or future clinical testing of our product candidates.

Success in preclinical testing and early clinical trials does not ensure that later clinical trials will be successful, and interim results of trials do
not necessarily predict final results. A number of companies in the pharmaceutical and biotechnology industry, including Kosan, have suffered significant setbacks in advanced clinical trials, even after reporting promising results in earlier trials.
Also, preclinical and clinical data can be interpreted in different ways, which could delay, limit or prevent further testing or regulatory approval.

We do not know whether clinical trials of our product candidates will begin on time or whether any of our clinical trials will be completed on schedule, or at all. We may plan and initiate clinical trials before final
data from earlier studies have been collected and analyzed because it takes a significant amount of time and effort to plan and initiate clinical trials and because of the length of time it takes to successfully develop a product candidate.
Consequently, we may need to modify, suspend, cancel or terminate clinical trials based on results from earlier studies. We also do not know whether clinical trials will indicate that an earlier-stage compound or formulation will be more appropriate
for clinical and commercial development than a compound or formulation that is at a later stage of clinical development, and therefore result in extended timelines as well as increased development costs. For example, Roche and we decided in February
2007 to cease development of KOS-862 in favor of further development of our second-generation epothilone product candidate, KOS-1584, and in May 2007, we decided to change the formulation for tanespimycin to our injectable suspension formulation,
which delayed the commencement of clinical trials. Negative or inconclusive results or adverse medical events during a clinical trial could cause a clinical trial to be suspended, terminated or repeated. Certain of the clinical trials of our product



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candidates are or may in the future be designed to include two stages, with the decision whether to proceed to the second stage dependent on results obtained
in the first stage. Failure to achieve predetermined response rates as defined in the protocol may result in the decision not to proceed into the second stage of the trial.

We have multiple product candidates in human clinical trials for the treatment of cancer. Anticancer drugs frequently have a narrow therapeutic window
between efficacy and toxicity. If unacceptable toxicity is observed in a clinical trial, the trial may be terminated at an early stage. For example, in June 2004, we discontinued a Phase 2 clinical study of KOS-862 in colorectal cancer due to
unanticipated cumulative drug toxicities in patients who had previously been treated with the cancer treatment oxaliplatin. We also observed a higher incidence of adverse events resulting in patient withdrawal in our Phase 2 clinical study of
KOS-862 in hormone-refractory prostate cancer than in our Phase 2 non-small cell lung cancer trial. We cannot predict whether any clinical trials of our product candidates will demonstrate toxicity issues or adverse events resulting in a significant
patient withdrawal.

Completion of clinical trials may take several years or more. The length of time generally varies substantially
according to the type, complexity, novelty and intended use of the product candidate. Our clinical trials may be suspended at any time if we, our collaborators, the FDA, or other regulatory authorities believe the patients participating in our
studies are exposed to unacceptable health risks or that study protocols or patient informed consents should be amended to reflect additional health risks, additional testing procedures or other changes. For example, in September 2005, we
temporarily suspended enrollment in our tanespimycin and alvespimycin clinical trials in connection with a request by the FDA to amend the protocols and patient informed consents for those trials. We provided amended protocols and informed consents
for those clinical trials to the FDA in response to certain of its requested changes, and we resumed enrollment in the clinical trials. The FDA may require further changes to these protocols and informed consents, which may have a material adverse
effect on the timing of, and our ability to conduct, the tanespimycin and alvespimycin clinical trials.

Our ability to commence or timely
complete clinical trials may be adversely affected by many factors, including:



| |  |  | ineffectiveness of the study compound, or perceptions by physicians that the compound is not effective for a particular indication;
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------



| |  |  | inability to manufacture sufficient quantities of compound for use in clinical trials;
| | - |  | --------------------------------------------------------------------------------------



| |  |  | a failure to obtain approval from the FDA, other regulatory authorities or an investigational sites institutional review board to conduct a clinical trial;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | inability to reach agreement with a sufficient number of investigational sites to conduct a study;
| | - |  | --------------------------------------------------------------------------------------------------



| |  |  | inability to obtain product liability insurance, including clinical trial insurance, that meets the requirements of a location outside the U.S.
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | the number of patients required, slower than expected rate of patient recruitment or inability to recruit a sufficient number of patients;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | the occurrence of adverse medical events, including death, during a clinical trial, even if caused by the advanced status of patients disease or medical
problems that are not related to our product candidates;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | inconclusive or negative results from a clinical trial;
| | - |  | -------------------------------------------------------



| |  |  | competing clinical trials in the same or similar indication;
| | - |  | ------------------------------------------------------------



| |  |  | third-party clinical investigators failing to perform our clinical trials on our anticipated schedule or consistent with a clinical trial protocol, and other
third-party organizations not performing data collection and analysis in a timely or accurate manner; and
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | a decision by the FDA or other regulatory authorities to require suspension or modification of a clinical trial.
| | - |  | ----------------------------------------------------------------------------------------------------------------



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Our product development costs will increase if we have delays in testing or approvals, if we need to
perform more or larger clinical trials than planned or if our clinical trials include more expensive testing or other procedures than planned. If the delays are significant, our financial results and the commercial prospects for our products will be
harmed, and our ability to become profitable will be adversely affected. If any clinical trials of our product candidates are not successful, our business, financial condition and results of operations will be harmed.

If we are not able to obtain required regulatory approvals, we will not be able to commercialize our product candidates, and our ability to generate revenue will be
materially impaired.

Our product candidates and the activities associated with their development, manufacture and commercialization are
subject to comprehensive regulation by the FDA and other regulatory agencies in the United States, and by comparable authorities in other countries. Our products may not be commercialized unless and until our partners or we obtain regulatory
approval from the FDA or foreign governmental authorities to do so. The process of obtaining regulatory approvals is expensive, often takes many years, if approval is obtained at all, and can vary substantially based upon the type, complexity,
novelty, safety and efficacy of the product candidates involved. We have not received regulatory approval to market any of our product candidates in any jurisdiction and, although our personnel have experience from working at other companies, we as
a company have no experience in preparing and filing the applications necessary to gain regulatory approvals to commercialize our products. This lack of experience may impede our ability to obtain FDA or other foreign regulatory approvals to
commercialize our products in a timely manner, if at all.

Changes in the regulatory approval policy during the development period, changes
in or the enactment of additional regulations or statutes, or changes in regulatory review for each submitted product application, may cause delays in the approval or rejection of an application for regulatory approval. Furthermore, the approval
procedure and the time required to obtain approval varies among countries and can involve additional testing beyond that required by the FDA. Approval by one regulatory authority does not ensure approval by regulatory authorities in other
jurisdictions.

The FDA and other regulatory authorities have substantial discretion in the approval process and may refuse to accept any
application or may decide that our data is insufficient for approval and require additional preclinical, clinical or other studies or modifications to the manufacturing processes or facilities or quality control procedures for our product
candidates. For example, we plan to commence during 2007 late-stage clinical trials of tanespimycin, and, if the results are favorable, we believe these clinical trials will support the filing of a New Drug Application (NDA) with the FDA
for the treatment of multiple myeloma. However, the FDA or other regulatory authorities may require additional data prior to accepting or approving an application for marketing approval for tanespimycin or other product candidates, which would
result in delays in potential FDA or other regulatory authority approval and additional costs, either of which may be too significant to continue development of tanespimycin or other product candidates. This risk is further compounded by any
changes during development of a product candidate, such as changes in manufacturing processes, formulations or dosing regimens.

Any
clinical trial may fail to produce results satisfactory to the FDA or other regulatory authorities. For example, we currently conduct, and expect to conduct in the future, clinical trials for our product candidates in countries outside of the United
States. The FDA or other regulatory authorities may reject data from clinical trials conducted in other countries if they are not conducted in accordance with applicable regulatory standards and procedures.



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We do not know whether clinical trials for our product candidates will demonstrate safety and efficacy
sufficient to obtain the requisite regulatory approvals or will result in marketable products. Our or our partners failure to adequately demonstrate the safety and efficacy of our products under development will prevent receipt of FDA and
foreign approvals and, ultimately, commercialization of our products.

We rely on third parties to conduct, supervise or monitor our clinical trials,
and those third parties may not perform satisfactorily.*

We do not have the ability to independently conduct clinical trials for our
products, and we rely on third parties such as our partners, contract research organizations, laboratory testing companies, medical institutions and clinical investigators to conduct, supervise or monitor our clinical trials. We have less control
over the timing and other aspects of these clinical trials than if we conducted them entirely on our own.

We are supervising and
monitoring on our own certain Phase 2 and Phase 2/3 clinical trials in our tanespimycin and alvespimycin development program. In 2007, we opened a broad, multinational registrational program for the treatment of multiple myeloma. The
tanespimycin in myeloma evaluation, or TIME program, includes a Phase 2/3 trial and a Phase 3 trial. We have not conducted a clinical trial that was intended to form the basis of an NDA (or foreign equivalent thereto) filing. Additionally, the TIME
program, and our planned Phase 2 trials with alvespimycin, are mulitnational trials. We have not conducted large clinical trials outside of North America. Consequently, we may not have the necessary capabilities to successfully execute and
complete these planned clinical trials in a manner that supports approval of our clinical candidates for their target indications in a timely manner, or at all Failure to commence or complete, or delays in our planned clinical trials would prevent
us from commercializing products, and thus seriously harm our business.

We also rely on Pfizer to conduct all clinical trials for our
motilin agonist program, including KOS-2187, and the NCI to conduct certain clinical trials for tanespimycin and alvespimycin, and we may rely on Roche to conduct future clinical trials for KOS-1584. We may rely on future partners to conduct
clinical trials for our product candidates. If any of these third parties do not successfully carry out their obligations or meet expected deadlines, clinical trials may be extended, delayed, suspended or terminated, data generated from clinical
trials may not be acceptable to the FDA or other regulatory authorities and our product candidates may not receive regulatory approval or be successfully commercialized.

We may not be able to obtain or maintain orphan drug exclusivity for our product candidates.

Some
jurisdictions, including Europe and the United States, may designate drugs for relatively small patient populations as orphan drugs. The FDA and the European Medicines Agency have granted orphan drug status to tanespimycin for the treatment of
multiple myeloma and chronic myelogenous leukemia. Orphan drug designation does not convey any advantage in, or shorten the duration of, the regulatory review and approval process, but does make the product eligible for orphan drug exclusivity and,
in the United States, specific tax credits. Generally, if a company receives the first marketing approval for a product with an orphan drug designation in the clinical indication for which it has such designation, the product is entitled to orphan
drug exclusivity. Orphan drug exclusivity means that another application to market the same drug for the same indication may not be approved, except in limited circumstances, for a period of up to ten years in Europe (reviewable after six years),
and for a period of seven years in the United States. This exclusivity, however, could block the approval of tanespimycin for the treatment of multiple myeloma or chronic myelogenous leukemia in the United States or Europe if a competitor obtains
approval before us of a product containing tanespimycin for these specific indications. Even if we obtain orphan drug exclusivity for any of our product candidates, we may not be able to maintain it. For example, if a competitive product is shown to
be clinically superior to our product, any orphan drug exclusivity we have obtained will not block the approval of such competitive product.



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Even if any of our product candidates receives regulatory approval, we may still face significant development and
regulatory difficulties.

Even if the FDA or other regulatory authorities approves a product candidate, the approval may impose
significant restrictions on the indicated uses, conditions for use, labeling, advertising, promotion, marketing and/or production of such product, and may impose ongoing requirements for post-approval studies, including additional research and
development and clinical trials. In addition, regulatory agencies subject a product, its manufacturer and the manufacturers facilities to continual review and periodic inspections. If a regulatory agency discovers previously unknown problems
with a product, including adverse events of unanticipated severity or frequency, or problems with the facility where the product is manufactured, a regulatory agency may impose restrictions on that product, our partners or us, including requiring
withdrawal of the product from the market.

If our partners, any of our product candidates that become approved for marketing by a
regulatory authority or we fail to comply with applicable regulatory requirements, a regulatory authority may take various actions, including:



| |  |  | issuing warning letters;
| | - |  | ------------------------



| |  |  | imposing civil or criminal penalties;
| | - |  | -------------------------------------



| |  |  | suspending regulatory approval;
| | - |  | -------------------------------



| |  |  | refusing to approve pending applications or supplements to approved applications filed by us or our partners;
| | - |  | -------------------------------------------------------------------------------------------------------------



| |  |  | imposing restrictions on operations, including costly new manufacturing requirements; or
| | - |  | ----------------------------------------------------------------------------------------



| |  |  | seizing or detaining products or requiring a product recall.
| | - |  | ------------------------------------------------------------

Any inability to protect our proprietary technologies could significantly harm our business and ability to successfully commercialize product candidates.

Our commercial success will depend in part on our ability to obtain patents and maintain adequate protection of other intellectual property for our technologies and product candidates in the United States and other
countries and prevent others from infringing our proprietary rights. If we are unable to adequately protect our intellectual property, competitors may be able to use our technologies and erode or negate any competitive advantage we may have.
Intellectual property laws vary from country to country, and the laws of a particular country may afford less intellectual property protection than another country.

Any patents that we or our partners own or license from third parties may not provide protection against competitors. We will be able to protect our proprietary rights from unauthorized use by third parties only to
the extent that our proprietary technologies and products are covered by valid and enforceable patents or are effectively maintained as trade secrets. However, our patent positions, as well as the patent positions of biotechnology companies and
other third parties, involve complex legal and factual questions, and, therefore, we cannot predict with certainty whether our patent applications will be allowed or any resulting patents will be valid and enforceable. Further, our patents or patent
applications or those of our licensors could be placed into interference, and we may lose our rights in such patents or applications. Patents may be challenged, even by our partners, held unenforceable, invalidated or circumvented. Certain of our
current exclusive license agreements restrict, and any future exclusive license agreements may restrict, our rights under patents and patent applications to certain fields of use, and therefore, we may not have the ability to prevent competitors
from developing and commercializing our product candidates or technologies in fields of use not covered by our exclusive license agreements.



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The degree of future protection for our proprietary rights is uncertain, and we cannot ensure that:



| |  |  | we or our licensors were the first to make the inventions covered by each of our patents or pending patent applications;
| | - |  | ------------------------------------------------------------------------------------------------------------------------



| |  |  | we or our licensors were the first to file patent applications for these inventions;
| | - |  | ------------------------------------------------------------------------------------



| |  |  | others will not independently develop similar or alternative technologies or duplicate any of our technologies;
| | - |  | ---------------------------------------------------------------------------------------------------------------



| |  |  | any of our or our licensors or partners pending patent applications will result in issued patents;
| | - |  | ----------------------------------------------------------------------------------------------------



| |  |  | any of our or our licensors or partners patents will be valid or enforceable;
| | - |  | -------------------------------------------------------------------------------



| |  |  | any patents issued to us or our licensors or collaborators will provide a basis for commercially viable products, will provide us with any competitive advantages or
will not be challenged by third parties;
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | we will develop additional proprietary technologies that are patentable; or
| | - |  | ---------------------------------------------------------------------------



| |  |  | the patents of others will not have an adverse effect on our business.
| | - |  | ----------------------------------------------------------------------

We apply for patents covering our technologies, product candidates, formulations and uses thereof, as we deem appropriate. However, we may fail to apply
for patents on important technologies or products in a timely fashion or at all. Our existing patents and any future patents we obtain may not be sufficiently broad to prevent others from practicing our technologies or from developing competing
products and technologies. For example, tanespimycin, the active pharmaceutical ingredient in the most advanced product candidate in our Hsp90 inhibitor program, was originally disclosed in a now-expired third party patent. Consequently, others can
develop products containing tanespimycin. We are aware of at least three other companies that have been developing product candidates containing or based on tanespimycin, and these companies have filed patent applications relating to their products
in development. Other competitors may currently be developing, or may in the future develop, products containing or based on tanespimycin. In addition, we generally are unable to control the patent prosecution of technology that we license from
others to the same degree as we would for our own technology. Further, some of our patents and patent applications for our motilide program have been assigned to Pfizer, and additional patents and patent applications may in the future be assigned to
Pfizer or other current or future partners. We generally are unable to control the filing, prosecution and maintenance of patent rights that we assign to partners to the same degree as we would if we maintained ownership of them.

In addition to patents, we rely on trade secrets and proprietary know-how. We have taken measures to protect our confidential information and trade
secrets. However, these measures may not provide adequate protection. We seek to protect our confidential information and trade secrets by entering into confidentiality agreements with employees, collaborators, consultants and others. Nevertheless,
parties may breach these agreements or competitors may otherwise obtain or independently develop our trade secrets.

Interference, opposition or similar
proceedings relating to our patents and patent applications are costly, and an unfavorable outcome could prevent us from commercializing our product candidates.*

We are aware of a significant number of patents and patent applications relating to aspects of our technologies and compounds filed by, and issued to, other parties. Others have filed patent applications or have been
granted patents claiming inventions also claimed or licensed by us, and we may have to participate in an interference or other proceeding before a patent agency or court to determine priority of invention or which party was first to invent and,
thus, has the right to a patent for these inventions. We believe one or more interferences may be declared between patents and applications we own or have exclusively licensed and patents and applications owned by Abbott Laboratories or Biotica
Technologies



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Ltd. relating to erythromycin polyketide synthase, or PKS, genes, methods for altering PKS genes, and erythromycin analogs. In April 2005, a hearing was held
at the European Patent Office to address an opposition filed by Biotica to one of our exclusively licensed patents related to the recombinant production of polyketides. The European Patent Office maintained, or upheld, the patent, but with narrowing
amendments. Both parties appealed this decision. In June 2007, the board of appeals of the European Patent Office held a hearing on the appeal and revoked the patent in its entirety. The revocation of this European patent is not expected to impact
our freedom to operate.

We are the exclusive licensee to two patent rights claiming our epothilone clinical candidate KOS-1584. One is a
granted patent and one is a patent application, each is owned by a different entity. In June 2007, the US Patent and Trademark Office declared an interference between the patent and patent application.

A proceeding or a lawsuit involving an interference or opposition could result in substantial cost to us even if the outcome is favorable, and if the
outcome is unfavorable, we could be required to license the other partys rights, on terms that may be unfavorable to us, or cease using the technology. We could incur additional cost because we are licensed under both partys patent
rights. An interference or opposition may also result in loss of claims based on patentability grounds raised in the interference or opposition. Although patent and intellectual property disputes in the biotechnology area are often settled through
licensing or similar arrangements, costs associated with these arrangements may be substantial and could include ongoing royalties. Furthermore, we cannot be certain that a license would be available to us on satisfactory terms, if at all. Companies
and others developing products that could compete with our product candidates, such as Novartis AG in the area of potential epothilone products, may be particularly unwilling to grant us a license at any price. If we are not able to obtain necessary
licenses, we may not be able to manufacture or commercialize our product candidates, which would materially harm our business, financial condition and results of operations.

Claims by third parties of intellectual property infringement would require us to spend time and money and could deprive us of valuable rights needed to develop or commercialize our products.

Our commercial success depends significantly on not infringing the patents and proprietary rights of other parties and not breaching any licenses that we
have entered into with regard to our technologies and products. Other parties may currently or in the future possess intellectual property rights covering product candidates that we or our partners are developing or desire to develop; methods of
treatment or administration involving our product candidates; formulations of our product candidates; and genes, gene fragments, cell lines, compounds and other technologies we use or may wish to use. Any infringement of patent rights or violation
of other proprietary rights may require us to obtain a license from another party, forego product development or commercialization or face lawsuits or other claims.

The biotechnology industry is characterized by extensive litigation regarding patents and other intellectual property rights. We are aware of patents and published patent applications that, if valid, and if we are
unsuccessful in circumventing or acquiring the rights to these patents, may block our ability to commercialize products based on the product candidates that we are developing or pursue our PKS gene manipulation and production technologies. We cannot
be sure that other parties have not filed for or obtained relevant patents that could affect our ability to obtain patents or operate our business. Others, including our partners, may challenge our patent or other intellectual property rights or sue
us for patent infringement, misappropriation of their intellectual property rights or breach of license agreements. We may be required to commence legal proceedings to resolve our patent or other intellectual property rights. An adverse
determination in any litigation or administrative proceeding to which we may become a party could subject us to significant liabilities, result in our patents being deemed invalid, unenforceable or revoked, require us to license disputed rights from
others or to cease using the disputed technology. In addition, our involvement in any of these proceedings may cause us to incur substantial costs and result in diversion of management and technical personnel.



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Other parties may obtain patents in the future and claim that our products or the use of our technologies
infringes these patents or that we are employing their proprietary technology without authorization. We could incur substantial costs and diversion of management and technical personnel in defending ourselves against any claims that the use of our
technologies infringes any patents, defending ourselves against any claim that we are employing any proprietary technology without authorization or enforcing our patents against others. Furthermore, parties making claims against us may be able to
obtain injunctive or other equitable relief that could effectively block our ability to develop, commercialize and sell products, and could result in the award of substantial damages against us. In the event of a successful claim of infringement
against us, we may be required to:



| |  |  | pay substantial damages;
| | - |  | ------------------------



| |  |  | stop producing certain products and using certain methods;
| | - |  | ----------------------------------------------------------



| |  |  | develop non-infringing products and methods; and
| | - |  | ------------------------------------------------



| |  |  | obtain one or more licenses from other parties.
| | - |  | -----------------------------------------------

We may not be able to obtain licenses from other parties at a reasonable cost, or at all. If we are not able to obtain necessary licenses at a reasonable cost or at all, we could encounter substantial delays in
product introductions while we attempt to develop alternative methods and products, which we may not be able to accomplish. Litigation or the failure to obtain licenses could prevent us from manufacturing or commercializing products and could
materially harm our business, financial condition and results of operation.

Manufacturing difficulties could delay or preclude commercialization of our
products and substantially increase our expenses.*

We currently use contract manufacturers to make tanespimycin and alvespimycin active
pharmaceutical ingredients. We currently formulate the final drug product for tanespimycin injectable suspension at our facilities and expect to use a contract manufacturer to do so in the future. We currently use a contract manufacturer to
formulate the final drug product for alvespimycin. The NCI is responsible for formulating the final drug product for alvespimycin and for formulating tanespimycin for their own trials under our cooperative research and development agreements, or
CRADAs. We maintain a limited inventory of tanespimycin and alvespimycin drug product at our facilities in Hayward, California, and we also maintain a limited inventory at the facilities of an outside contractor. The NCI is not obligated to maintain
an inventory of either the active pharmaceutical ingredient or formulated drug product for alvespimycin or the formulation of tanespimycin being studied by the NCI under our CRADA. In our epothilone program, we rely on contract manufacturers for the
active pharmaceutical ingredient for KOS-1584. Drug product for KOS-1584 is formulated by an outside contractor. We maintain limited inventories of formulated drug product for KOS-1584 at our facilities in Hayward, California and at the facilities
of an outside contractor.

If any of our or our contract manufacturers manufacturing or inventory facilities encounter delays, are
destroyed or otherwise become unavailable to us, then the clinical development of our product candidates or submissions for their regulatory approval, and therefore commercialization, could be delayed or precluded. Adverse effects would be
particularly acute if problems arise with our sole sourcing or inventory relationships. Alternative qualified production capacity may not be available on a timely basis or at all because manufacturing processes for our product candidates are complex
and may be subject to a lengthy regulatory approval process.



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A number of factors could cause prolonged interruptions in the manufacturing and supply of our product
candidates, including:



| |  |  | the failure of a supplier to provide raw materials or intermediates used for manufacture of our product candidates;
| | - |  | -------------------------------------------------------------------------------------------------------------------



| |  |  | equipment malfunctions or failures;
| | - |  | -----------------------------------



| |  |  | the failure to manufacture in accordance with current good manufacturing practices, FDA or other regulatory requirements;
| | - |  | -------------------------------------------------------------------------------------------------------------------------



| |  |  | the delay of product shipments due to U.S. custom regulations or third-party carriers used to transport our product candidates, and damage to our product candidates
while they are in transit;
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | changes in FDA or other regulatory authority requirements or standards that require modifications to the manufacturing processes or facilities used in the
production of our product candidates;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | action by the FDA or other regulatory authorities to suspend production of one or more of our product candidates; or
| | - |  | --------------------------------------------------------------------------------------------------------------------



| |  |  | difficulties in scaling-up production of our product candidates for large clinical trials or commercial supply.
| | - |  | ---------------------------------------------------------------------------------------------------------------

While our manufacturing personnel have extensive experience from working at other companies, we as a company have no experience manufacturing products
for commercial sale. We may encounter difficulties in scaling-up our manufacturing processes and equipment. We may not be able to achieve such scale-up in a timely manner or at a commercially reasonable cost, if at all. In addition, our facilities
in Hayward, California are located within the San Francisco Bay Area, an area where earthquakes periodically occur. They are also located in a designated flood zone. Our access to any raw materials, intermediates, active pharmaceutical ingredients
or formulated drug product for our product candidates sourced or inventoried through our facilities in Hayward, California may be subject to interruption, damage or loss in the event of an earthquake or flood.

As discussed above, we rely upon outside contractors to manufacture and supply to us raw materials, intermediates, active pharmaceutical ingredients and
formulated drug product for our product candidates. Our dependence upon others for the manufacture of our product candidates and their components may adversely affect our ability to continue clinical development of our product candidates in a timely
manner and may adversely affect any future profit margins and our ability to commercialize any products that we may develop on a timely and competitive basis. Dependence on contract manufacturers involves a number of additional risks, many of which
are outside of our control, including:



| |  |  | failure of a contract manufacturer to manufacture products to our specifications or to deliver products in the quantities, timeframe or manner that we require;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | a decision by the FDA or other regulatory authorities not to approve our use of a particular contract manufacturer to supply our products;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | intellectual property rights to any improvements in a manufacturing process or new manufacturing processes being owned by or shared with a contract manufacturer;
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | termination of an agreement with a contract manufacturer or increased prices charged by a contract manufacturer; or
| | - |  | -------------------------------------------------------------------------------------------------------------------



| |  |  | a contract manufacturer declaring bankruptcy or otherwise going out of business.
| | - |  | --------------------------------------------------------------------------------

Any of these factors could cause us to delay or suspend clinical trials, regulatory submissions or commercialization of our products and could result in
significantly increased costs.

In addition, our future contract manufacturers may not be in the United States, and we currently utilize
contract manufacturers located outside the United States. Consequently, we may face additional manufacturing difficulties due to a number of potential factors, including importation and customs issues, political uncertainties and a potentially
limited ability to enforce our contractual rights against parties not located within the United States.



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We face intense competition from pharmaceutical companies, biotechnology companies and academic groups.*

We face, and will continue to face, intense competition from organizations such as biotechnology and pharmaceutical companies, as well
as academic and research institutions and government agencies, that are pursuing competing technologies and products. These organizations may develop or currently possess technologies or products that are superior alternatives to ours. For example,
companies with competing Hsp90 inhibitors include Biogen Idec Inc., which has initiated Phase 1 clinical trials in solid tumors with its intravenous formulation of tanespimycin and its oral synthetic Hsp90 inhibitor and has announced that it expects
to initiate Phase 2 clinical trials in 2007; Infinity Pharmaceuticals, which, in collaboration with MedImmune, Inc. (recently acquired by AstraZeneca), has initiated a Phase 1/2 clinical trial of its intravenous Hsp90 inhibitor in non-small cell
lung cancer and a Phase 1 clinical trial in gastrointestinal stromal tumors and has announced its plans to initiate clinical trials with its oral formulation in the second half of 2007; Abraxis BioScience, Inc., which has announced its plans to file
an IND in the second half of 2007 for its albumin-bound nanoparticle formulation of tanespimycin; Vernalis plc, which has announced that it has selected an intravenous and an oral Hsp90 inhibitor preclinical development candidate in collaboration
with Novartis AG and expects to initiate Phase 1 clinical trials in mid-2007; Serenex, Inc., which has announced plans to initiate Phase 1 clinical trials for its oral Hsp90 inhibitor in the second quarter of 2007; and Synta Pharmaceuticals Corp.,
which announced plans to file an IND for its Hsp90 inhibitor in 2007, as well as other companies reported to be pursuing Hsp90 inhibitors. Competing epothilones in clinical development include those being developed by Bristol-Myers Squibb Company,
which is reported to have filed an NDA; Novartis AG, which is reported to be in Phase 3 clinical trials; and Schering AG, which is reported to be in Phase 2 clinical trials. Gastrointestinal motility competitors include Chugai Pharmaceuticals, whose
motilide agonist is reported to be in Phase 2 clinical trials. Thus, it is possible that, even if we are successful in developing any of our product candidates, one or more compounds of our competitors will be approved and marketed before our own.
This could place our partners and us at a significant disadvantage and could prevent us from realizing significant commercial benefit from such compounds. Further, our competitors in the polyketide gene-engineering field may be more effective at
implementing their technologies to develop commercial products or may hold or develop patents or other proprietary rights that may prevent us from practicing our technologies and pursuing our programs.

We also face and will continue to face intense competition from other companies for partnering arrangements with pharmaceutical and biotechnology
companies, for establishing relationships with academic and research institutions and for licenses to additional technologies. These competitors, either alone or with their collaborative partners, may succeed in developing technologies or products
that are superior to ours.

Any products that we develop through our technologies will compete in multiple, highly competitive markets.
Development of pharmaceutical products requires significant investment and resources. Many of the organizations competing with us in the markets for such products have greater capital resources, research and development and marketing staffs,
facilities and capabilities, and greater experience in discovery and developing drugs, obtaining regulatory approvals and product manufacturing and marketing. Accordingly, our competitors may succeed in more rapidly developing and marketing
technologies and products that are more effective than our technologies and products or that would render our products or technologies obsolete or noncompetitive.



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We believe that our ability to successfully compete will depend on, among other things:



| |  |  | our ability to develop novel product candidates with attractive pharmaceutical properties and to secure and protect intellectual property rights based on our
innovations;
| | - |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | the efficacy and safety of our product candidates;
| | - |  | --------------------------------------------------



| |  |  | the speed at which we and our partners develop our product candidates;
| | - |  | ----------------------------------------------------------------------



| |  |  | our and our partners ability to design and successfully execute appropriate clinical trials;
| | - |  | ---------------------------------------------------------------------------------------------



| |  |  | our and our partners ability to obtain regulatory approvals to market our product candidates and the timing and scope of any regulatory approvals;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | our and our partners ability to manufacture commercial quantities and sell any product candidates that are approved for marketing;
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------



| |  |  | acceptance of future products by physicians and other healthcare providers; and
| | - |  | -------------------------------------------------------------------------------



| |  |  | the development of effective pricing and reimbursement strategies.
| | - |  | ------------------------------------------------------------------

If we face product liability claims, these claims will divert our managements time and we will incur litigation costs, and if we are held liable, our business, financial condition and results of operation may
be materially harmed.

We face an inherent business risk of liability claims in the event that the use of our potential products in
clinical trials or otherwise, or any other products manufactured in our facility, results in personal injury or death. Even though we have obtained product liability insurance, it may not be sufficient to cover claims that may be made against us.
Product liability insurance is expensive, difficult to obtain and may not be available on acceptable terms, if at all. Any claims against us, regardless of their merit, could materially and adversely affect our business, financial condition and
results of operation, because litigation related to these claims would strain our financial resources in addition to consuming the time and attention of our management. If we are sued for any injuries caused by our products or products manufactured
at our facility, our liability could exceed our total assets.

We use hazardous chemicals and radioactive and biological materials in our business. Any
claims relating to improper handling, storage or disposal of these materials could be time consuming and costly.

Our research and
development processes involve the controlled use of hazardous materials, including hazardous chemicals and radioactive and biological materials. Some of these materials may be novel, including bacteria with novel properties and bacteria that produce
biologically active compounds. Our operations also produce hazardous waste products. We cannot eliminate the risk of accidental contamination or discharge and any resultant injury from these materials. Federal, state and local laws and regulations
govern the use, manufacture, storage, handling, shipment and disposal of these materials. We could be subject to civil damages in the event of an improper or unauthorized release of, or exposure of individuals to, hazardous materials. In addition,
we could be sued for injury or contamination that results from our use or the use by third parties or our partners of these materials, and our liability may exceed our insurance coverage and total assets. Compliance with environmental laws and
regulations may be expensive, and current or future environmental regulations may impair our research, development or commercialization efforts. In the event we do not comply with any of these laws or regulations, we may incur significant fines, our
governmental licenses or permits may be revoked or we may face additional penalties, any of which could harm our business.



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We have a stockholders rights plan and anti-takeover provisions in our corporate charter documents that may result in
outcomes with which you do not agree.

Our board of directors has the authority to issue up to 10,000,000 shares of preferred stock and
to determine the rights, preferences, privileges and restrictions of those shares without further vote or action by our stockholders. The rights of the holders of any preferred stock that may be issued in the future may adversely affect the rights
of the holders of common stock. The issuance of preferred stock could make it more difficult for third parties to acquire a majority of our outstanding voting stock.

Our certificate of incorporation provides for staggered terms for the members of the board of directors and prevents our stockholders from acting by written consent. These provisions and other provisions of our bylaws
and of Delaware law applicable to us could delay or make more difficult a merger, tender offer or proxy contest involving us. This could reduce the price that investors might be willing to pay for shares of our common stock and result in the market
price being lower than it would be without these provisions. In addition, these provisions may frustrate or prevent any attempts by our stockholders to replace or remove our current management by making it more difficult for stockholders to replace
members of our board of directors. This is because our board of directors is responsible for appointing the members of our management team.

We have adopted a rights agreement under which all stockholders have the right to purchase shares of a new series of preferred stock at an exercise price of $70.00 per one one-hundredth of a share, if a person acquires more than 20% of our
common stock. The rights plan could make it more difficult for a person to acquire a majority of our outstanding voting stock. The rights plan could also reduce the price that investors might be willing to pay for shares of our common stock and
result in the market price being lower than it would be without the rights plan. In addition, the existence of the rights plan itself may deter a potential acquirer from acquiring us. As a result, either by operation of the rights plan or by its
potential deterrent effect, mergers and acquisitions of us that our stockholders may consider in their best interests may not occur.

Our stock price
has been, and may continue to be, extremely volatile.*

The trading price of our common stock has been, and is likely to continue to be,
highly volatile. During the period from July 1, 2006 through June 30, 2007, our common stock traded between $2.88 and $7.35 on the Nasdaq Global Market. The trading price of our common stock could be subject to wide fluctuations in price
in response to various factors, many of which are beyond our control, including:



| |  |  | delay or failure in initiating, conducting, completing or analyzing clinical trials or unsatisfactory design of or data from these trials by our collaborators or
us;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | announcements of data from clinical trials, new partnering arrangements or other developments that do not meet the expectations of analysts, investors or other
third parties;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | developments in clinical trials for potentially competitive product candidates;
| | - |  | -------------------------------------------------------------------------------



| |  |  | changes in the United States or foreign health care systems or regulations;
| | - |  | ---------------------------------------------------------------------------



| |  |  | regulatory approvals for competitive product candidates or delays or failures by our partners or us in obtaining regulatory approvals for our product candidates;
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | new products or services introduced or announced by us or our competitors;
| | - |  | --------------------------------------------------------------------------



| |  |  | announcements of technological developments in research by us or our competitors;
| | - |  | ---------------------------------------------------------------------------------



| |  |  | published reports by securities analysts;
| | - |  | -----------------------------------------



| |  |  | announcements of expirations, terminations or amendments of collaborations, licenses or government research grants, or announcements that we have entered into new
collaboration, licensing or similar arrangements;
| | - |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | departures of key personnel;
| | - |  | ----------------------------



| |  |  | developments or disputes as to patent or other proprietary rights;
| | - |  | ------------------------------------------------------------------



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| |  |  | litigation or an unfavorable outcome in litigation;
| | - |  | ---------------------------------------------------



| |  |  | sales of our common stock;
| | - |  | --------------------------



| |  |  | announcements of, and actual or anticipated fluctuations in, our financial results; and
| | - |  | ---------------------------------------------------------------------------------------



| |  |  | economic and other external factors, disasters or crises.
| | - |  | ---------------------------------------------------------

In addition, the stock market in general, and the Nasdaq Global Market and the market for biotechnology companies in particular, have experienced extreme price and volume fluctuations that have often been unrelated or
disproportionate to the operating performance of those companies. These broad market and industry factors may seriously harm the market price of our common stock, regardless of our operating performance. In the past, following periods of volatility
in the market price of a company's securities, securities class-action litigation has often been instituted against that company. If this type of litigation were instituted against us, we would be faced with substantial costs and management's
attention and resources would be diverted, which could in turn seriously harm our business, financial condition and results of operations.

We expect
that our quarterly and annual results of operations will fluctuate, and this fluctuation could cause our stock price to decline, creating investor losses.

Our quarterly and annual operating results have fluctuated in the past and are likely to do so in the future. These fluctuations could cause our stock price to fluctuate significantly or decline. Some of the factors
that could cause our operating results to fluctuate include:



| |  |  | expiration or termination of partnering arrangements, which may not be renewed or replaced;
| | - |  | -------------------------------------------------------------------------------------------



| |  |  | the success rate of our or our partners efforts leading to milestone payments and royalties under our partnering arrangements with Roche or Pfizer or any
future collaboration or license agreements;
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | the progress of our product candidates in clinical trials, and therefore, the timing of expenses for those clinical trials;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------



| |  |  | the timing and willingness of partners to develop and commercialize our products;
| | - |  | ---------------------------------------------------------------------------------



| |  |  | general and industry specific economic conditions, which may affect our partners' research and development expenditures; and
| | - |  | ----------------------------------------------------------------------------------------------------------------------------



| |  |  | costs and expenses related to any litigation or administrative proceedings in which we may be involved.
| | - |  | -------------------------------------------------------------------------------------------------------

If our revenues decline or do not grow due to expiration, termination or amendment of current or future collaboration agreements or licenses, failure to
obtain new contracts or other factors, we may not be able to reduce our operating expenses correspondingly. In addition, we expect operating expenses to continue to increase. Failure to achieve anticipated levels of revenues could therefore
significantly harm our operating results for a particular fiscal period.

Due to the possibility of fluctuations in our revenues and
expenses, we believe that quarter-to-quarter or year-to-year comparisons of our operating results are not a good indication of our future performance. Our operating results in some quarters or years may not meet the expectations of stock market
analysts and investors. In that case, our stock price may decline.

If we are unable to favorably assess the effectiveness of internal controls over
financial reporting, or if our independent auditors are unable to provide an unqualified attestation report on our assessment, our stock price could be adversely affected.

Pursuant to Section 404 of the Sarbanes-Oxley Act of 2002, on an annual basis, our management is required to report on, and our independent auditors
to attest to, the effectiveness of our internal controls over financial reporting. The rules governing the standards that must be met for management to make its



40



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##### Table of Contents

annual assessment are complex and require significant documentation and testing. While our internal controls over financial reporting were deemed effective
by both our management and our independent auditors as of December 31, 2006, there may be changes in our systems, processes or operations that will affect the effectiveness of internal controls in the future. Because of its inherent
limitations, internal control over financial reporting may not prevent or detect misstatements. Our future assessments of internal controls may continue to result in increased expenses and the devotion of significant management resources. If we
cannot favorably assess the effectiveness of our internal controls over financial reporting in the future, or if our independent auditors are unable to provide an unqualified attestation report on our internal controls over financial reporting,
investor confidence and our stock price could be adversely affected.



41



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##### Table of Contents

|Item 4: | Submission of Matters to a Vote of Security Holders
|------- | ---------------------------------------------------

The Company held its Annual Meeting of Stockholders on May 24, 2007. The results of the voting were as follows:



| | a) | To elect two directors Charles, J. Homcy, M.D. and Chaitan S. Khosla, Ph.D., to serve as Class A Directors for a term that expires at the Annual Meeting of Stockholders in
2010, or until their respective successors are duly elected and qualified. Directors whose term of office as a director continued after the meeting are Bruce A. Chabner, M.D., Peter Davis, Ph.D., Jean Deleage, Ph.D., Kevan Clemens, Ph.D.,
Christopher T. Walsh, Ph.D., and Robert G. Johnson, M.D., Ph.D. The number of votes cast as to the election of the Class A directors of the Company at the Annual Meeting of Stockholders is as follows:
| | -- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



|                         |  |                       |  |                 |  |                 
|------------------------ |  | --------------------- |  | --------------- |  | ----------------
|Candidate                |  | Shares Voted in Favor |  | Shares Withheld |  | Broker Non-Votes
|Charles J. Homcy, M.D.   |  | 37,126,964            |  | 183,424         |  |                
|Chaitan S. Khosla, Ph.D. |  | 37,104,581            |  | 205,807         |  |                



| | b) | To ratify the selection of Ernst & Young LLP as independent registered public accounting firm of the Company for its fiscal year ending December 31, 2007.
| | -- | ----------------------------------------------------------------------------------------------------------------------------------------------------------



|                      |  |                |  |         |  |                  
|--------------------- |  | -------------- |  | ------- |  | -----------------
|Shares Voted In Favor |  | Shares Against |  | Abstain |  | Broker Non- Votes
|37,135,291            |  | 164,788        |  | 10,309  |  |                 



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##### Table of Contents

|Item 6: | Exhibits
|------- | --------



|            |  |                                                                                                                                                                                                                                                           
|----------- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Exhibit No. |  |                                                                                                                                                                                                                                                           
|            | 
|3.1         |  | Amended and Restated Certificate of Incorporation. (1)                                                                                                                                                                                                    
|            | 
|3.2         |  | Amended and Restated Bylaws of Registrant. (2)                                                                                                                                                                                                            
|            | 
|4.1         |  | Form of Specimen Common Stock Certificate. (3)                                                                                                                                                                                                            
|            | 
|4.2         |  | Third Amended and Restated Registration Rights Agreement, dated March 30, 2000, between Registrant and certain stockholders. (3)                                                                                                                          
|            | 
|4.3         |  | Registrants Certificate of Designation of Series A Junior Preferred Stock. (4)                                                                                                                                                                           
|            | 
|4.4         |  | Warrant for the purchase of shares of common stock, dated July 19, 2006, issued by the Registrant to Kingsbridge Capital Limited. (5)                                                                                                                     
|            | 
|4.5         |  | Registration Rights Agreement, dated July 19, 2006, by and between the Registrant and Kingsbridge Capital Limited. (5)                                                                                                                                    
|            | 
|31.1        |  | Certification required by Rule 13a-14(a) or Rule 15d-14(a)                                                                                                                                                                                                
|            | 
|31.2        |  | Certification required by Rule 13a-14(a) or Rule 15d-14(a)                                                                                                                                                                                                
|            | 
|32.1        |  | Certification by the Chief Executive Officer and the Chief Financial Officer of Kosan Biosciences Incorporated, as required by Rule 13a-14(b) or Rule 15d-14(b) and Section 1350 of Chapter 63 of
Title 18 of the United Stated Code (18 U.S.C. 1350). (6)

---

|(1) | Incorporated herein by reference to an exhibit of our quarterly report on Form 10-Q for the period ended June 30, 2001.
|--- | -----------------------------------------------------------------------------------------------------------------------

|(2) | Incorporated herein by reference to an exhibit of our quarterly report on Form 10-Q for the period ended June 30, 2003.
|--- | -----------------------------------------------------------------------------------------------------------------------

|(3) | Incorporated herein by reference to an exhibit of our Registration Statement on Form S-1, Registration No. 333-33732.
|--- | ---------------------------------------------------------------------------------------------------------------------

|(4) | Incorporated by reference to an exhibit of our current report on Form 8-K filed on October 15, 2001.
|--- | ----------------------------------------------------------------------------------------------------

|(5) | Incorporated by reference to an exhibit of our current report on Form 8-K filed on July 25, 2006.
|--- | -------------------------------------------------------------------------------------------------

|(6) | This certification accompanies the Quarterly Report on Form 10-Q to which it relates, pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, and is not deemed
filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of Kosan Biosciences Incorporated under the Securities Act or the Exchange Act (whether made before or after the date of the Quarterly
Report on Form 10-Q), irrespective of any general incorporation language contained in such filing.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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##### Table of Contents

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to
be signed on its behalf by the undersigned, thereunto duly authorized.



|               |  |                                |  |                                                  
|-------------- |  | ------------------------------ |  | -------------------------------------------------
|               |  | Kosan Biosciences Incorporated
|               |  |                               
|August 9, 2007 |  | By:                            |  | /s/ Robert G. Johnson, Jr.                       
|               |  |                                |  | Robert G. Johnson, Jr., M.D., Ph.D.              
|               |  |                                |  | President and Chief Executive Officer            
|               |  |                               
|August 9, 2007 |  | By:                            |  | /s/ Gary S. Titus                                
|               |  |                                |  | Gary S. Titus                                    
|               |  |                                |  | Senior Vice President and Chief Financial Officer



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EXHIBIT INDEX



|            |  |                                                                                                                                                                                                                                                           
|----------- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Exhibit No. |  |                                                                                                                                                                                                                                                           
|3.1         |  | Amended and Restated Certificate of Incorporation. (1)                                                                                                                                                                                                    
|            | 
|3.2         |  | Amended and Restated Bylaws of Registrant. (2)                                                                                                                                                                                                            
|            | 
|4.1         |  | Form of Specimen Common Stock Certificate. (3)                                                                                                                                                                                                            
|            | 
|4.2         |  | Third Amended and Restated Registration Rights Agreement, dated March 30, 2000, between Registrant and certain stockholders. (3)                                                                                                                          
|            | 
|4.3         |  | Registrants Certificate of Designation of Series A Junior Preferred Stock. (4)                                                                                                                                                                           
|            | 
|4.4         |  | Warrant for the purchase of shares of common stock, dated July 19, 2006, issued by the Registrant to Kingsbridge Capital Limited. (5)                                                                                                                     
|            | 
|4.5         |  | Registration Rights Agreement, dated July 19, 2006, by and between the Registrant and Kingsbridge Capital Limited. (5)                                                                                                                                    
|            | 
|31.1        |  | Certification required by Rule 13a-14(a) or Rule 15d-14(a)                                                                                                                                                                                                
|            | 
|31.2        |  | Certification required by Rule 13a-14(a) or Rule 15d-14(a)                                                                                                                                                                                                
|            | 
|32.1        |  | Certification by the Chief Executive Officer and the Chief Financial Officer of Kosan Biosciences Incorporated, as required by Rule 13a-14(b) or Rule 15d-14(b) and Section 1350 of Chapter 63 of
Title 18 of the United Stated Code (18 U.S.C. 1350). (6)

---

|(1) | Incorporated herein by reference to an exhibit of our quarterly report on Form 10-Q for the period ended June 30, 2001.
|--- | -----------------------------------------------------------------------------------------------------------------------

|(2) | Incorporated herein by reference to an exhibit of our Quarterly Report on Form 10-Q for the period ended June 30, 2003.
|--- | -----------------------------------------------------------------------------------------------------------------------

|(3) | Incorporated herein by reference to an exhibit of our Registration Statement on Form S-1, Registration No. 333-33732.
|--- | ---------------------------------------------------------------------------------------------------------------------

|(4) | Incorporated by reference to an exhibit of our current report on Form 8-K filed on October 15, 2001.
|--- | ----------------------------------------------------------------------------------------------------

|(5) | Incorporated by reference to an exhibit of our current report on Form 8-K filed on July 25, 2006.
|--- | -------------------------------------------------------------------------------------------------

|(6) | This certification accompanies the Quarterly Report on Form 10-Q to which it relates, pursuant to Section 906 of the Sarbanes Oxley Act of 2002, and is not deemed
filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of Kosan Biosciences Incorporated under the Securities Act or the Exchange Act (whether made before or after the date of the Quarterly
Report on Form 10-Q), irrespective of any general incorporation language contained in such filing.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------