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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 March 31, 2006.

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; 34,554,753 shares outstanding at April 30, 2006.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

Form 10-Q

Quarter Ended March 31, 2006

INDEX



|               |  |                                                                                                   |  |     
|-------------- |  | ------------------------------------------------------------------------------------------------- |  | ----
|               |  |                                                                                                   |  | Page
|PART I.        |  | FINANCIAL INFORMATION                                                                             |  |     
|               |  |                                                                                                  
|Item 1:        |  | Condensed Financial Statements and Notes:                                                         |  |     
|               |  |                                                                                                  
|               |  | Condensed Balance Sheets as of March 31, 2006 (unaudited) and December 31, 2005                   |  | 3   
|               |  |                                                                                                  
|               |  | Condensed Statements of Operations (unaudited) for the three months ended March 31, 2006 and 2005 |  | 4   
|               |  |                                                                                                  
|               |  | Condensed Statements of Cash Flows (unaudited) for the three months ended March 31, 2006 and 2005 |  | 5   
|               |  |                                                                                                  
|               |  | Notes to Condensed Financial Statements                                                           |  | 6   
|               |  |                                                                                                  
|Item 2:        |  | Managements Discussion and Analysis of Financial Condition and Results of Operations             |  | 17  
|               |  |                                                                                                  
|Item 3:        |  | Quantitative and Qualitative Disclosures About Market Risk                                        |  | 26  
|               |  |                                                                                                  
|Item 4:        |  | Controls and Procedures                                                                           |  | 26  
|               |  |                                                                                                  
|PART II.       |  | OTHER INFORMATION                                                                                 |  |     
|               |  |                                                                                                  
|Item 1A:       |  | Risk Factors                                                                                      |  | 27  
|               |  |                                                                                                  
|Item 5:        |  | Other Information                                                                                 |  | 44  
|               |  |                                                                                                  
|Item 6:        |  | Exhibits                                                                                          |  | 44  
|               | 
|SIGNATURES     |  | 45                                                                                               
|               | 
|CERTIFICATIONS |  |                                                                                                  



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##### Table of Contents

PART I. FINANCIAL INFORMATION

Item 1:Condensed Financial Statements and Notes

KOSAN BIOSCIENCES INCORPORATED

CONDENSED BALANCE SHEETS

(in thousands)



|                                                       |  |               |          |   |                      |   |          |  
|------------------------------------------------------ |  | ------------- | -------- | - | -------------------- | - | -------- | -
|                                                       |  | March 31,2006 |          |   | December 31,2005 (1) |  
|                                                       |  | (unaudited)   |          |   |                      |  
|Assets                                                 |  |               |          |   |                      |   |          |  
|Current assets:                                        |  |               |          |   |                      |   |          |  
|Cash and cash equivalents                              |  | $             | 14,524   |   |                      | $ | 18,750   |  
|Short-term investments                                 |  |               | 29,983   |   |                      |   | 35,427   |  
|Accounts receivable                                    |  |               | 2,013    |   |                      |   | 3,319    |  
|Prepaid and other current assets                       |  |               | 1,290    |   |                      |   | 1,192    |  
|                                                       |  |               |          |   |                      |   |          |  
|Total current assets                                   |  |               | 47,810   |   |                      |   | 58,688   |  
|Restricted cash                                        |  |               | 949      |   |                      |   | 949      |  
|Property and equipment, net                            |  |               | 5,617    |   |                      |   | 6,061    |  
|Other assets and notes receivable from related parties |  |               | 331      |   |                      |   | 299      |  
|                                                       |  |               |          |   |                      |   |          |  
|Total assets                                           |  | $             | 54,707   |   |                      | $ | 65,997   |  
|                                                       |  |               |          |   |                      |   |          |  
|Liabilities and Stockholders Equity                   |  |               |          |   |                      |   |          |  
|Current liabilities:                                   |  |               |          |   |                      |   |          |  
|Accounts payable                                       |  | $             | 1,984    |   |                      | $ | 2,639    |  
|Accrued liabilities                                    |  |               | 6,572    |   |                      |   | 6,460    |  
|Current portion of deferred revenue                    |  |               | 3,277    |   |                      |   | 3,277    |  
|Current portion of equipment loans                     |  |               | 1,733    |   |                      |   | 1,854    |  
|                                                       |  |               |          |   |                      |   |          |  
|Total current liabilities                              |  |               | 13,566   |   |                      |   | 14,230   |  
|Deferred revenue, less current portion                 |  |               | 8,056    |   |                      |   | 8,876    |  
|Equipment loans, less current portion                  |  |               | 1,625    |   |                      |   | 1,785    |  
|Stockholders equity:                                  |  |               |          |   |                      |   |          |  
|Common stock                                           |  |               | 29       |   |                      |   | 29       |  
|Additional paid-in capital                             |  |               | 172,931  |   |                      |   | 172,083  |  
|Accumulated other comprehensive income                 |  |               | (82      | ) |                      |   | (169     | )
|Accumulated deficit                                    |  |               | (141,418 | ) |                      |   | (130,837 | )
|                                                       |  |               |          |   |                      |   |          |  
|Total stockholders equity                             |  |               | 31,460   |   |                      |   | 41,106   |  
|                                                       |  |               |          |   |                      |   |          |  
|Total liabilities and stockholders equity             |  | $             | 54,707   |   |                      |   | $ 65,997 |  
|                                                       |  |               |          |   |                      |   |          |  

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|(1) | The balance sheet data at December 31, 2005 has been derived from the audited financial statements at that date.
|--- | ----------------------------------------------------------------------------------------------------------------

See accompanying notes.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

CONDENSED STATEMENTS OF OPERATIONS

(unaudited)

(in thousands, except
per share data)



|                                                                                                          |  |                             |         |   |      |   |        |  
|--------------------------------------------------------------------------------------------------------- |  | --------------------------- | ------- | - | ---- | - | ------ | -
|                                                                                                          |  | Three Months EndedMarch 31, |        
|                                                                                                          |  | 2006                        |         |   | 2005 |  
|Revenues:                                                                                                 |  |                             |         |   |      |   |        |  
|Contract revenue                                                                                          |  | $                           | 2,732   |   |      | $ | 2,616  |  
|Grant revenue                                                                                             |  |                             | 229     |   |      |   | 259    |  
|                                                                                                          |  |                             |         |   |      |   |        |  
|Total revenues                                                                                            |  |                             | 2,961   |   |      |   | 2,875  |  
|                                                                                                          |  |                            
|Operating expenses:                                                                                       |  |                             |         |   |      |   |        |  
|Research and development (including charges for stock-based compensation of $342 and $77, respectively)   |  |                             | 11,350  |   |      |   | 9,655  |  
|General and administrative (including charges for stock-based compensation of $193 and $39, respectively) |  |                             | 2,559   |   |      |   | 1,541  |  
|                                                                                                          |  |                             |         |   |      |   |        |  
|Total operating expenses                                                                                  |  |                             | 13,909  |   |      |   | 11,196 |  
|                                                                                                          |  |                             |         |   |      |   |        |  
|Loss from operations                                                                                      |  |                             | (10,948 | ) |      |   | (8,321 | )
|Other income, net                                                                                         |  |                             | 367     |   |      |   | 306    |  
|                                                                                                          |  |                             |         |   |      |   |        |  
|Net loss                                                                                                  |  | $                           | (10,581 | ) |      | $ | (8,015 | )
|                                                                                                          |  |                             |         |   |      |   |        |  
|                                                                                                          |  |                            
|Basic and diluted net loss per common share                                                               |  | $                           | (0.36   | ) |      | $ | (0.28  | )
|                                                                                                          |  |                             |         |   |      |   |        |  
|Shares used in computing basic and diluted net loss per common share                                      |  |                             | 29,403  |   |      |   | 29,099 |  

See accompanying notes.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

CONDENSED STATEMENTS OF CASH FLOWS

(unaudited)

(in thousands)



|                                                                           |  |                             |         |   |      |   |        |  
|-------------------------------------------------------------------------- |  | --------------------------- | ------- | - | ---- | - | ------ | -
|                                                                           |  | Three Months EndedMarch 31, |        
|                                                                           |  | 2006                        |         |   | 2005 |  
|Operating activities                                                       |  |                             |         |   |      |   |        |  
|Net loss                                                                   |  | $                           | (10,581 | ) |      | $ | (8,015 | )
|Adjustment to reconcile net loss to net cash used in operating activities: |  |                             |         |   |      |   |        |  
|Depreciation and amortization                                              |  |                             | 603     |   |      |   | 637    |  
|Amortization of investment premiums and discounts                          |  |                             | 11      |   |      |   | 250    |  
|Amortization of stock-based compensation                                   |  |                             | 535     |   |      |   | 115    |  
|Changes in assets and liabilities:                                         |  |                             |         |   |      |   |        |  
|Accounts receivable                                                        |  |                             | 1,306   |   |      |   | 1,951  |  
|Prepaid and other current assets                                           |  |                             | (98     | ) |      |   | 2      |  
|Other assets and notes receivable from related parties                     |  |                             | (32     | ) |      |   | 31     |  
|Accounts payable and accrued liabilities                                   |  |                             | (543    | ) |      |   | (1,273 | )
|Deferred revenue                                                           |  |                             | (820    | ) |      |   | (819   | )
|                                                                           |  |                             |         |   |      |   |        |  
|Net cash used in operating activities                                      |  |                             | (9,619  | ) |      |   | (7,121 | )
|                                                                           |  |                             |         |   |      |   |        |  
|Investing activities                                                       |  |                             |         |   |      |   |        |  
|Acquisition of property and equipment                                      |  |                             | (159    | ) |      |   | (263   | )
|Purchase of investments                                                    |  |                             | (6,130  | ) |      |   | (7,000 | )
|Proceeds from maturity of investments                                      |  |                             | 11,650  |   |      |   | 18,614 |  
|                                                                           |  |                             |         |   |      |   |        |  
|Net cash provided by investing activities                                  |  |                             | 5,361   |   |      |   | 11,351 |  
|                                                                           |  |                             |         |   |      |   |        |  
|Financing activities                                                       |  |                             |         |   |      |   |        |  
|Proceeds from issuance of common stock                                     |  |                             | 313     |   |      |   | 316    |  
|Proceeds from equipment loans                                              |  |                             | 270     |   |      |   | 944    |  
|Principal payments under equipment loans                                   |  |                             | (551    | ) |      |   | (533   | )
|                                                                           |  |                             |         |   |      |   |        |  
|Net cash provided by financing activities                                  |  |                             | 32      |   |      |   | 727    |  
|                                                                           |  |                             |         |   |      |   |        |  
|Net increase (decrease) in cash and cash equivalents                       |  |                             | (4,226  | ) |      |   | 4,957  |  
|Cash and cash equivalents at beginning of period                           |  |                             | 18,750  |   |      |   | 13,777 |  
|                                                                           |  |                             |         |   |      |   |        |  
|Cash and cash equivalents at end of period                                 |  | $                           | 14,524  |   |      | $ | 18,734 |  
|                                                                           |  |                             |         |   |      |   |        |  

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 biotechnology company advancing two new classes of anticancer agents through clinical
development. The Companys heat shock protein 90, or Hsp90, inhibitor, KOS-953, is in Phase I and II clinical trials, primarily for multiple myeloma and HER2 positive breast cancer. KOS-953 is the Companys proprietary formulation of
17-AAG, a geldanamycin analog. In addition, intravenous and oral formulations of a second-generation Hsp90 inhibitor, KOS-1022, are currently in Phase I clinical trials. These compounds have a novel mechanism of action targeting multiple pathways
involved in cancer cell growth and survival. The Company is collaborating with the National Cancer Institute (NCI), a component of the National Institutes of Health, in the development of 17-AAG and other analogs of geldanamycin for the
treatment of cancer.

Kosan is also developing KOS-862 in Phase II clinical trials in breast cancer. KOS-862 is an epothilone with a
mechanism of action similar to taxanes, one of the most successful classes of anti-tumor agents. Kosans follow-on epothilone, KOS-1584, is in Phase I clinical trials. 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. The Company has selected KOS-2187 as a clinical product candidate in this program.

Kosan also has additional research programs for cancer that are undergoing preclinical evaluation.

Kosan has funded its operations primarily through sales of common stock and convertible preferred stock, 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 March 31, 2006, and for the three months ended March 31, 2006 and 2005, 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, 2005 filed with the Securities and Exchange
Commission on March 16, 2006.



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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 and the severity to 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 three months ended March 31, 2006 and 2005, 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 March 31, 2006 and December 31, 2005. 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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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 under research grants from the National Institutes of Health and the National Institute of Standards and Technology. The arrangements may include upfront 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 and determinable; and (iv) collectability is reasonably assured.

The Company recognizes license and other upfront and initial fees pursuant to research and development collaboration agreements over the estimated
research and development term of the respective agreement. These estimated terms are reviewed on a periodic basis and updated if the underlying assumptions are modified. 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
collaborative 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 collaboration 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, external manufacturing and other outside service providers. Expenses related to clinical trials generally are accrued based on the level of patient
enrollment and activity according to the protocol. The Company monitors patient enrollment level and related activity 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
payments, upfront and initial 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)

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, less shares subject to repurchase. Diluted net loss 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.

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 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. Compensation related to the grants of stock options to
non-employees is recorded in accordance with SFAS 123 and EITF 96-18. The measurement of stock-based compensation to non-employees is subject to periodic adjustment as the underlying awards vest. See Note 3 Stock-Based Compensation for further
information.

2. Research and Development Agreements

Roche

Effective September 2002, the Company entered into a research and development collaboration agreement (the
Agreement) with Roche. Under the terms of the Agreement, Roche has worldwide exclusive rights to market and sell KOS-862 and other epothilones (including KOS-1584) in the field of oncology, and the Company will co-develop and has the
right to co-promote the product in the United States. The Agreement provides for the Company to receive payments for the reimbursement of research and development expenditures, funding of a back-up program, achievement of clinical, regulatory and
commercial milestones, development activities and royalties on sales of collaboration products. In addition, the Company has the opportunity to increase its royalties through a buy-in at a later stage of clinical development and by co-promotion of
products resulting from the collaboration. Effective July 1, 2004, the Company entered into an amendment to the Agreement that provided for the reimbursement of



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

2. Research and Development Agreements (continued)

Roche (continued)

costs that exceeded the previously stipulated amounts set forth in the Agreement. On March 27, 2006, the Company
entered into a letter agreement with Roche replacing a particular at-risk milestone payment obligation in the Agreement with an obligation by Roche to reimburse the Company for certain patent expenses up to a specified amount at the time set forth
in the letter agreement. The letter agreement had no effect on revenue recognized by the Company in the current quarter or historically. For the three months ended March 31, 2006 and 2005, the Company recognized revenue related to the Agreement
of approximately $2.7 million and $2.6 million, respectively.

License Agreements

The Company has collaborative and license agreements with several academic, government and medical institutions. Included in research and development
expenses were total payments made under these agreements of approximately $77,000 and $140,000 for the three months ended March 31, 2006 and 2005, respectively.

3. Stock-Based Compensation

At March 31, 2006, the Company had the following
stock-based compensation plans:

1996 Stock Option Plan

The Companys 1996 Stock Option Plan (the 1996 Plan) provides for the granting of incentive stock options and nonstatutory stock options to employees, officers, directors and consultants of the
Company. The maximum number of aggregate shares that may be optioned and sold under the 1996 Plan is 9,475,000 as of March 31, 2006. The 1996 Plan provides for an annual increase of shares on January 1 of each year that may be the lesser
of 1,125,000 shares, 5% of the outstanding shares on such date or a lesser amount as determined by the Companys board. Incentive stock options may be granted with exercise prices not less than fair value, and nonstatutory stock options may be
granted with exercise prices not less than 85% of the fair value on the date of grant. The fair value of grants is determined by the closing market price of the Companys common stock as listed on any established stock exchange on the date of
grant. Stock options granted to a stockholder owning more than 10% of voting stock of the Company may be granted with an exercise price of not less than 110% of the fair value on the date of grant. Options expire no later than ten years from the
date of the grant. The number of shares, terms and exercise period are determined by the Companys board of directors. Options generally vest at 25% per year over a four-year period. The 1996 Plan will expire in June 2006, unless
terminated in accordance with the provisions of the 1996 Plan.



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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

2000 Employee Stock Purchase Plan

The Companys 2000 Employee Stock Purchase Plan (the Purchase
Plan) permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering 2000 Employee Stock Purchase Plan periods. The maximum number of shares available for issuance pursuant to the Purchase
Plan is 750,000, as of March 31, 2006. The Purchase Plan provides for an annual increase of shares on January 1 of each year equal to the lesser of 150,000 shares, 0.75% of the outstanding shares on such date or such amount as determined
by the Companys board. The price at which the stock is purchased is equal to the lower of 85% of the fair market value of the common stock on the first day of the offering or 85% of the fair market value of the Companys common stock on
the purchase date. Each offering under the Purchase Plan is for a six-month period.

2000 Non-Employee Directors Plan

The Companys 2000 Non-Employee Directors Stock Option Plan (the Directors Plan) provides for the granting of nonstatutory
stock options to directors of the Company. The maximum number of shares of common stock available for issuance pursuant to the Directors Plan is 500,000, as of March 31, 2006. The Directors Plan provides for an annual increase of
shares on January 1 of each year equal to the lesser of 150,000 shares, 0.75% of the outstanding shares on such date or a lesser amount as determined by the Companys board. Under the Directors Plan, as amended, each non-employee
director who becomes a director of the Company will be automatically granted a non-statutory stock option to purchase 20,000 shares of common stock on the date on which such person first becomes a director, and such option will vest over four years.
Beginning with the 2002 Annual Stockholders Meeting and each year thereafter, each non-employee director will automatically be granted, one day following each years annual meeting of stockholders, a non-statutory option to purchase 5,000
shares of common stock, which will vest one day before the annual meeting of stockholders subsequent to the date of grant. The exercise price of options under the Directors Plan will be equal to the fair market value of the common stock on the
date of grant. The maximum term of the options granted under the Directors Plan is ten years.

Prior to January 1, 2006, the
Company provided pro forma disclosure amounts in accordance with SFAS No. 148, Accounting for Stock-Based CompensationTransition and Disclosure (SFAS 148), as if the fair value method defined by SFAS 123 had been
applied to its stock-based compensation. Effective January 1, 2006, the Company adopted the fair value recognition provisions of SFAS 123R, using the modified prospective transition method and therefore has not restated prior periods
results. Under this transition method, stock-based compensation expense for the first quarter of 2006 included compensation expense 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 provisions of SFAS 123. Stock-based compensation expense for all share-based payment 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.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

As a result of adopting SFAS 123R, the Companys net loss for the three months ended March 31, 2006 was approximately $442,000 greater than if the Company had continued to account for stock-based compensation under Accounting
Principles Board Opinion No. 25. The impact on both basic and diluted earnings per share for the three months ended March 31, 2006 was $0.02 per share. Due to the Companys lack of earnings history, no income tax benefit or cash flow
effect is recognized as any resulting deferred tax asset is fully offset by a valuation allowance.

The following table presents the
calculation of basic and diluted net loss per share (in thousands, except per share data) had the Company applied the fair value recognition provisions under SFAS 123 in 2005:



|                                                                                                        |  |                                  |        |  
|------------------------------------------------------------------------------------------------------- |  | -------------------------------- | ------ | -
|                                                                                                        |  | Three Months EndedMarch 31, 2005 |       
|Net loss attributable to common stockholders, as reported                                               |  | $                                | (8,015 | )
|Plus:Stock-based employee compensation expense included in reported net loss                            |  |                                  |       |  
|Less: Stock-based employee compensation expense determined under fair value based method for all awards |  |                                  | (608   | )
|                                                                                                        |  |                                  |        |  
|Pro forma net loss                                                                                      |  | $                                | (8,623 | )
|                                                                                                        |  |                                  |        |  
|Basic and diluted net loss per common share:                                                            |  |                                  |        |  
|As reported                                                                                             |  | $                                | (0.28  | )
|Pro forma                                                                                               |  | $                                | (0.30  | )

The Company uses the Black-Scholes-Merton closed-form model to value stock-based compensation
expense. Separate groups of employees that have similar historical exercise behavior are considered separately for valuation purposes. The table below presents the valuation assumptions used to determine stock-based compensation expense under the
provisions of SFAS 123R. Expected term is based on the simplified method allowed under the provisions of Staff Accounting Bulletin (SAB) No. 107 and may only be applied through December 31, 2007. The risk-free interest rate is
based on the U.S. Treasury zero coupon issues with an equivalent remaining term at the time of the option grant. Expected volatility is based on the Companys historical volatility and other factors.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)



|                         |  |               |   |  |               |  
|------------------------ |  | ------------- | - |  | ------------- | -
|                         |  | March 31,2006 |   |  | March 31,2005 |  
|Expected term (in years) |  | 6.25          |   |  | 4.00          |  
|Risk-free interest rate  |  | 4.71          |   |  | 4.00          |  
|Expect volatility        |  | 67            | % |  | 64            | %
|Expected dividends       |  |              |   |  |              |  

A summary of stock option activity and changes through the period ending March 31, 2006 is as
follows:



|                                              |                |                 |   |  |                               |            |                                          |                |                          |   |      
|--------------------------------------------- | -------------- | --------------- | - |  | ----------------------------- | ---------- | ---------------------------------------- | -------------- | ------------------------ | - | -----
|Options                                       |                | Number ofShares |   |  | Weighted-AverageExercisePrice |            | Weighted-AverageRemainingContractualTerm |                | AggregateInstrinsicValue
|                                              | (in thousands) |                 |   |  |                               | (in years) |                                          | (in thousands)
|Outstanding at December 31, 2005              |                | 4,902           |   |  | $                             | 7.57       |                                          |                |                          |   |      
|Granted                                       |                | 155             |   |  | $                             | 4.65       |                                          |                |                          |   |      
|Forfeited or expired                          |                | (509            | ) |  | $                             | 6.89       |                                          |                |                          |   |      
|Exercised                                     |                | (34             | ) |  | $                             | 3.53       |                                          |                |                          |   |      
|                                              |                |                 |   |  |                               |            |                                          |                |                          |   |      
|Outstanding at March 31, 2006                 |                | 4,514           |   |  | $                             | 7.58       |                                          | 7.23           |                          | $ | 1,309
|                                              |                |                 |   |  |                               |            |                                          |                |                          |   |      
|Vested and expected to vest at March 31, 2006 |                | 4,299           |   |  | $                             | 7.65       |                                          | 7.13           |                          | $ | 1,202
|Exercisable at March 31, 2006                 |                | 2,818           |   |  | $                             | 8.05       |                                          | 6.31           |                          | $ | 741  

The weighted-average grant-date fair value of options granted during the quarter ending
March 31, 2006 and 2005 was $3.02 and $2.42, respectively. The total intrinsic value of options exercised during the quarter ending March 31, 2006 and 2005 was approximately $73,000 and $7,000, respectively.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

A summary of the nonvested shares as of December 31, 2005, and changes during the period ended March 31, 2006, is presented below:



|                               |                |                 |   |  |                                     |     
|------------------------------ | -------------- | --------------- | - |  | ----------------------------------- | ----
|Nonvested shares               |                | Number ofShares |   |  | Weighted-AverageGrantDate FairValue
|                               | (in thousands) |                 |   | 
|Nonvested at December 31, 2005 |                | 2,183           |   |  | $                                   | 6.98
|Granted                        |                | 155             |   |  | $                                   | 4.65
|Vested                         |                | (132            | ) |  | $                                   | 7.04
|Forfeited or expired           |                | (510            | ) |  | $                                   | 6.89
|                               |                |                 |   |  |                                     |     
|Nonvested at March 31, 2006    |                | 1,696           |   |  | $                                   | 6.79
|                               |                |                 |   |  |                                     |     

As of March 31, 2006, there was approximately $3.2 million of total unrecognized compensation
cost related to nonvested share-based compensation arrangements granted under the option plans. The cost is expected to be recognized over a weighted-average period of 2.9 years.



4. Comprehensive Loss

For the three months ended March 31, 2006 and 2005, comprehensive loss was as follows (in thousands):



|                                                        |  |                             |         |   |      |   |        |  
|------------------------------------------------------- |  | --------------------------- | ------- | - | ---- | - | ------ | -
|                                                        |  | Three Months EndedMarch 31, |        
|                                                        |  | 2006                        |         |   | 2005 |  
|Net loss                                                |  | $                           | (10,581 | ) |      | $ | (8,015 | )
|Unrealized gain (loss) on available-for-sale securities |  |                             | 87      |   |      |   | (74    | )
|                                                        |  |                             |         |   |      |   |        |  
|Comprehensive loss                                      |  | $                           | (10,494 | ) |      | $ | (8,089 | )
|                                                        |  |                             |         |   |      |   |        |  



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

5. 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, of which $2.6 million had been utilized as
of March 31, 2006. 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 9.07%. Obligations under the loans are secured by the assets financed under the loans.



6. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):



|                                 |  |               |       |                  |   |      
|-------------------------------- |  | ------------- | ----- | ---------------- | - | -----
|                                 |  | March 31,2006 |       | December 31,2005
|Research and development-related |  | $             | 3,855 |                  | $ | 3,115
|Compensation-related             |  |               | 693   |                  |   | 1,438
|Professional services            |  |               | 1,008 |                  |   | 899  
|Facilities-related               |  |               | 768   |                  |   | 767  
|Other                            |  |               | 248   |                  |   | 241  
|                                 |  |               |       |                  |   |      
|                                 |  | $             | 6,572 |                  | $ | 6,460
|                                 |  |               |       |                  |   |      



7. Line of Credit Facility

In July 2005, the Company entered into a $35.0 million secured line of credit facility with Silicon Valley Bank (Bank), a subsidiary of SVB
Financial Group. Under the terms of the agreement, the Company has the option of making up to two draws through May 31, 2006, with a minimum initial draw of $15.0 million. In the event the Company draws down less than $35.0 million by
May 31, 2006, the Company will be required to pay 0.75% of the unused amount of the credit facility. The credit facility contains financial covenants, including covenants requiring the Company to maintain: a ratio of unrestricted cash and cash
equivalents, investments and eligible accounts to the aggregate principal amount outstanding under the line of credit of at least 1.3 to 1; at least six months remaining months liquidity, which is calculated by dividing (i) cash and
cash equivalents, investments and eligible accounts by (ii) the Companys cash utilization calculated on a rolling three-month basis; and aggregate balances in its investment and operating accounts with Bank or SVB Asset Management equal
to the lesser of (a) the amount of the aggregate outstanding principal amount under the line of credit plus $5.0 million; and (b) 85% of the Companys aggregate cash and investment account balances. As of March 31, 2006, the
Company had not made any draws on the facility.



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##### Table of Contents

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

8. Restructuring Charge

In March
2006, the Company implemented and completed a corporate restructuring, reflecting a realignment of research priorities and corporate operations to support its clinical product candidates and pipeline opportunities. As a result, the Company reduced
its workforce by 39 positions, from 119 to 80 full-time employees, primarily in research and general and administrative.

The reduction in
workforce resulted in a one-time severance-related charge of approximately $0.6 million, all of which was recognized in the three months ended March 31, 2006. Approximately $50,000 of such costs were accrued at March 31, 2006.



9. Subsequent Event

In April 2006, the Company completed a public offering of 5,100,000 shares of its common stock at a public offering price of $5.00 per share. The
aggregate proceeds of the offering, net of offering costs, were approximately $23.9 million.



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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 the timing of announcements of clinical data and initiation of clinical trials;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------



| |  |  | the progress and results of our research and development programs, including clinical testing;
| | - |  | ----------------------------------------------------------------------------------------------



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



| |  |  | revenues from existing and new collaborations;
| | - |  | ----------------------------------------------



| |  |  | product development;
| | - |  | --------------------



| |  |  | 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 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.

Overview

We are a biotechnology company advancing two new classes of anticancer agents, Hsp90 inhibitors and epothilones, through clinical development. Following
is the status of our product candidates.



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##### Table of Contents

Hsp90 Inhibitors



| |  |  | KOS-953. KOS-953 is our proprietary formulation of 17-AAG, a geldanamycin analog. KOS-953 is in Phase I and II clinical trials, primarily for multiple myeloma and HER2 positive
breast cancer. In 2006, we expect to announce interim results and the recommended Phase II dose from our Phase Ib clinical study of KOS-953 in combination with Velcade® in patients with multiple myeloma. We also expect to announce interim results from our Phase Ib/II clinical trial of
KOS-953 in combination with Herceptin® in patients
with HER2 positive breast cancer in mid-2006. If our multiple myeloma Phase Ib results are favorable, we intend to initiate a Phase II/III registration study of KOS-953 in combination with Velcade® in patients with relapsed refractory multiple myeloma in late 2006 or early 2007.
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | KOS-1022. Intravenous and oral formulations of a second-generation Hsp90 inhibitor, KOS-1022, are currently in Phase I clinical trials. Intravenous KOS-1022 is also being studied in
a Phase Ib clinical trial in combination with Herceptin®. In mid-2006, we expect to announce interim results from the initial Phase I clinical trial of intravenous KOS-1022. If the final results from this Phase I clinical trial, and our Phase Ib/II clinical trial of KOS-953
in combination with Herceptin® in patients with
HER2 positive breast cancer, are favorable, we intend to initiate late-stage clinical trials of intravenous KOS-1022 in combination with Herceptin® in patients with HER2 positive breast cancer in 2007. We anticipate that KOS-953 will be developed for use in multiple myeloma and KOS-1022 will be
developed for other indications such as HER2 positive breast cancer.
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Epothilones



| |  |  | KOS-862. KOS-862 is being evaluated in a Phase II clinical trial in patients with metastatic breast cancer. In addition, KOS-862 is being evaluated in the Phase II portion of a
Phase Ib/II clinical trial in combination with Herceptin® in patients with HER2 positive locally advanced or metastatic breast cancer. We expect that interim results from the Phase Ib/II clinical trial of KOS-862 in combination with Herceptin® will be announced in mid-2006.
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | KOS-1584. KOS-1584 is our second-generation epothilone anticancer candidate that is being evaluated in two dose-escalating Phase I clinical trials in patients with solid tumors. We
expect to announce interim results from our KOS-1584 Phase I clinical trials in mid-2006.
| | - |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Our epothilone program is
partnered with Roche through a global development and commercialization agreement. Roche is funding all of the current KOS-862 and KOS-1584 clinical trials. We anticipate that Roche will pursue late-stage clinical development of only one of our
epothilone product candidates, if any. We anticipate that Roche will make a decision in this regard in 2006.

We also have a motilin
receptor agonist program for gastrointestinal motility. We plan to file an Investigational New Drug, or IND, application with the Food and Drug Administration, or FDA, or an application for clinical trial authorization with regulatory authorities in
Europe for our clinical candidate, KOS-2187, in 2006.

We also have additional research 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.

In March 2006, we implemented a corporate restructuring, reflecting a realignment of research priorities and corporate operations to support our clinical product candidates and pipeline opportunities.



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##### Table of Contents

As a result, we reduced our workforce by 39 positions, from 119 to 80 full-time employees, primarily in research and general and administration. The
reduction in workforce resulted in a one-time severance related charge of approximately $0.6 million, all of which was recognized in the three months ended March 31, 2006.

We have incurred significant losses since our inception. As of March 31, 2006, our accumulated deficit was approximately $141.4 million. We expect
to incur additional operating losses over the next several years as we continue to advance our clinical product candidates into and through clinical trials.

Critical Accounting Policies

Critical accounting policies are those that require significant judgment and/or estimates by
management at the time that the financial statements are prepared such that materially different results might have been reported if other assumptions had been made. We consider certain accounting policies related to revenue recognition, clinical
trial accruals and stock-based compensation to be critical policies. 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
our making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. The basis of our current estimates or assumptions has not changed since we filed our Annual Report on Form 10-K for the year
ended December 31, 2005 with the Securities and Exchange Commission on March 16, 2006. Since December 31, 2005, we have not made any significant changes to either our policies or any significant estimates or assumptions.

Stock-based Compensation

Effective January 1,
2006, we adopted the fair value recognition provisions of SFAS 123R, using the modified prospective transition method and therefore we did not restate prior periods results. Under this transition method, stock-based compensation expense for
the first quarter of 2006 included compensation expense 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 provisions
of SFAS 123. Stock-based compensation expense for all stock-based payment awards granted after December 31, 2005 is based on the grant-date fair value estimated in accordance with the provisions of SFAS 123R. We recognize 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.

We use the Black-Scholes-Merton closed-form model to value stock-based compensation expense. Separate groups of employees that have similar historical
exercise behavior are considered separately for valuation purposes. Expected term is based on the simplified method allowed under the provisions of SAB No. 107 and may only be applied through December 31, 2007. The risk-free interest rate
is based on the U.S. Treasury zero coupon issues with an equivalent remaining term at the time of the option grant. Expected volatility is based on the historical volatility of our stock price and other factors.

Results of Operations

Revenues

Revenues for the quarters ended March 31, 2006 and 2005 were approximately $3.0 million and $2.9 million, respectively. Revenues in both periods
consisted primarily of contract revenues recognized under our development and commercialization agreement with Roche and funded research related to government grant awards.



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##### Table of Contents

|                                  |  |                             |       |          |   |       |  |     |  
|--------------------------------- |  | --------------------------- | ----- | -------- | - | ----- |  | --- | -
|                                  |  | Three Months EndedMarch 31, |       | % Change |  
|(In thousands, except percentage) |  | 2006                        |       | 2005     |  
|Contract revenue                  |  | $                           | 2,732 |          | $ | 2,616 |  | 4   | %
|Grant revenue                     |  |                             | 229   |          |   | 259   |  | -12 | %
|                                  |  |                             |       |          |   |       |  |     |  
|Total revenues                    |  | $                           | 2,961 |          | $ | 2,875 |  | 3   | %
|                                  |  |                             |       |          |   |       |  |     |  

The increase in revenues of approximately 3%, or $0.1 million, for the three months ended
March 31, 2006, compared to the same period last year was primarily due to higher funding in KOS-1584 for the progress of Phase I trials in solid tumors and the manufacture of clinical materials, partially offset by the conclusion of certain
KOS-862 Phase I trials and the KOS-862 trial in non-small cell lung cancer. We expect to receive a one-time reimbursement for certain patent expenses in 2006. 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.

Research and Development Expenses

Our research and development expenses were approximately $11.4 million and $9.7 million for the three months ended March 31, 2006 and 2005,
respectively. Our research and development expenses 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, external manufacturing 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.



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##### Table of Contents

|                                       |  |                             |        |          |   |       |                           |    |   |  |   |        
|-------------------------------------- |  | --------------------------- | ------ | -------- | - | ----- | ------------------------- | -- | - |  | - | -------
|                                       |  | Three Months EndedMarch 31, |        | % Change |   |       | Inception -March 31, 2006
|(In thousands, except for percentages) |  | 2006                        |        | 2005     |   |      
|Clinical development                   |  |                             |        |          |   |       |                           |    |   |  |   |        
|Epothilones                            |  | $                           | 2,427  |          | $ | 2,203 |                           | 10 | % |  | $ | 51,968 
|Hsp90 inhibitors                       |  |                             | 4,089  |          |   | 2,404 |                           | 70 | % |  |   | 19,163 
|                                       |  |                             |        |          |   |       |                           |    |   |  |   |        
|Total clinical development             |  |                             | 6,516  |          |   | 4,607 |                           | 41 | % |  |   | 71,131 
|Research and preclinical (1)           |  |                             | 4,834  |          |   | 5,048 |                           | -4 | % |  |   | 143,095
|                                       |  |                             |        |          |   |       |                           |    |   |  |   |        
|Total research and development         |  | $                           | 11,350 |          | $ | 9,655 |                           | 18 | % |  | $ | 214,226
|                                       |  |                             |        |          |   |       |                           |    |   |  |   |        

---

|(1) | Research and preclinical constitutes internal research and development costs for our early stage programs in the areas of cancer, gastrointestinal motility and
infectious disease as well as technology development. Expenses for the three months ended March 31, 2006 and 2005 includes allocated personnel-related expenses of approximately $2.3 million and $2.4 million, allocated facility-related expenses
of approximately $1.3 million and $1.6 million and allocated lab consumables of $204,000 and $360,000, respectively. Expenses for the period from inception through March 31, 2006 includes allocated personnel-related expenses of approximately
$70.7 million, allocated facility-related expense of approximately $34.6 million and allocated lab consumables of $9.9 million.
|--- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

The increase of 18%, or approximately $1.7 million, in research and development expenses for the three months ended March 31, 2006 compared to the same period in 2005 was the result of the following:



| |  |  | approximately $1.9 million in increased clinical costs, primarily due to the advancement of our clinical product candidates in the Hsp90 inhibitor program, including costs
associated with clinical trials of KOS-953 and KOS-1022 and the initiation of the Phase I multicenter clinical trial of the oral formulation of KOS-1022, and implementation of SFAS123R, partially offset by
| | - |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | approximately $0.2 million in lower preclinical costs, primarily related to a decrease in expenses associated with the development of early stage product candidates due to the
selection of KOS-2187 as a clinical product candidate and realignment of our research priorities, offset by costs associated with our March 2006 restructuring and implementation of SFAS 123R.
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Our research and development employees decreased from 98 employees as of December 31, 2005 to 65 employees as of March 31, 2006, principally as
a result of the March 2006 corporate restructuring. The costs associated with the corporate restructuring were recorded in the three-month period ended March 31, 2006. We expect our research and development expenses to decrease over the next
several quarters primarily due to reduced employee-related expenses as a result of the corporate restructuring implemented in March 2006.



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##### Table of Contents

General and Administrative Expenses



|                                       |  |                             |       |          |   |       |  |    |  
|-------------------------------------- |  | --------------------------- | ----- | -------- | - | ----- |  | -- | -
|                                       |  | Three Months EndedMarch 31, |       | % Change |  
|(In thousands, except for percentages) |  | 2006                        |       | 2005     |  
|General and administrative             |  | $                           | 2,559 |          | $ | 1,541 |  | 66 | %
|                                       |  |                             |       |          |   |       |  |    |  

For the three months ended March 31, 2006, the increase of 66% or approximately $1.0 million
in general and administrative expenses compared to the same period in 2005 was due to severance related payments to our former Chief Executive Officer in the first quarter of 2006, costs associated with our March 2006 corporate restructuring and the
implementation of SFAS123R.

Our general and administrative employees decreased from 24 as of December 31, 2005 to 15 as of
March 31, 2006, primarily as a result of our March 2006 restructuring. The costs associated with the corporate restructuring and non-recurring severance related payments to our former Chief Executive Officer were recorded in the three-month
period ended March 31, 2006. We expect our general and administrative expenses to decrease over the next several quarters as we recognize reduced employee-related expenses as a result of the corporate restructuring implemented in March 2006.

Other Income, Net



|                                       |  |                             |     |   |          |   |     |   |  |     |  
|-------------------------------------- |  | --------------------------- | --- | - | -------- | - | --- | - |  | --- | -
|                                       |  | Three Months EndedMarch 31, |     |   | % Change |  
|(In thousands, except for percentages) |  | 2006                        |     |   | 2005     |   |    
|Interest income                        |  | $                           | 433 |   |          | $ | 383 |   |  | 13  | %
|Interest expense                       |  |                             | (66 | ) |          |   | (77 | ) |  | -14 | %
|                                       |  |                             |     |   |          |   |     |   |  |     |  
|Other income, net                      |  | $                           | 367 |   |          | $ | 306 |   |  | 20  | %
|                                       |  |                             |     |   |          |   |     |   |  |     |  

Interest income increased to approximately $433,000 for the three months ended March 31,
2006, from approximately $383,000 for the same period in 2005. The increase resulted from higher returns in the current rising interest rate environment, partially offset by lower average investment balances in the first three months of 2006
compared to the same period in 2005.

Interest expense decreased to approximately $66,000 for the three months ended March 31, 2006
from approximately $77,000 for the same period last year. The decrease resulted from the lower average debt balances in the first three months of 2006, partially offset by higher average interest rates on more recent debt-financing drawdowns. We
expect that our interest expense will increase in the future as a result of additional property and equipment-related debt financing combined with anticipated higher-interest rates and fees associated with our credit facility.



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##### Table of Contents

Liquidity and Capital Resources

Since inception, we have financed our operations primarily through sales of our convertible preferred stock and common stock, contract payments received under our corporate collaboration agreements and government
grant awards, interest income and equipment financing arrangements. As of March 31, 2006, we had received approximately $151.0 million from the sale of convertible preferred stock and common stock, approximately $106.1 million from contract
payments received under our corporate collaboration agreements and government grant awards, approximately $16.3 million from interest income and approximately $14.0 million from equipment financing arrangements. As of March 31, 2006, we had
approximately $45.5 million in cash and investments, compared to approximately $55.1 million as of December 31, 2005. In April 2006, we raised an additional approximately $23.9 million of net proceeds in a public offering of our common stock.
Our funds are currently invested in U.S. Treasury and government agency obligations and corporate obligations.

Cash used in operating
activities was approximately $9.6 million for the three months ended March 31, 2006, compared to approximately $7.1 million for the same period last year. Our net loss of approximately $10.6 million for the three months ended March 31,
2006 was partially offset by non-cash charges of approximately $1.2 million related to stock-based compensation, depreciation and amortization of investment premiums and discounts. Cash used in operating activities for the same period in 2005 was
primarily used to fund the net loss of $8.0 million, partially offset by non-cash charges of approximately $1.0 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 $159,000 for the three months ended March 31, 2006,
compared to approximately $263,000 for the same period in 2005 largely due to lower capital spending for the three months ended March 31, 2006 as compared to the comparable period in 2005.

Cash provided by financing activities was approximately $32,000 for the three months ended March 31, 2006, compared to approximately $727,000 for
the three months ended March 31, 2005. Financing activities for the three months ended March 31, 2006 included approximately $270,000 of equipment debt financing and approximately $313,000 in proceeds from the sale of our common stock
related to stock option exercises and stock purchases made under our 2000 Employee Stock Purchase Plan, offset by approximately $551,000 of scheduled payments on new and existing debt. Financing activities for the three months ended March 31,
2005 included approximately $944,000 of equipment debt financing and approximately $316,000 in proceeds from the sale of our common stock related to stock option exercises and stock purchases made under our 2000 Employee Stock Purchase Plan, offset
by approximately $533,000 of scheduled payments on new and existing debt.

In April 2004, we entered into a $3.5 million equipment line of
credit agreement for facility improvements and capital purchases, which expired in April 2006. As of March 31, 2006, we had utilized approximately $2.6 million of the line of credit. We expect to obtain a new equipment line of credit for
facility improvements and capital purchases.

In July 2005, we entered into a loan and security agreement with Silicon Valley Bank for a
$35.0 million line of credit. Under the terms of the agreement, we have the option of making up to two draws through May 31, 2006, with a minimum initial draw of $15.0 million. In the event we draw down less than $35.0 million by May 31,
2006, we will be required to pay 0.75% of the unused amount of the credit facility. The credit facility contains financial covenants, including covenants requiring us to maintain: a ratio of unrestricted cash and cash equivalents, investments and
eligible accounts to the aggregate principal amount outstanding under the line of credit of at least 1.3 to 1; at least six months remaining months liquidity, which is calculated by dividing (i) cash and cash equivalents,
investments and eligible



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accounts by (ii) our cash utilization calculated on a rolling three-month basis; and aggregate balances in its investment and operating accounts with
Bank or SVB Asset Management equal to the lesser of (a) the amount of the aggregate outstanding principal amount under the line of credit plus $5.0 million; and (b) 85% of the our aggregate cash and investment account balances. As of
March 31, 2006, we had not utilized the line of credit.

On April 4, 2006, we completed a public offering of 5,100,000 shares of
common stock, at a price of $5.00 per share. The aggregate proceeds of the offering, net of offering costs, were approximately $23.9 million.

We believe that our existing cash and investment securities and anticipated cash flow from our existing collaboration with Roche, including the net proceeds to us from our public offering in April 2006, will be sufficient to support our
current operating plan into the second half of 2007, although 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 collaborations, our rights and obligations under any new collaboration agreements and our ability to generate revenues under any new collaborations;
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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



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



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



| |  |  | our ability to maintain or extend our existing collaborations with Roche 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.
| | - |  | --------------------------------------------------------

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
public or private sale of equity securities, debt financings, additional collaboration or licensing arrangements, government grant awards or any combination of the foregoing or other arrangements. In September 2003, we filed a registration statement
on Form S-3 to offer to sell common stock in one or more offerings up to a dollar amount of $75.0 million. In December 2003, we completed a registered direct offering of 3,115,000 shares of common stock at a price of $9.00 per share. We received
approximately $26.0 million in net proceeds after placement agent fees and other offering costs. In December 2004, we filed a second registration statement on Form S-3 to offer to sell common stock and/or warrants in one or more offerings up to a
dollar amount of $50.0 million. After giving effect to our April 2006 public offering, we may, pursuant to the two registration statements, sell approximately $71.5 million of additional common stock in the future.



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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 March 31, 2006, our obligations and commitments to make future payments under contracts,
such as debt and lease agreements, and under contingent commitments were as follows:



|                                   |  |       |        |                 |   |           |  |           |       |              |   |       |  |   |      
|---------------------------------- |  | ----- | ------ | --------------- | - | --------- |  | --------- | ----- | ------------ | - | ----- |  | - | -----
|(In thousands)                     |  | Total |        | Less than 1Year |   | 1-3 Years |  | 4-5 Years |       | After 5Years
|Equipment financing obligations    |  | $     | 3,677  |                 | $ | 1,930     |  | $         | 1,634 |              | $ | 113   |  | $ |     
|Operating leases                   |  |       | 8,857  |                 |   | 1,635     |  |           | 2,697 |              |   | 2,234 |  |   | 2,291
|Purchase obligations               |  |       | 799    |                 |   | 799       |  |           |      |              |   |      |  |   |     
|                                   |  |       |        |                 |   |           |  |           |       |              |   |       |  |   |      
|Total contractual cash obligations |  | $     | 13,333 |                 | $ | 4,364     |  | $         | 4,331 |              | $ | 2,347 |  | $ | 2,291
|                                   |  |       |        |                 |   |           |  |           |       |              |   |       |  |   |      



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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. 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%, as of March 31, 2006 rates, the fair value of our portfolio would decline by approximately $88,000 on that date.

The following table represents the fair value balance of our cash, cash equivalents and short-term and long-term investments that are subject to interest rate risk by year of expected maturity and average interest
rates as of March 31, 2006 (dollars in thousands):



|                          |  |      |        |   |      |   |       |  
|------------------------- |  | ---- | ------ | - | ---- | - | ----- | -
|                          |  | 2006 |        |   | 2007 |  
|Cash and cash equivalents |  | $    | 14,524 |   |      |   |      |  
|Average interest rate     |  |      | 3.95   | % |      |   |      |  
|                          |  |     
|Short-term investments    |  | $    | 27,022 |   |      | $ | 2,961 |  
|Average interest rate     |  |      | 3.43   | % |      |   | 5.12  | %
|                          |  |     
|Long-term investments     |  | $    | 949    |   |      |   |      |  
|Average interest rate     |  |      | 0.75   | % |      |   |      |  

We did not hold any derivative instruments as of March 31, 2006, and we have never held such
instruments in the past. In addition, we had outstanding debt, consisting of borrowings under equipment financings, of $3.4 million as of March 31, 2006, with a range of interest rates from 6.31% to 9.07%.

Item 4:Controls and Procedures

Evaluation of Disclosure Controls and Procedures. Based on their
evaluation as of March 31, 2006, our chief executive officer and principal financial officer and accounting 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)) are effective to ensure that information required to be disclosed by us in reports that we file or submit under the Exchange Act is recorded, processed, summarized and
reported within the time periods specified by the Securities and Exchange Commissions rules and forms.

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 March 31, 2006 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 principal financial officer and accounting 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. Further, the design of a control system must reflect the fact that there are resource constraints, and
the benefits of controls must be considered relative to their costs. 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, 2006.

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

We commenced operations in 1996 and are still in an early stage of development. We have not commercialized any products,
and we have incurred significant losses to date. As of March 31, 2006, we had an accumulated deficit of approximately $141.4 million. To date, our revenues have been primarily from collaborations 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 have incurred net losses since our inception, including a net loss of approximately $10.6 million for
the three months ended March 31, 2006. 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 drug candidates and research
programs. The amount of time necessary to successfully commercialize any of our drug 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 clinical 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.

In July 2005, we entered into a $35.0 million line of credit facility. However, we may not be able to borrow funds under this credit facility if we are
not able to meet various conditions, covenants and representations, including covenants requiring us to maintain:



| |  |  | a ratio of unrestricted cash and cash equivalents, investments and eligible accounts to the aggregate principal amount outstanding under the line of credit of at least 1.3 to 1;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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| |  |  | at least six months remaining months liquidity, which is calculated by dividing (a) cash and cash equivalents, investments and eligible accounts by (b) our
cash burn calculated on a rolling three-month basis; and
| | - |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | aggregate balances in our investment and operating accounts with Silicon Valley Bank or SVB Asset Management equal to the lesser of (a) the amount of the aggregate outstanding
principal amount under the line of credit plus $5.0 million; and (b) 85% of our aggregate cash and investment account balances.
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

We may need to raise additional capital in order to stay in compliance with these covenants. Further debt financing, if available, may subject us to similar or additional 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 - Liquidity and
Capital Resources.

We believe that our existing cash and investment securities and anticipated cash flow from our existing
collaboration with Roche, including the net proceeds from our public offering in April 2006, will be sufficient to support our current operating plan into at least the second half of 2007, although 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 collaborations, our rights and obligations under any new collaboration agreements and our ability to generate revenues under any new collaborations;
| | - |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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



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



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



| |  |  | our ability to maintain or extend our existing collaborations with Roche 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.
| | - |  | --------------------------------------------------------



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Our workforce reduction announced in March 2006 may have an adverse impact on our ability to make significant progress
on our clinical and research programs or otherwise harm our business.*

In March 2006, we announced a workforce reduction of 39
employees to reflect a realignment of our research priorities and corporate operations to support our clinical product candidates and pipeline opportunities. Workforce and expense reductions have resulted in a reduction in the scope of our research
programs, and further reductions could result in the reduced scope of, and progress on, our clinical and research programs. In addition, employees, whether or not directly affected by a reduction, may seek alternative employment. Although our
employees are required to sign a confidentiality agreement at the time of hire, the confidential nature of certain proprietary information may not be maintained in the course of any such future employment. We may face litigation from former
employees who were affected by the workforce reduction. In addition, the implementation of expense reduction programs has resulted in, and may in the future result in, the diversion of efforts of our executive management team and other key
employees, which could adversely affect our business.



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 scientific personnel to perform future research, manufacturing and development work, as well as key management personnel with expertise in clinical testing and finance, among other areas, will
be critical to our success. None of our employees have employment commitments for any fixed period of time and could leave our employment at will. In the past, we have experienced turnover among our management, including the recent resignations of
our former Chief Executive Officer and Chief Financial Officer. In addition, in March 2006, we experienced a restructuring that resulted in a workforce reduction. We may have difficulty attracting required personnel as a result of a perceived risk
of future workforce and expense reductions, or otherwise. 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 pursue collaborations and develop our product candidates and research programs, which would likely have an adverse effect on our business.

Our 1996 Stock Option Plan will expire on June 12, 2006. Regulations implemented by the NASDAQ National Market require stockholder
approval for all stock option plans, and regulations implemented by the New York Stock Exchange, or NYSE, prohibit NYSE member organizations from giving a proxy to vote on equity-compensation plans unless the beneficial owner of the shares has given
voting instructions. As a result of these regulations, we will not be able to grant stock options to employees after our 1996 Stock Option Plan expires unless our stockholders approve a new plan. We have asked stockholders to approve the 2006 Equity
Incentive Plan at our 2006 annual meeting of stockholders. If the 2006 Equity Incentive Plan is not approved, we may incur increased compensation costs, be required to change our equity compensation strategy or find it difficult to attract, retain
and motivate employees, each of which could materially and adversely affect our business.



If our current collaborations are unsuccessful or if conflicts develop with our collaborators, 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 corporate research and commercialization collaboration with Roche in the field of epothilones. 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 2006. We may not be able to
maintain or extend these collaborations or license agreements on acceptable terms, if at all. If we do not



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maintain, extend or replace our corporate collaboration with Roche, our research and development efforts 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 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 or timing of resources that our collaborators devote to our programs or potential products, nor the scope, content and timing of the efforts that they conduct or permit under the collaborations. As a result, we do not know if our
collaborators 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 collaborators 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 beyond limited funding for certain Kosan activities specified under the contract, 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. For example, we anticipate that Roche will pursue late-stage clinical development of only one of our epothilone product candidates, if any. We anticipate that Roche will make a decision in this
regard in 2006. Roche may elect to cease development of KOS-862, our lead epothilone compound, in favor of further development of KOS-1584, which is currently in Phase I clinical trials. In our collaboration with the NCI, we do not control the
selection, conduct, timing and resources provided to clinical trials of geldanamycin analogs 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. We also do not know whether our current collaborative partners or future collaborative 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 collaborative arrangements with us. In addition, business combinations or significant changes in a collaborators business strategy may adversely affect a
collaborators willingness or ability to continue the collaboration with us.

Failure by our corporate 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 collaborators fail to conduct the collaborative 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 collaboration agreements are terminated, we may be required to seek new collaborators 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
collaborators 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.



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If we fail to enter into new collaborative agreements 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 collaborative arrangements
will ultimately be successful. There have been, and may continue to be, a significant number of business combinations among large pharmaceutical companies that have resulted, and may continue to result, in a reduced number of potential future
corporate collaborators, which may limit our ability to find partners who will work with us in developing and commercializing our drug candidates. If we do not enter into new collaborative agreements, we may be required to curtail, suspend or
terminate research and development programs 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 an early stage of development, and substantial additional effort
and expense will be necessary for development.

Our drug candidates are in early 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 clinical testing, before we can submit any application for regulatory approval.

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 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 animal and human 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 II clinical study of 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 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.



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We do not know whether clinical trials of our product candidates (including ongoing and anticipated
clinical trials of KOS-953, KOS-1022, KOS-862, KOS-1584, KOS-2187 or other product candidates) will begin on time or whether any of our clinical trials will be completed on schedule, or at all. We also do not know whether clinical trials will
indicate that an earlier-stage compound, such as KOS-1584 or KOS-1022, will be more appropriate for commercial development than a related compound that is at a later stage of clinical development, such as KOS-862 or KOS-953, respectively. Negative
or inconclusive results or adverse medical events during a clinical trial could cause a clinical trial to be suspended, repeated or terminated. Certain of the clinical trials of our product 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, KOS-953, KOS-1022,
KOS-862 and KOS-1584. 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 II 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 II clinical study of KOS-862 in hormone-refractory prostate cancer than in our Phase II non-small cell lung cancer trial. We cannot predict whether future trials of KOS-862, or other compounds,
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 drug 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 KOS-953 and KOS-1022 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. While the FDA has not agreed or objected to these
changes, 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 KOS-953 and KOS-1022 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;
| | - |  | --------------------------------------------------------------------------------------------------



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



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



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



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| |  |  | 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 governmental authorities to require suspension or modification of a clinical trial.
| | - |  | ------------------------------------------------------------------------------------------------------------------

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 collaborators 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 products. 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 (including ongoing and anticipated
clinical trials of KOS-953, KOS-1022, KOS-862, KOS-1584 or other product candidates) will demonstrate safety and efficacy sufficient to obtain the requisite regulatory approvals or will result in marketable products. Our 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 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 contract research organizations, medical institutions and clinical investigators to perform
this function. We also rely on Roche to conduct certain clinical trials for KOS-862 and potentially KOS-1584 and the NCI to conduct certain clinical trials for 17-AAG and KOS-1022. We may rely on future collaborators 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, 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 17-AAG 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 KOS-953 if a competitor
obtains approval before us of a product containing 17-AAG for the treatment of multiple myeloma or chronic myelogenous leukemia in the United States or Europe. 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.

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 collaborators or us, including requiring withdrawal of the product from the
market.



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If any of our approved products, our collaborators 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 collaborators;
| | - |  | ------------------------------------------------------------------------------------------------------------------



| |  |  | 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 products 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.

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, the patent positions of biotechnology companies, including our patent position, 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. In particular, two of our exclusively licensed patents, which cover KOS-862 and its use, are the subject of an interference proceeding with a patent application concerning
epothilones C, D, E, and F filed by Gesellschaft für Biotechnologische Forschung, or GBF. This patent application is licensed to BMS. Patents may be challenged, held unenforceable, invalidated or circumvented. Thus, any patents that we own or
license from third parties may not provide protection against competitors.

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 pending patent applications will result in issued patents;
| | - |  | ---------------------------------------------------------------------------------------



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



| |  |  | any patents issued to us or our licensors and 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, drug candidates, formulations and uses thereof, as we deem appropriate. However, we may fail to apply for patents on important technologies or products in a



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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, 17-AAG, the active pharmaceutical ingredient in the most advanced product candidate in our Hsp90 inhibitor program, KOS-953, was originally disclosed in a now-expired
third party patent. Consequently, others can develop products containing 17-AAG. We are aware of at least two other companies that have been developing product candidates containing or based on 17-AAG, 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 17-AAG. 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.

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.



Patents related to one of our product candidates, KOS-862, are the subject of an interference in
the U.S. involving patent rights licensed to us. Oppositions have been filed in Europe against patents granted to a third party that purportedly cover this compound. These proceedings or any other proceedings are costly, and an unfavorable outcome
could prevent us from commercializing this compound.*

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. Two of our exclusively licensed patents, which cover
KOS-862 and its use, are the subject of an interference proceeding with a patent application concerning epothilones C, D, E and F filed by GBF. GBF has licensed this patent application to BMS, one of our competitors. In connection with the
anticipated settlement of the interference proceeding, we have entered into a cross-license agreement with BMS under which both parties have agreed to grant the other a co-exclusive, worldwide license to epothilone D, or KOS-862. Further, 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 Novartis AG relating to epothilone biosynthetic genes; and patents and applications owned by Abbott
Laboratories or Biotica Technologies Ltd. relating to erythromycin 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. Although the written decision has not yet been issued, at the hearing, the patent was maintained (upheld), but with narrowing amendments; the time for Biotica
to appeal the ruling has not yet expired. In addition, the European Patent Office has recently granted patents to GBF, which have been licensed to BMS and which, if valid in individual European countries, would cover KOS-862 in those countries.
Formal oppositions to these patents have been filed with the European Patent Office.

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. 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



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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, which could 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 drug candidates that we are developing or desire to develop; methods of treatment or administration involving our drug candidates; formulations of our drug 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 drug candidates that we are developing or pursue our polyketide synthase, or 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 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.

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



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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 two manufacturers to make 17-AAG, which is the active pharmaceutical ingredient in KOS-953, and KOS-1022. We formulate
the final drug product for KOS-953 at our own facility and through contract manufacturers. We formulate the final drug product for KOS-1022 through contract manufacturers. The NCI currently formulates drug product for the formulation of 17-AAG being
studied by it under our cooperative research and development agreement, or CRADA. We maintain a limited inventory of KOS-953 and KOS-1022 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 the formulation of 17-AAG or KOS-1022 being studied by the NCI under our CRADA. In our epothilone
program, we are the sole manufacturer of the active pharmaceutical ingredient for KOS-862, and we rely on contract manufacturers for the active pharmaceutical ingredient for KOS-1584. We use a single outside contractor to formulate drug product for
KOS-862, and drug product for KOS-1584 is formulated at our facilities and by an outside contractor. We maintain limited inventories of formulated drug product for KOS-862 and KOS-1584 at our facilities in Hayward, California and at the facilities
of an outside contractor. Limited inventories of formulated drug product for KOS-862 are also maintained by Roche.

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.

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



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



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



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



| |  |  | the delay of product shipments due to U.S. custom regulations or third-party carriers used to transport our products, and damage to our products 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
products;
| | - |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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



| |  |  | difficulties in scaling-up production of our products 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



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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. Our access to any key intermediates, active pharmaceutical ingredient or
formulated drug product for our product candidates sourced or inventoried solely through our facilities in Hayward, California may be subject to interruption in the event of an earthquake.

As discussed above, we rely upon outside contractors to manufacture and supply to us key intermediates, active pharmaceutical ingredients and formulated
drug product for our product candidates. Our dependence upon others for the manufacture of our product candidates and components thereof may adversely affect our ability to continue in a timely manner clinical development of our product candidates
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 or timeframe 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.

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

We face, and will continue to face, intense competition from organizations such as large 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 Conforma Therapeutics, which has initiated Phase I clinical trials with its formulation of 17-AAG in solid tumors and chronic myelogenous leukemia and has announced
that it expects to be acquired by Biogen Idec Inc. in the second quarter of 2006, Infinity Pharmaceuticals, which has initiated Phase I clinical trials of its Hsp90 inhibitor in multiple myeloma and gastrointestinal stromal tumors, and Vernalis plc,
which has announced plans to enter clinical trials of its Hsp90 inhibitor in the second half of 2006 in collaboration with Novartis AG, as well as other companies reported to be pursuing Hsp90 inhibitors. Competing epothilones in clinical
development include those being developed by BMS (reported to be in Phase III clinical trials), Novartis AG (reported to be in Phase III clinical trials) and Schering AG (reported to be in Phase II clinical trials). Gastrointestinal motility
competitors include Chugai Pharmaceuticals, whose motilide agonist is reported to be in Phase II clinical



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trials. 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. Some of these competitors have entered into collaborations with leading companies within our target
markets to produce polyketides for commercial purposes.

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.

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



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



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



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



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



| |  | the timing and scope of regulatory approvals;
| | - | ---------------------------------------------



| |  | our ability to manufacture and sell commercial quantities of future products to the market;
| | - | -------------------------------------------------------------------------------------------



| |  | 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 in the future 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 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



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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 collaborators
of these materials, and our liability may exceed our 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.

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.

Some of our existing stockholders can
exert control over us and may not make decisions that are in the best interest of all stockholders.*

Our officers, directors and their
affiliates together controlled approximately 13% of our outstanding common stock as of March 31, 2006. As a result, these stockholders, if they act together, are able to exert a significant degree of influence over our management and affairs
and over matters requiring stockholder approval, including the election of directors and approval of significant corporate transactions. This concentration of ownership may delay or prevent a change in control of us and might affect the market price
of our common stock, even when a change may be in the best interests of all stockholders. In addition, the interests of this concentration of ownership may not always coincide with our interests or the interests of other stockholders, and
accordingly, they could cause us to enter into transactions or agreements, which we would not otherwise consider.

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 April 1, 2005 through March 31, 2006, our common stock traded between $3.96



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and $9.77 on the NASDAQ National 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:



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



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



| |  |  | 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 collaborators or us in obtaining regulatory approvals for our product candidates;
| | - |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------



| |  |  | new products or services introduced or announced 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;
| | - |  | ------------------------------------------------------------------



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



| |  |  | sales of our common stock, including sales by previous executive officers and other former employees or consultants;
| | - |  | --------------------------------------------------------------------------------------------------------------------



| |  |  | 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 National Market and the market for biotechnology companies in particular, has 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 results of operations will fluctuate, and this fluctuation could cause our stock price to decline, creating investor losses.

Our quarterly 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 research contracts with collaborators or government research grants, which may not be renewed or replaced;
| | - |  | ---------------------------------------------------------------------------------------------------------------------------------------



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



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



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



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



42



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##### Table of Contents

If our revenues decline or do not grow due to expiration, termination or amendment of current or future
collaboration agreements, licenses or government research grants, 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 comparisons of our operating results are not a good indication of our future performance. Our operating results
in some quarters may not meet the expectations of stock market analysts and investors. In that case, our stock price would probably decline.

Changes in
the accounting treatment of stock options will continue to adversely affect our results of operations.*

We adopted SFAS 123R on
January 1, 2006, under which we are required to record additional compensation expense related to stock options and other share-based payments in 2006 and beyond. The impact on our earnings resulting from this new standard will have a negative
impact on our reported results of operations compared to the results we have reported under prior accounting standards on stock options and other share-based payments.

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 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, 2005, there may be changes in our
systems, processes or operations that will effect 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 assessment, investor confidence and our stock price could be adversely affected.



43



---

##### Table of Contents

Item 5:Other Information

None.

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)                                                                                                                                                                           
|            | 
|10.28       |  | Consulting Agreement between Registrant and Susan M. Kanaya dated January 1, 2006.(5)*                                                                                                                                                                    
|            | 
|10.29       |  | Form of Level I Change of Control and Severance Benefit Agreement. (5)*                                                                                                                                                                                   
|            | 
|10.30       |  | Form of Level II Change of Control and Severance Benefit Agreement. (5)*                                                                                                                                                                                  
|            | 
|10.31       |  | First Amendment to Loan and Security Agreement between Registrant and Silicon Valley Bank dated as of February 21, 2006. (5)                                                                                                                              
|            | 
|10.32       |  | Employment Agreement between Registrant and Petrus Timmermans, Ph.D. dated December 6, 2004.(5)*                                                                                                                                                          
|            | 
|10.33       |  | Compensation Information for Executive Officers. (5)*                                                                                                                                                                                                     
|            | 
|10.34       |  | Separation and Consulting Agreement between Registrant and Daniel V. Santi, M.D., Ph.D. dated March 20, 2006.*                                                                                                                                            
|            | 
|10.35       |  | Letter agreement among Registrant, Hoffmann-La Roche Inc. and F. Hoffmann-La Roche Ltd. dated March 27, 2006.+                                                                                                                                            
|            | 
|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 annual report on Form 10-K for the year ended December 31, 2005.
|--- | ---------------------------------------------------------------------------------------------------------------

|(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.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|+ | Confidential treatment has been requested for certain portions of this exhibit. Omitted portions have been filed separately with the Securities and Exchange Commission.
|- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|* | Represents a management or director compensation plan.
|- | ------------------------------------------------------



44



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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
|             |  |                               
|May 10, 2006 |  | By:                            |  | /s/ Robert G. Johnson, Jr.                                  
|             |  |                                |  | Robert G. Johnson, Jr., M.D., Ph.D.                         
|             |  |                                |  | Chief Executive Officer                                     
|             |  |                               
|May 10, 2006 |  | By:                            |  | /s/ David L. Johnson                                        
|             |  |                                |  | David L. Johnson                                            
|             |  |                                |  | Principal Accounting Officer and Principal Financial Officer



45



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##### Table of Contents

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)                                                                                                                                                                           
|            | 
|10.28       |  | Consulting Agreement between Registrant and Susan M. Kanaya dated January 1, 2006.(5)*                                                                                                                                                                    
|            | 
|10.29       |  | Form of Level I Change of Control and Severance Benefit Agreement. (5)*                                                                                                                                                                                   
|            | 
|10.30       |  | Form of Level II Change of Control and Severance Benefit Agreement. (5)*                                                                                                                                                                                  
|            | 
|10.31       |  | First Amendment to Loan and Security Agreement between Registrant and Silicon Valley Bank dated as of February 21, 2006. (5)                                                                                                                              
|            | 
|10.32       |  | Employment Agreement between Registrant and Petrus Timmermans, Ph.D. dated December 6, 2004.(5)*                                                                                                                                                          
|            | 
|10.33       |  | Compensation Information for Executive Officers.(5)*                                                                                                                                                                                                      
|            | 
|10.34       |  | Separation and Consulting Agreement between Registrant and Daniel V. Santi, M.D., Ph.D. dated March 20, 2006.*                                                                                                                                            
|            | 
|10.35       |  | Letter agreement among Registrant, Hoffmann-La Roche Inc. and F. Hoffmann-La Roche Ltd. dated March 27, 2006.+                                                                                                                                            
|            | 
|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.(7) 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 annual report on Form 10-K for the year ended December 31, 2005.
|--- | ---------------------------------------------------------------------------------------------------------------

|(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.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|+ | Confidential treatment has been requested for certain portions of this exhibit. Omitted portions have been filed separately with the Securities and Exchange Commission.
|- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|* | Represents a management or director compensation plan.
|- | ------------------------------------------------------