KOSAN BIOSCIENCES INCORPORATED

            THIRD AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT

                           DATED AS OF MARCH 30, 2000






                                TABLE OF CONTENTS




                                                                                                               PAGE
                                                                                                         
Section 1 Restrictions on Transferability of Securities; Compliance with Securities Act...........................2

1.1      CERTAIN DEFINITIONS......................................................................................2

1.2      RESTRICTIONS ON TRANSFERABILITY..........................................................................3

1.3      RESTRICTIVE LEGENDS......................................................................................3

1.4      NOTICE OF PROPOSED TRANSFERS.............................................................................4

1.5      REQUESTED REGISTRATION...................................................................................5

1.6      COMPANY REGISTRATION.....................................................................................7

1.7      REGISTRATION ON FORM S-3.................................................................................8

1.8      EXPENSES OF REGISTRATION.................................................................................9

1.9      INDEMNIFICATION.........................................................................................10

1.10     Information of Holder; Copies of Prospectus.............................................................12

1.11     OBLIGATIONS OF THE COMPANY..............................................................................12

1.12     RULE 144 REPORTING......................................................................................13

1.13     TRANSFER OF REGISTRATION RIGHTS.........................................................................14

1.14     STANDOFF AGREEMENT......................................................................................14

Section 2 Miscellaneous..........................................................................................14

2.1      TRANSFER; SUCCESSORS AND ASSIGNS........................................................................14

2.2      GOVERNING LAW...........................................................................................15

2.3      COUNTERPARTS............................................................................................15

2.4      TITLES AND SUBTITLES....................................................................................15

2.5      NOTICES.................................................................................................15

2.6      TERMINATION.............................................................................................15


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                               TABLE OF CONTENTS
                                  (CONTINUED)

                                                                                                               PAGE

2.7      SEVERABILITY............................................................................................15

2.8      ENTIRE AGREEMENT........................................................................................15

2.9      MODIFICATIONS AND AMENDMENTS............................................................................15

2.10     ADDITIONAL REGISTRATION RIGHTS..........................................................................16

EXHIBIT A - HOLDERS




                                      -ii-



         THIS THIRD AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT (the
"Agreement") is made as of the 30th day of March, 2000 by and among Kosan
Biosciences Incorporated, a California corporation (the "Company") and the
persons and entities listed on EXHIBIT A attached hereto. Capitalized terms used
herein and not otherwise defined shall have the meanings ascribed to them in
Section 1.1 hereof.

         WHEREAS, concurrently with the execution and delivery of this
Agreement, certain of the Holders are acquiring shares of the Company's Series C
Preferred Stock pursuant to a Series C Preferred Stock Purchase Agreement of
even date herewith (the "Series C Purchase Agreement");

         WHEREAS, the Company sold shares of its Series A Preferred Stock to
certain of the Holders pursuant to a Series A Preferred Stock Purchase Agreement
dated as of January 31, 1997 (the "Series A Purchase Agreement");

         WHEREAS, in connection with the Series A Purchase Agreement, the
Company and certain of the Holders entered into an Amended and Restated
Registration Rights Agreement dated as of January 31, 1997, pursuant to which
the Company granted such Holders certain registration rights with respect to the
Company's Common Stock to be acquired upon the conversion of the Series A
Preferred Stock;

         WHEREAS, the Company sold shares of its Series B Preferred Stock to
certain of the Holders pursuant to a Series B Preferred Stock Purchase Agreement
dated as of April 3, 1998 (the "Series B Purchase Agreement");

         WHEREAS, in connection with the Series B Purchase Agreement, the
Company and certain of the Holders entered into a Second Amended and Restated
Registration Rights Agreement dated as of April 3, 1998 (the "Prior Registration
Rights Agreement"), pursuant to which the Company granted such Holders certain
registration rights with respect to the Company's Common Stock to be acquired
upon the conversion of the Series B Preferred Stock;

         WHEREAS, the Company wishes to amend and restate the Prior Registration
Rights Agreement to grant to those persons and entities that are purchasing
Series C Preferred Stock pursuant to the Series C Purchase Agreement certain
registration rights with respect to the Company's Common Stock; and

         WHEREAS, pursuant to Section 2.9 of the Prior Registration Rights
Agreement, the Prior Registration Rights Agreement may be modified or amended
only with the written consent of the Company and the Holders (as such term is
defined in the Prior Registration Rights Agreement) holding at least seventy
percent (70%) of the Registrable Securities (as such term is defined in the
Prior Registration Rights Agreement) then subject to the Prior Registration
Rights Agreement, and those persons and entities listed on the signature pages
hereof constitute the Holders of at least seventy percent (70%) of the
Registrable Securities now subject to the Prior Registration Rights Agreement.



         NOW, THEREFORE, the parties agree as follows:

                                   SECTION 1
                 RESTRICTIONS ON TRANSFERABILITY OF SECURITIES;
                         COMPLIANCE WITH SECURITIES ACT

         1.1 CERTAIN DEFINITIONS. As used in this Agreement, the following terms
shall have the following respective meanings:

         "AFFILIATE" shall mean any person controlling, controlled by or under
common control with another person. "Control" shall mean the right to direct the
management or policies of the person or to elect or appoint a majority of its
managerial body (such as the board of directors of a corporation, the managers
of a limited liability company, the general partners of a partnership, the
trustees of a trust, etc.), or the ownership of more than 50% of the equity
interests of the person. "Person" shall mean any individual, corporation,
company, partnership, trust, association or other legal entity.

         "COMMISSION" shall mean the Securities and Exchange Commission or any
other federal agency at the time administering the Securities Act.

         "CONVERSION SHARES" shall mean the shares of the Company's Common Stock
issued or issuable upon the conversion of Shares that are convertible
securities.

         "HOLDER" shall mean any holder of Registrable Securities listed on
EXHIBIT A, including any person holding Registrable Securities to whom the
rights under this Section 1 have been transferred in accordance with Section
1.13 hereof.

         "INITIATING HOLDERS" shall mean any Holders of at least twenty-five
(25%) of the Registrable Securities.

         "REGISTRABLE SECURITIES" means (i) Conversion Shares and (ii) any
Common Stock of the Company issued or issuable with respect to such Conversion
Shares upon any stock split, stock dividend, recapitalization or similar event
or any Common Stock otherwise issued with respect to the such shares; PROVIDED,
HOWEVER, that Common Stock or other securities shall only be treated as
Registrable Securities if and so long as they have not been (A) sold to or
through a broker or dealer or underwriter in a public distribution or a public
securities transaction, or (B) sold (other than pursuant to Section 1.13 hereof)
after the Company's Common Stock is registered under the Securities Exchange Act
of 1934, as amended, in a transaction exempt from the registration and
prospectus delivery requirements of the Securities Act under Rule 144 thereof
(or any comparable exemption) so that all transfer restrictions and restrictive
legends with respect thereto, if any, are removed upon the consummation of such
sale.

         "RESTRICTED SECURITIES" shall mean the Shares and the Conversion
Shares.


                                      -2-


         The terms "REGISTER," "REGISTERED" and "REGISTRATION" refer to a
registration effected by preparing and filing a registration statement in
compliance with the Securities Act and the declaration or ordering of the
effectiveness of such registration statement.

         "REGISTRATION EXPENSES" shall mean all expenses incurred by the Company
in complying with Sections 1.5, 1.6 and 1.7 hereof, including, without
limitation, all registration, qualification and filing fees, printing expenses,
escrow fees, fees and disbursements of counsel for the Company, blue sky fees
and expenses, and the expense of any special audits incident to or required by
any such registration (but excluding the compensation of regular employees of
the Company which shall be paid in any event by the Company).

         "SECURITIES ACT" shall mean the Securities Act of 1933, as amended, or
any similar federal statute and the rules and regulations of the Commission
thereunder, all as the same shall be in effect at the time.

         "SELLING EXPENSES" shall mean all underwriting discounts, selling
commissions and stock transfer taxes applicable to the securities registered by
any of the Holders and all fees and disbursements of counsel for such Holders
(as limited by Section 1.8).

         "SHARES" shall mean the shares of the Company's Common Stock, the
shares of the Company's Series A Preferred Stock, the shares of the Company's
Series B Preferred Stock and the shares of the Company's Series C Preferred
Stock, in each case listed opposite the name of each Holder on EXHIBIT A
attached hereto.

         References in this Section 1 to the Company's "commercially reasonable
efforts" with respect to a registration shall not be construed so as to permit
the Company to delay or refuse to undertake a registration due to the (i)
expense to the Company of such registration, (ii) timing of such registration,
or (iii) involvement of the Company's management or other resources in such
registration.

         1.2 RESTRICTIONS ON TRANSFERABILITY. The Restricted Securities shall
not be sold, assigned, transferred or pledged except upon the conditions
specified in this Section 1, which conditions are intended to ensure compliance
with the provisions of the Securities Act. Each Holder will cause any proposed
purchaser, assignee, transferee or pledgee of any of the Restricted Securities,
to agree to take and hold such Restricted Securities subject to the provisions
and upon the conditions specified in this Section 1, including without
limitation those imposed upon Holders under Section 1.13.

         1.3 RESTRICTIVE LEGENDS.



         (a) Each certificate representing (i) the Shares, (ii) the Conversion
Shares and (iii) any other securities issued in respect of the Shares or the
Conversion Shares upon any stock split, stock dividend, recapitalization,
merger, consolidation or similar event, shall (unless otherwise permitted by the
provisions of Section 1.4 below) be stamped or otherwise imprinted with a legend
in substantially the following form (in addition to any legend required under
applicable state securities laws):


                                      -3-


                  THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED
                  FOR INVESTMENT AND NOT FOR DISTRIBUTION, AND HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. SUCH
                  SHARES MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR
                  HYPOTHECATED OR OTHERWISE TRANSFERRED IN THE ABSENCE OF A
                  REGISTRATION STATEMENT IN EFFECT WITH RESPECT THERETO UNDER
                  SUCH ACT UNLESS SOLD PURSUANT TO RULE 144 OF SUCH ACT OR
                  UNLESS THE SALE IS OTHERWISE EXEMPT FROM REGISTRATION. UNLESS
                  SUCH SHARES ARE SOLD PURSUANT TO RULE 144 OF THE SECURITIES
                  ACT, THE COMPANY MAY REQUEST A WRITTEN OPINION OF COUNSEL,
                  WHICH OPINION AND COUNSEL ARE ACCEPTABLE TO THE COMPANY, TO
                  THE EFFECT THAT REGISTRATION IS NOT REQUIRED IN CONNECTION
                  WITH ANY SUCH SALE, PLEDGE OR HYPOTHECATION OR OTHER TRANSFER.
                  THIS CERTIFICATE MUST BE SURRENDERED TO THE COMPANY OR ITS
                  TRANSFER AGENT AS A CONDITION PRECEDENT TO THE SALE, PLEDGE,
                  HYPOTHECATION OR OTHER TRANSFER OF ANY INTEREST IN ANY OF THE
                  SHARES REPRESENTED BY THIS CERTIFICATE.

                  THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO
                  RESTRICTIONS ON TRANSFER CONTAINED IN AN AGREEMENT BETWEEN THE
                  COMPANY AND THE SHAREHOLDER, A COPY OF WHICH IS ON FILE WITH
                  THE SECRETARY OF THE COMPANY.

         (b) Each Holder consents to the Company making a notation on its
records and giving instructions to any transfer agent of the Restricted
Securities in order to implement the restrictions on transfer established in
this Section 1.

         1.4 NOTICE OF PROPOSED TRANSFERS. Each Holder by acceptance of
Restricted Securities agrees to comply in all respects with the provisions of
this Section 1.4. Prior to any proposed sale, assignment, transfer or pledge
of any Restricted Securities, unless there is in effect a registration
statement under the Securities Act covering the proposed transfer, the holder
thereof shall give written notice to the Company of such holder's intention
to effect such transfer, sale, assignment or pledge. Each such notice shall
describe the manner and circumstances of the proposed transfer, sale,
assignment or pledge in sufficient detail and, if the Company reasonably so
requests, shall be accompanied at such holder's expense by either (i) an
opinion of legal counsel which shall be reasonably satisfactory to the
Company, which opinion shall be addressed to the Company, to the effect that
the proposed transfer of the Restricted Securities may be effected without
registration under the Securities Act, or (ii) a "no action" letter from the
Commission to the effect that the transfer of such securities without
registration will not result in a recommendation by the staff of the
Commission that action be taken with respect thereto, whereupon the holder of
such Restricted Securities shall be entitled to transfer such Restricted
Securities in accordance with the terms of the notice delivered by the holder
to the Company. Each certificate or other writing evidencing the Restricted
Securities transferred as above provided shall bear, except if such transfer
is made pursuant to Rule 144, the appropriate restrictive legend set forth in
Section 1.3 above, except that


                                      -4-



such certificate shall not bear such restrictive legend if in the opinion of
counsel for such holder and the Company such legend is not required in order
to establish compliance with any provisions of the Securities Act. It is
agreed that the Company will not require opinions of counsel for transactions
made pursuant to Rule 144. Notwithstanding the provisions of this Section
1.4, no such opinion of counsel shall be necessary for a transfer by a Holder
which is (A) a partnership to its partners or former partners in accordance
with partnership interests, (B) a corporation to its shareholders in
accordance with their interest in the corporation, (C) a limited liability
company to its members or former members in accordance with their interest in
the limited liability company, or (D) to the Holder's or Affiliate's family
member or to a trust for the benefit of an individual Holder, provided, that
in all cases the transferee will be subject to the terms of this Section 1.4
to the same extent as if such transferee were an original Holder hereunder.

         1.5 REQUESTED REGISTRATION.

         (a) REQUEST FOR REGISTRATION. In case the Company shall receive from
Initiating Holders a written request that the Company effect any registration,
qualification or compliance with respect to the Registrable Securities, the
anticipated aggregate offering price of which is at least $5,000,000, the
Company will:

                  (i) promptly give written notice of the proposed registration,
qualification or compliance to all other holders of Registrable Securities; and

                  (ii) as soon as practicable, use its commercially reasonable
efforts to effect such registration, qualification or compliance (including,
without limitation, the execution of an undertaking to file post-effective
amendments, appropriate qualification under applicable blue sky or other state
securities laws and appropriate compliance with applicable regulations issued
under the Securities Act and any other governmental requirements or regulations)
as may be so requested and as would permit or facilitate the sale and
distribution of all or such portion of such Registrable Securities as are
specified in such request, together with all or such portion of the Registrable
Securities of any Holder joining in such request as are specified in a written
request received by the Company within thirty (30) days after receipt of such
written notice from the Company; PROVIDED, HOWEVER, that the Company shall not
be obligated to take any action to effect any such registration, qualification
or compliance pursuant to this Section 1.5:

                                    (A) In any particular jurisdiction in which
the Company would be required to execute a general consent to service of process
in effecting such registration, qualification or compliance unless the Company
is already subject to service in such jurisdiction and except as may be required
by the Securities Act;

                                    (B) prior to the earlier of twelve (12)
months from the date of this Agreement or one hundred and eighty (180) days
immediately following the effective date of the registration statement
pertaining to a firm commitment underwritten initial public offering of
securities of the Company;


                                      -5-



                                    (C) beginning at any time when the Company
delivers notice to the holders of Registrable Securities within thirty (30) days
of any registration request of its bona fide intention to file a registration
statement with the Commission pertaining to a firm commitment underwritten
initial public offering of securities of the Company within ninety (90) days of
such request and ending on the earlier of the abandonment or consummation of
such offering;

                                    (D) during the one hundred and eighty (180)
days immediately following the effective date of the registration statement
pertaining to a firm commitment underwritten initial public offering of
securities of the Company (other than a registration of securities in a Rule 145
transaction or with respect to an employee benefit plan);

                                    (E) after the Company has effected three
such registrations pursuant to this subparagraph 1.5(a), such registrations have
been declared or ordered effective and the securities offered pursuant to such
registrations have been sold; or

                                    (F) if the Company shall furnish to Holders
a certificate signed by the President of the Company stating that in the good
faith judgment of the Board of Directors it would be seriously detrimental to
the Company or its shareholders for a registration statement to be filed in the
near future, provided that the Company's obligation to use its best efforts to
register, qualify or comply under this Section 1.5 shall be deferred for a
period not to exceed ninety (90) days, and provided, further, that the Company
shall not exercise its right under this clause to defer such obligation more
than once in any twelve (12) month period.

         Subject to the foregoing clauses (A) through (F), the Company shall
file a registration statement covering the Registrable Securities so requested
to be registered as soon as practicable after receipt of the request or requests
of the Initiating Holders.

                  (b) UNDERWRITING. In the event that the Initiating Holders
intend to distribute their Registrable Securities covered by their request by
means of an underwriting, they shall so advise the Company as part of their
request made pursuant to this Section 1.5 and the Company shall so advise the
Holders as part of the notice given pursuant to Section 1.5(a)(i). In such
event, the right of any Holder to registration pursuant to Section 1.5 shall be
conditioned upon such Holder's participation in the underwriting arrangements
required by this Section 1.5(b) and the inclusion of such holder's Registrable
Securities in the underwriting to the extent requested and to the extent
provided herein.

         The Company (together with all Holders proposing to distribute their
securities through such underwriting) shall, upon request by the managing
underwriter selected for such underwriting by a majority in interest of the
Initiating Holders (which managing underwriter shall be reasonably acceptable
to the Company), enter into any reasonable agreement requested by the
managing underwriter in connection with the offering including, but not
limited to, an underwriting agreement in customary form with the managing
underwriter. Notwithstanding any other provision of this Section 1.5, if the
managing underwriter determines that marketing factors require a limitation
of the number of shares to be underwritten, the managing underwriter may
limit or exclude the securities to be included in such registration by any
Holder exercising its rights pursuant to this Section 1.5; provided that if
any exclusion or limitation of securities is so required, the securities to
be included

                                      -6-



shall be apportioned as follows: first, among the Holders of Registrable
Securities participating in such registration pursuant to the exercise of
their rights in this Section 1.5 in proportion to the number of shares of
Registrable Securities held by such Holders, second, to the Company, and
third, to any other holders of securities of the Company entitled to
participate and participating in such registration ("Other Holders") in
proportion to the number of shares of the Company's Common Stock (or
equivalents thereof) held by such Other Holders. No securities or Registrable
Securities excluded from the underwriting by reason of the underwriter's
marketing limitation shall be included in such registration.

         If any Holder disapproves of the terms of the underwriting, such person
may elect to withdraw therefrom by written notice to the Company, the managing
underwriter and the Initiating Holders. The Registrable Securities so withdrawn
shall also be withdrawn from registration, and such Registrable Securities shall
not be transferred in a public distribution prior to ninety (90) days after the
effective date of such registration; PROVIDED, HOWEVER, that, if by the
withdrawal of such Registrable Securities a greater number of Registrable
Securities held by other Holders may be included in such registration (up to the
maximum of any limitation imposed by the underwriters), then the Company shall
offer to all Holders who have included Registrable Securi ties in the
registration the right to include additional Registrable Securities in the same
proportion used in determining the underwriter limitation in this Section
1.5(b).

         1.6 COMPANY REGISTRATION.

                  (a) NOTICE OF REGISTRATION. If at any time or from time to
time the Company shall determine to register any of its securities in connection
with the sale thereof for cash, either for its own account or the account of a
security holder or holders exercising their respective demand registration
rights, other than (i) a registration relating solely to employee benefit plans,
or (ii) a registration relating solely to a Commission Rule 145 transaction, the
Company will:

                           (A) promptly give to each Holder written notice
thereof; and

                           (B) include in such registration (and any related
qualification under blue sky laws or other compliance), and in any underwriting
involved therein, all the Registrable Securities specified in a written request
or requests, made within thirty (30) days after receipt of such written notice
from the Company by any Holder.

                  (b) UNDERWRITING. If the registration of which the Company
gives notice is for a registered public offering involving an underwriting, the
Company shall so advise the Holders as a part of the written notice given
pursuant to Section 1.6(a)(i). In such event, the right of any Holder to
registration pursuant to Section 1.6 shall be conditioned upon such Holder's
participation in such underwriting and the inclusion of Registrable Securities
in the underwriting to the extent provided herein. All Holders proposing to
distribute their securities through such underwriting shall (together with the
Company and the other holders distributing their securities through such
underwriting) enter into an underwriting agreement in customary form with the
managing underwriter selected for such underwriting by the Company (or by the
holders who have demanded such registration). Notwithstanding any other
provision of this Section 1.6, if the managing underwriter determines that

                                      -7-



marketing factors require a limitation of the number of shares to be
underwritten, the managing underwriter may limit or exclude the Registrable
Securities to be included in such registration prior to the exclusion from
such registration of any securities to be sold by the Company or any party
exercising demand registration rights with respect to such registration;
provided that if any exclusion or limitation of Registrable Securities is so
required, the securities to be included shall be apportioned as follows:
first, to the Company, second, among the Holders of Registrable Securities
participating in such registration in proportion to the number of shares of
Registrable Securities held by such Holders, and third, among any Other
Holders of securities of the Company entitled to participate and
participating in such registration in proportion to the number of shares of
the Company's Common Stock (or equivalents thereof) held by such Other
Holders. In no event will shares of any Other Holders be included in such
registration which would reduce the number of shares which may be included by
Holders without the written consent of Holders of not less than seventy
percent (70%) of the Registrable Securities proposed to be sold in the
offering. If any Holder or Other Holder dis approves of the terms of any such
underwriting, he may elect to withdraw therefrom by written notice to the
Company and the managing underwriter. Any securities excluded or withdrawn
from such underwriting shall be withdrawn from such registration. All
priorities with respect to demand registrations shall be governed by Section
1.5 hereof.

                  (c) RIGHT TO TERMINATE REGISTRATION. The Company shall have
the right to terminate or withdraw any registration initiated by the Company
under this Section 1.6 prior to the effectiveness of such registration whether
or not any Holder has elected to include securities in such registration.

         1.7 REGISTRATION ON FORM S-3.

                  (a) If any Holder requests that the Company file a
registration statement on Form S-3 (or any successor form to Form S-3) for a
public offering of shares of the Registrable Securities the anticipated
aggregate offering price of not less than $2,000,000, and the Company is a
registrant entitled to use Form S-3 to register the Registrable Securities for
such an offering, the Company shall use its commercially reasonable efforts to
cause such Registrable Securities to be registered for the offering on such
form; PROVIDED, HOWEVER, that the Company shall not be required to effect more
than two such registration pursuant to this Section 1.7 in any twelve-month
period. The Company will (i) promptly give written notice of the proposed
registration to all other Holders and (ii) as soon as practicable, use its
commercially reasonable efforts to effect such registration (including, without
limitation, the execution of an undertaking to file post-effective amendments,
appropriate qualification under applicable blue sky or other state securities
laws and appropriate compliance with applicable regulations issued under the
Securities Act and any other governmental requirements or regulations) as may be
so requested and as would permit or facilitate the sale and distribution of all
or such portion of such Registrable Securities as are specified in such request,
together with all or such portion of the Registrable Securities of any Holder or
Holders joining in such request as are specified in a written request received
by the Company within thirty (30) days after receipt of such written notice from
the Company. If the registration is for a public offering involving an
underwriting, the substantive provisions of Sec tion 1.5(b) shall be applicable
to each registration initiated under this Section 1.7.

                                      -8-



                  (b) Notwithstanding the foregoing, the Company shall not be
obligated to take any action pursuant to this Section 1.7: (i) in any
particular jurisdiction in which the Company would be required to execute a
general consent to service of process in effecting such registration,
qualification or compliance unless the Company is already subject to service
in such jurisdiction and except as may be required by the Securities Act;
(ii) beginning at any time when the Company delivers notice to the Holders of
the Company's bona fide intention to effect the filing of a registration
statement (other than with respect to a registration statement relating to a
Rule 145 transaction, an offering solely to employees or any other
registration which is not appropriate for the registration of Registrable
Securities) with the Commission within ninety (90) days of such notice and
ending on the earlier of the abandonment or consummation of such offering;
(iii) during the period starting with the date forty-five (45) days prior to
the filing of, and ending on a date sixty (60) days following the effective
date of, a registration statement (other than with respect to a registration
statement relating to a Rule 145 transaction, an offering solely to employees
or any other registration which is not appropriate for the registration of
Registrable Securities), provided that the Company is actively employing in
good faith all reasonable efforts to cause such registration statement to
become effective; or (iv) if the Company shall furnish to such Holder a
certificate signed by the President of the Company stating that in the good
faith judgment of the Board of Directors it would be seriously detrimental to
the Company or its shareholders for registration statements to be filed in
the near future or for any disclosure to be made that, in the opinion of the
Board of Directors duly advised by counsel, is required to be made in
connection with the sale of Registrable Securities pursuant to such
registration, provided that the Company's obligation to use its commercially
reasonable efforts to file a registration statement shall be deferred for a
period not to exceed ninety (90) days from the receipt of the request to file
such registration by such Holder, and provided, further, that the Company
shall not exercise its right under this clause to defer such obligation more
than once in any twelve-month period.

1.8      EXPENSES OF REGISTRATION.


                  (a) All Registration Expenses incurred in connection with any
registration pursuant to Sections 1.5 or 1.6, and the reasonable cost of one
special legal counsel to represent all of the Holders together, shall be borne
by the Company. All Registration Expenses incurred in connection with the first
two registrations pursuant to Section 1.7, excluding the expense of counsel for
the Holders, shall be borne by the Company. The Company shall not be required to
pay the Registration Expenses of any registration proceeding begun pursuant to
Section 1.5 if the request for such registration has been subsequently withdrawn
by the Initiating Holders, unless the Holders of at least seventy percent (70%)
of the Registrable Securities agree to forfeit their right to one demand
registration pursuant to Section 1.5. Notwithstanding the foregoing, however, if
at the time of the withdrawal the Holders have learned of a material adverse
change in the condition, business or prospects of the Company from that known to
the Holders at the time of their request, of which the Company had knowledge at
the time of the request, then the Holders shall not be required to pay any of
said Registration Expenses or to forfeit the right to one demand registration.

                                      -9-



                  (b) All Selling Expenses incurred in connection with a
registration pursuant to Section 1.7 shall be borne pro-rata by the Holder or
Holders requesting the registration on Form S-3 according to the number of
Registrable Securities in such registration.

                  (c) Notwithstanding the provisions of Section 1.8(a), if, as a
condition of registration or qualification of any offering in any state or
jurisdiction in which the Company or any underwriter determines in good faith
that it wishes to offer securities registered in an offering to which this
Agreement applies, it is required that offering expenses be allocated in a
manner different from that provided in Section 1.8(a), the offering expenses
shall be allocated in whatever permitted manner is most nearly in compliance
with the provisions of this Agreement, so long as such allocation does not
materially reduce the net proceeds received by any Holder.

         1.9 INDEMNIFICATION.

                  (a) To the extent permitted by law, the Company will indemnify
each Holder, each of its officers and directors and partners, and each person
controlling such Holder within the meaning of Section 15 of the Securities Act,
with respect to which registration, qualification or compliance has been
effected pursuant to this Section 1, and each underwriter, if any, and each
person who controls any underwriter within the meaning of Section 15 of the
Securities Act, against all expenses, claims, losses, damages or liabilities (or
actions in respect thereof), including any of the foregoing incurred in
settlement of any litigation, commenced or threatened, arising out of or based
on any untrue statement (or alleged untrue statement) of a material fact
contained in any registration statement, prospectus, offering circular or other
document, or any amendment or supplement thereto, incident to any such
registration, qualification or compliance, or based on any omission (or alleged
omission) to state therein a material fact required to be stated therein or
necessary to make the statements therein, in light of the circumstances in which
they were made, not misleading, or any violation (or alleged violation) by the
Company of the Securities Act or any rule or regulation promulgated under the
Securities Act applicable to the Company in connection with any such
registration, qualification or compliance, and the Company will reimburse or pay
for the account of each such Holder, each of its officers and directors, and
each person controlling such Holder, each such underwriter and each person who
controls any such underwriter, for any legal and any other expenses reasonably
incurred (as and when incurred) in connection with investigating, preparing or
defending any such claim, loss, damage, liability or action, provided that the
Company will not be liable in any such case to the extent that any such claim,
loss, damage, liability or expense arises out of or is based on any untrue
statement or omission or alleged untrue statement or omission, made in reliance
upon and in conformity with written information furnished to the Company by an
instrument duly executed by such Holder, controlling person or underwriter and
stated to be specifically for use therein.

                  (b) To the extent permitted by law, each Holder will, if
Registrable Securities held by such Holder are included in the securities as to
which such registration, qualification or compliance is being effected,
indemnify the Company, each of its directors and officers, each underwriter, if
any, of the Company's securities covered by such a registration statement, each
person who controls the Company or such underwriter within the meaning of
Section 15 of the

                                     -10-



Securities Act, and each other such Holder, each of its officers and
directors and each person controlling such Holder within the meaning of
Section 15 of the Securities Act, against all claims, losses, damages and
liabilities (or actions in respect thereof) arising out of or based on any
untrue statement (or alleged untrue statement) of a material fact contained
in any such registration statement, prospectus, offering circular or other
document, or any omission (or alleged omission) to state therein a material
fact required to be stated therein or necessary to make the statements
therein not misleading, and will reimburse or pay for the account of the
Company, such Holders, such directors, officers, persons, underwriters or
control persons for any legal or any other expenses reasonably incurred (as
and when incurred) in connection with investigating or defending any such
claim, loss, damage, liability or action, in each case to the extent, but
only to the extent, that such untrue statement (or alleged untrue statement)
or omission (or alleged omission) is made in such-registration statement,
prospectus, offering circular or other document in reliance upon and in
conformity with written information furnished to the Company by an instrument
duly executed by such Holder and stated to be specifically for use therein;
provided, however, that the liability of a Holder for indemnification under
this Section 1.9(b) shall not exceed the net proceeds from the offering
received by such Holder, unless such liability arises out of or is based on
willful misconduct of such Holder.

                  (c) Each party entitled to indemnification under this
Section 1.9 (the "Indemnified Party") shall give notice to the party required
to provide indemnification (the "Indemnifying Party") promptly after such
Indemnified Party has actual knowledge of any claim as to which indemnity may
be sought and shall permit the Indemnifying Party to assume the defense of
any such claim or any litigation resulting therefrom, provided that counsel
for the Indemnifying Party, who shall conduct the defense of such claim or
litigation, shall be approved by the Indemnified Party (which approval shall
not unreasonably be withheld), and the Indemnified Party may participate in
such defense at such party's expense; PROVIDED, HOWEVER, that an Indemnified
Party shall have the right to retain its own counsel, with the fees and
expenses to be paid by the Indemnifying Party, if representation of such
Indemnified Party by the counsel retained by the Indemnifying Party would be
inappropriate due to actual or potential differing interests between such
Indemnified Party and any other party represented by such counsel in such
proceeding. The failure of any Indemnified Party to give notice as provided
herein shall not relieve the Indemnifying Party of its obligations under this
Section 1 except to the extent that the failure to give such notice is
materially prejudicial to an Indemnifying Party's ability to defend such
action. No Indemnifying Party, in the defense of any such claim or
litigation, shall, except with the consent of each Indemnified Party, consent
to entry of any judgment or enter into any settlement (i) that does not
include as an unconditional term thereof the giving by the claimant or
plaintiff to such Indemnified Party of a release from all liability in
respect to such claim or litigation, or (ii) that contains or requires any
admission of fault on the part of an Indemnified Party. No Indemnifying Party
shall be liable for indemnification hereunder with respect to any settlement
or consent to judgment in connection with any claim or litigation to which
these indemnification provisions apply that has been entered into without the
prior consent of the Indemnifying Party (which consent will not be
unreasonably withheld).

                                     -11-



                  (d) If the indemnification provided for in this Section 1.9 is
held by a court of competent jurisdiction to be unavailable to an Indemnified
Party with respect to any losses, claims, damages or liabilities referred to
herein, the Indemnifying Party, in lieu of indemnifying such Indemnified Party
hereunder, shall to the extent permitted by applicable law contribute to the
amount paid or payable by such Indemnified Party as a result of such loss,
claim, damage or liability in such proportion as is appropriate to reflect the
relative fault of the Indemnifying Party on the one hand and of the Indemnified
Party on the other in connection with the untrue statement (or alleged untrue
statement) or omission (or alleged omission) that resulted in such loss, claim,
damage or liability, as well as any other relevant equitable considerations. The
relative fault of the Indemnifying Party and of the Indemnified Party shall be
determined by a court of law by reference to, among other things, whether the
untrue or alleged untrue statement of a material fact or the omission to state a
material fact relates to information supplied by the Indemnifying Party or by
the Indemnified Party and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such statement or omission;
provided, that in no event shall any contribution by a Holder hereunder exceed
the net proceeds from the offering received by such Holder.

                  (e) The obligations of the Company and the Holders under
this Section 1.9 shall survive the completion of any offering of Registrable
Securities in a registration statement pursuant to this Section 1.

         1.10 INFORMATION OF HOLDER; COPIES OF PROSPECTUS. Each Holder of
Registrable Securities included in any registration shall furnish to the Company
such information regarding such Holder, the Registrable Securities held by such
Holder and the distribution proposed by such Holder as the Company may
reasonably request in writing and as shall be required and typically provided by
selling shareholders in a like situation in connection with any registration,
qualification or compliance referred to in this Section 1. In connection with
any such registration, the Company shall furnish to such Holder or Holders such
numbers of copies as may be reasonably requested in order to facilitate the
disposition of Registrable Securities owned by such Holder, of any prospectus or
preliminary prospectus prepared in conformity with the Securities Act.

         1.11 OBLIGATIONS OF THE COMPANY. Whenever required under this Section 1
to effect the registration of any Registrable Securities, the Company shall, as
soon as practicable:

                  (a) Prepare and file with the Commission a registration
statement with respect to such Registrable Securities and use its commercially
reasonable efforts to cause such registration statement to become effective, and
upon the request of the Holders of a majority of the Registrable Securities
registered thereunder, keep such registration statement effective for a period
of up to one hundred twenty (120) days or until the distribution contemplated in
the Registration Statement has been completed.

                  (b) Prepare and file with the Commission such amendments and
supplements to such registration statement and the prospectus used in connection
with such registration statement as may be necessary to comply with the
provisions of the Securities Act with respect to the disposition of all
securities covered by such registration statement.

                                      -12-



                  (c) Use its commercially reasonable efforts to register and
qualify the securities covered by such registration statement under such other
securities or blue sky laws of such jurisdictions as shall be reasonably
requested by the Holders, provided that the Company shall not be required in
connection therewith or as a condition thereto to file a general consent to
service of process in any such states or jurisdictions.

                  (d) Notify each Holder of Registrable Securities covered by
such registration statement at any time when a prospectus relating thereto is
required to be delivered under the Securities Act of the happening of any event
as a result of which the prospectus included in such registration statement, as
then in effect, includes an untrue statement of a material fact or omits to
state a material fact or omits to state a material fact required to be stated
therein or necessary to make the statements therein not misleading in the light
of the circumstances then existing.

                  (e) Furnish to the Holders participating in such registration
and, if applicable, to the underwriters of the securities being registered, such
reasonable number of copies of the registration statement, preliminary
prospectus, final prospectus, accountant's comfort letters, opinions of counsel
and such other documents as such underwriters may reasonably request in order to
facilitate the public offering of such securities.

                  (f) In the event of an underwritten public offering, enter
into and perform its obligations under an underwriting agreement, in usual and
customary form, with the managing underwriter of such offering. Each Holder
participating in such underwriting shall also enter into and perform its
obligations under such an agreement.

                  (g) File such listing applications as may be reasonably
necessary in connection with the sale of such Registrable Securities.

         1.12 RULE 144 REPORTING. With a view to making available the benefits
of certain rules and regulations of the Commission, which may permit the sale of
the Restricted Securities to the public without registration, after such time as
a public market exists for the Common Stock of the Company, the Company agrees
to use its commercially reasonable effort to:

                  (a) Make and keep public information available, as those terms
are understood and defined in Rule 144 under the Securities Act at all times
after the effective date that the Company becomes subject to the reporting
requirements of the Securities Exchange Act of 1934, as amended (the "Exchange
Act");

                  (b) File with the Commission in a timely manner all reports
and other documents required of the Company under the Securities Act and the
Exchange Act;

                  (c) So long as a Holder owns any Restricted Securities, to
furnish to the Holder forthwith upon request a written statement by the
Company as to its compliance with the reporting requirements of said Rule
144, a copy of the most recent annual or quarterly report of the Company and
such other reports and documents of the Company and other information in the
possession of or reasonably obtainable by the Company as a Holder may
reasonably request in availing itself of any

                                      -13-



rule or regulation of the Commission allowing a Holder to sell any such
securities without registration; and

                  (d) Take such action, including the voluntary registration of
its Common Stock under Section 12 of the 1934 Act, as is necessary to enable the
Holders to utilize Form S-3 for the sale of their Registrable Securities, such
action to be taken as soon as practicable after the end of the fiscal year in
which the first registration statement filed by the Company for the offering of
its securities to the general public is declared effective.

         1.13 TRANSFER OF REGISTRATION RIGHTS. The rights to cause the Company
to register securities granted Holders under Sections 1.5, 1.6, and 1.7 may be
assigned or otherwise conveyed to a transferee or assignee in connection with
any transfer or assignment of Registrable Securities by a Holder (together with
any Affiliate) provided that such transfer may otherwise be effected in
accordance with applicable securities laws, the Holder effecting such transfer
shall comply with the requirements of Section 1.4 of this Agreement, the
transferee shall agree to be bound by all of the provisions of this Section 1,
such transfer does not violate any agreements by and among the Company and such
Holders or any agreements among such Holders, and such transferee or assignee is
a wholly owned subsidiary, constituent partner (including retired and limited
partners) or Affiliate of such Holder, is any family member of any individual
Holder, is a trust for the benefit of any individual Holder, or acquires from
such Holder at least 150,000 shares of the Company's Registrable Securities
subject to this Agreement (as adjusted for any stock split or combination), or a
lesser amount provided such transferee or assignee acquires all of the shares of
the Company's capital stock subject to this Agreement then held by such Holder,
provided in each case that the Company is given written notice by such
transferee at the time of said transfer stating the name and address of said
transferee and said transferee's agreement to be bound by this Agreement.

         1.14 STANDOFF AGREEMENT. So long as the Company has complied in all
material respects with the terms of this Agreement, each Holder agrees in
connection with the firm commitment initial underwritten public offering of the
Company's securities, upon request of the Company or the underwriters managing
any underwritten offering of the Company's securities, not to sell, make any
short sale of, loan, grant any option for the purchase of or otherwise dispose
of any Registrable Securities (other than those included in the registration)
without the prior written consent of the Company or such underwriters, as the
case may be, for such period of time (not to exceed one hundred eighty (180)
days) from the effective date of such registration as may be requested by the
Company or such managing underwriters, provided that each of the Company's
officers, directors and holders of at least one percent (1%) of the Company's
voting securities shall have agreed to be bound by the same restrictions in
connection with the Company's initial public offering.

                                   SECTION 2

                                 MISCELLANEOUS

         2.1 TRANSFER; SUCCESSORS AND ASSIGNS. Except as the transferability of
rights is expressly limited herein, the terms and conditions of this Agreement
shall inure to the benefit of and be binding upon the respective successors and
assigns of the parties. Nothing in this Agreement,

                                      -14-



express or implied, is intended to confer upon any party other than the
parties hereto or their respective successors and assigns any rights,
remedies, obligations or liabilities under or by reason of this Agreement,
except as expressly provided in this Agreement.

         2.2 GOVERNING LAW. This Agreement shall be governed by and construed
under the laws of the state of California as applied to agreements among
California residents entered into and to be performed entirely within
California.

         2.3 COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

         2.4 TITLES AND SUBTITLES. The titles and subtitles used in this
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

         2.5 NOTICES. All notices and other communications required or permitted
hereunder shall be in writing and shall be mailed by registered or certified
mail, postage prepaid, or otherwise delivered by hand or by messenger, addressed
(a) if to a Holder, at such address as such Holder shall have furnished to the
Company in writing, or (b) if to the Company, at such address as the Company
shall have furnished in writing to the Holder to the attention of the President.
A notice shall be effective when actually delivered by hand or messenger, or, if
to a domestic addressee, five (5) business days after deposit in the mail as
aforesaid, or, if to an international addressee, sent by express messenger
specifying not more than three days' delivery.

         2.6 TERMINATION. This Agreement shall terminate with respect to any
Holder when such Holder may sell all of its Registrable Securities under Rule
144 without limitation as to volume.

         2.7 SEVERABILITY. If any provision of this Agreement, or the
application thereof, will for any reason and to any extent be invalid or
unenforceable, the remainder of this Agreement and application of such provision
to other persons or circumstances will be interpreted so as to reasonably effect
the intent of the parties hereto. The parties further agree to replace such void
or unenforceable provision of this Agreement with a valid and enforceable
provision that will achieve, to the extent possible, the economic, business and
other purposes of the void or unenforceable provision.

         2.8 ENTIRE AGREEMENT. This Agreement constitutes the full and entire
understanding and agreement between the parties with regard to the subjects
hereof.

         2.9 MODIFICATIONS AND AMENDMENTS. This Agreement may be modified or
amended only with the written consent of the Company and the Holders holding at
least seventy percent (70%) of the Registrable Securities then subject to this
Agreement. Any waiver by a party of its rights hereunder shall be effective only
if evidenced by a written instrument executed by such party. In no event shall
such waiver of any rights hereunder constitute the waiver of such rights in any
future instance unless the waiver so specifies in writing. Each Holder
acknowledges that by the operation of this Section 2.9 the Holders of seventy
percent (70%) of the Registrable Securities interests may

                                     -15-



have the right and power to diminish or eliminate all rights of such Holder
under this Agreement. Each Holder agrees that its consent to amend this
Agreement shall not be required in the event the Company desires to amend
this Agreement to include in the definition of Holder suppliers, lessors or
commercial lending institutions that acquire Registrable Securities after the
date hereof, so long as the rights so granted are not inconsistent with or
superior to any rights granted to the Holders under this Agreement.

         2.10 ADDITIONAL REGISTRATION RIGHTS. Without the prior written consent
of the Holders of at least seventy percent (70%) of the Registrable Securities,
the Company shall not grant registration rights other than in compliance with
Section 2.9 above and shall not enter into any agreement with respect to
registration rights that is inconsistent with the terms of this Agreement.

                  [Remainder of Page Intentionally Left Blank]



                                     -16-



         IN WITNESS WHEREOF, the parties have executed this Third Amended and
Restated Registration Rights Agreement as of the date first above written.

"COMPANY"                            KOSAN BIOSCIENCES INCORPORATED

                                     By:    /s/ Daniel V. Santi
                                            ------------------------------

                                     Title:  Chief Executive Officer
                                            ------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



"HOLDERS"                         ALTA CALIFORNIA PARTNERS, L.P.

                                  By: Alta California Management Partners, L.P.

                                  By:   /s/ Jean Deleage
                                        ------------------------------
                                  Title: General Partner


                                  ALTA EMBARCADERO PARTNERS, LLC

                                  By:   /s/ Jean Deleage
                                        ------------------------------
                                  Title: Member


(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                  CV SOFINNOVA VENTURE PARTNERS III

                                  By: Sofinnova Management L.P.

                                  By:   /s/ Michael F. Powell
                                        ------------------------------

                                  Title: Managing Director
                                        ------------------------------
                                         Sofinnova Management III, LLC
                                         (General Partner)



                                  SOFINNOVA CAPITAL II  F.C.P.R.

                                  By:   /s/ Michael F. Powell
                                        ------------------------------

                                  Title:
                                        ------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                  WALDEN EDB PARTNERS, L.P.

                                  By:  Walden Management, L.P., General Partner

                                  By:   /s/ Lip Bu Tan
                                        ------------------------------

                                  Title: General Partner
                                        ------------------------------


                                  WALDEN TECHNOLOGY VENTURES II, L.P.

                                  By:  Walden Technology Partners, L.P.,
                                  General Partner

                                  By:   [ILLEGIBLE]
                                        ------------------------------

                                  Title: General Partner
                                        ------------------------------


                                  WALDEN-SBIC, L.P.

                                  By:   [ILLEGIBLE]
                                        ------------------------------

                                  Title: General Partner
                                        ------------------------------


                                  WALDEN EDB PARTNERS II, L.P.

                                  By:  Walden Management LLC, General Partner

                                  By:   /s/ Lip Bu Tan
                                        ------------------------------

                                  Title: General Partner
                                        ------------------------------

(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                  THE GOLDMAN SACHS GROUP, INC.

                                  By:   /s/ Eric M. Mindich
                                        ------------------------------

                                  By:
                                        ------------------------------

                                  Title: Attorney-in-Fact
                                        ------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                  LOMBARD ODIER & CIE

                                  By:   /s/ Carmela Gokok
                                        ------------------------------

                                  Title: Assistant Vice President
                                        ------------------------------


                                  By:   /s/ Alexander Meyer
                                        ------------------------------

                                  Title: Assistant Vice President
                                        ------------------------------

(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                  S.R. ONE, LIMITED

                                  By:
                                        ------------------------------

                                  By:   /s/ Raymond Whitaker
                                        ------------------------------

                                  Title: Vice President
                                        ------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)


                                  /s/ Daniel V. Santi
                                  ------------------------------
                                  DANIEL V. SANTI


(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                  AG-BIOTECH CAPITAL, LLC

                                  By:   Veridian Management, LLC
                                        ------------------------------

                                  By:   /s/ Helene S. Cohen
                                        ------------------------------

                                  Title: President/Manager
                                        ------------------------------

(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     DEUTSCHE ASSET MANAGEMENT (NAVAP)

                                     By:   [ILLEGIBLE]
                                           -------------------------------

                                     Title:[ILLEGIBLE]
                                           -------------------------------
                                            DEUTSCHE ASSET MANAGEMENT
                                            INVESTMENT GESELLSCHAFT mbtt

                                     DEUTSCHE VERMOEGENSBILDUNGS
                                     GESELLSCHAFT m.b.H.

                                     By:   /s/ Daniel Endrikat
                                           -------------------------------
                                           Daniel Endrikat

                                     Title:
                                           -------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     THE FRANKLIN BIOTECHNOLOGY DISCOVERY FUND

                                     By:
                                           -------------------------------

                                     By:   /s/ Murray L. Simpson
                                           -------------------------------

                                     Title: Vice President
                                           -------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     INVEMED FUND, L.P.

                                     By:
                                           -------------------------------

                                     By:   /s/ Cristina H. Kepner
                                           -------------------------------

                                     Title:Executive Vice President of Invemed
                                           -------------------------------
                                           Associates LLC and General Partner
                                           of Invemed Fund, L.P.

(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     CRISTINA H. KEPNER
                                     /s/ Cristina H. Kepner
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                     G. ALLEN MEBANE
                                     /s/ G. Allen Mebane
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                     THOMAS TEAGUE
                                     /s/ Thomas Teague by [ILLEGIBLE]
                                     -------------------------------------
                                     Attorney-in-Fact

(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                     BRUCE M. LANGONE
                                     /s/ Bruce M. Langone
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                     JAMES MCGIBBON
                                     /s/ James McGibbon
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     ED HERLIHY
                                     /s/ Ed Herlihy
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     KENNETH LANGONE
                                     /s/ Kenneth Langone
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     JOHN BARAN
                                     /s/ John Baran
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     BALDWIN SMITH, JR.
                                     /s/ Baldwin Smith, Jr.
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     ADAM CHIN
                                     /s/ Adam Chin
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)



                                     ANDREW R. TAUSSIG
                                     /s/ Andrew R. Taussig
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)




                                     VIREN MEHTA
                                     /S/ Viren Mehta
                                     -------------------------------------


(Signature Page to the Third Amended and Restated Registration Rights Agreement)





                                    EXHIBIT A
                                     HOLDERS



                    "HOLDER"                                                  "REGISTRABLE SECURITIES"
- -------------------------------------------------   ---------------------------------------------------------------------
                                                                            SERIES A         SERIES B          SERIES C
                                                           COMMON           PREFERRED        PREFERRED         PREFERRED
                                                           STOCK             STOCK            STOCK             STOCK
                                                        -----------       -------------    -------------     -------------
                                                                                                 
Alta California Partners, L.P.                             52,263            462,968          237,009            23,654

Alta Embarcadero Partners, LLC                              1,552             13,224            5,415               540

Chiron Corporation                                         35,877            238,096                0                 0

Daniel V. Santi                                           216,498            229,761           24,244                 0

Chaitan Khosla                                             22,857              7,413                0                 0

Walden-SBIC, L.P.                                               0            178,572           24,793             5,499

Walden Technology Ventures II, L.P.                             0             35,714            4,960             1,100

Walden EDB Partners, L.P.                                       0             47,619            6,611             1,466

CV Sofinnova Venture Partners III                               0            190,476           24,242                 0

Sofinnova Capital II F.C.P.R.                                   0                  0           36,364                 0

Parvin Anand                                                1,794             11,905                0                 0

Joseph T. Fitzpatrick, Trustee of the Joseph T.
Fitzpatrick Trust U/A/D 9/8/88                                717              4,762                0                 0

Jeffrey A. Golden                                             538              3,572                0                 0

John F. Hamilton and Carol Leonard,
as community property                                         538              3,572                0                 0

Kathryn M. Ivanetich                                          179              1,191                0                 0




                    "HOLDER"                                                  "REGISTRABLE SECURITIES"
- -------------------------------------------------   ---------------------------------------------------------------------
                                                                            SERIES A         SERIES B          SERIES C
                                                           COMMON           PREFERRED        PREFERRED         PREFERRED
                                                           STOCK             STOCK            STOCK             STOCK
                                                        -----------       -------------    -------------     -------------
                                                                                                 
Jacobson, Silverstein, Winslow Architects                     359              2,381                0                 0

George Robert Johnson                                         359              2,381                0                 0

Deborah Kass                                                  359              2,381                0                 0

James Huger Richardson                                        538              3,572                0                 0

Kathy Houser Richardson                                       359              2,381                0                 0

Howard J. Schaeffer                                           359              2,381                0                 0

Robert M. Stroud                                              359              2,381                0                 0

WS Investment Company '96A                                    717              4,762                0                 0

The Goldman Sachs Group, Inc.
One New York Plaza, 50th Floor
New York, NY  10004
Attn:  Robert Granovsky
WITH A COPY TO:
The Goldman Sachs Group, Inc.
One New York Plaza, 37th Floor
New York, NY  10004
Attn: John Berton                                               0                  0          303,030                 0

Lombard Odier & Cie                                             0                  0          363,636            58,065

S.R. One, Limited                                               0                  0          303,030            16,129

Ag-Biotech Capital, LLC                                         0                  0          484,848            16,129

Deutsche Asset Management                                       0                  0                0           161,291

Deutsche Vermoegensbildungs Gesellschaft m.b.H.                                                                  96,774

The Franklin Biotechnology Discovery Fund                       0                 0                 0           387,097




                    "HOLDER"                                                  "REGISTRABLE SECURITIES"
- -------------------------------------------------   ---------------------------------------------------------------------
                                                                            SERIES A         SERIES B          SERIES C
                                                           COMMON           PREFERRED        PREFERRED         PREFERRED
                                                           STOCK             STOCK            STOCK             STOCK
                                                        -----------       -------------    -------------     -------------
                                                                                                 
Invemed Fund, L.P.                                              0                 0                 0           12,904

Cristina H. Kepner                                              0                 0                 0            2,420

G. Allen Mebane                                                 0                 0                 0              806

Thomas Teague                                                   0                 0                 0              645

Bruce M. Langone                                                0                 0                 0              806

James McGibbon                                                  0                 0                 0              968

Ed Herlihy                                                      0                 0                 0              806

Kenneth Langone                                                 0                 0                 0            8,873

John Baran                                                      0                 0                 0              806

Baldwin Smith, Jr.                                              0                 0                 0              806

Adam Chin                                                       0                 0                 0              806

Andrew R. Taussig                                               0                 0                 0            1,612

Viren Mehta                                                     0                  0                0            3,226

                                           TOTAL          336,222          1,451,195        1,818,182          803,228
                                                          =======          =========        =========          =======