<PAGE>

                                                                    EXHIBIT 10.4

              FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT


     This Fourth Amended And Restated Stockholders' Agreement (the "Agreement"),
dated as of September 3, 1999, is by and among Dendreon Corporation, a Delaware
corporation (the "Company"), those stockholders of the Company set forth on
Exhibit A hereto and the purchasers of the Company's Series E Preferred Stock
set forth on Exhibit B hereto (the "Purchasers").

                                   Recitals

     Whereas, the Company and the Purchasers are entering into a Series E
Preferred Stock Purchase Agreement as of the date hereof (the "Stock Purchase
Agreement"), and in connection therewith, the Company is making certain
covenants with such Purchasers and has agreed to grant to such Purchasers
certain rights;

     Whereas, the Company and certain of the parties hereto entered into an
Amended and Restated Stockholders' Agreement, dated July 10, 1998 (the
"Stockholders' Agreement"); and

     Whereas, in order to induce further investment in the Company on the terms
and conditions contemplated in the Stock Purchase Agreement and in this
Agreement, the Company and the parties hereto have agreed to amend the covenants
and rights heretofore granted pursuant to the Stockholders' Agreement, as set
forth herein.

     Now, Therefore, in consideration of the foregoing and of the respective
covenants and undertakings hereunder and pursuant to the Stock Purchase
Agreement, the parties hereto do hereby agree as follows:

SECTION 1. Definitions.

     As used in this Agreement, the following terms shall have the following
respective meanings:

     "Business Day" shall mean any day other than: (i) Saturday or Sunday, or
(ii) a day on which the New York Stock Exchange is closed.

     "By-laws" shall mean the By-laws of the Company, as amended.

     "Capital Stock" shall mean any: (i) shares of Common Stock, Preferred Stock
or any other equity security of the Company, (ii) debt securities convertible
into or exchangeable for any equity security of the Company or (iii) options,
warrants or other rights to subscribe for, purchase or otherwise acquire any
such equity security or debt security of the Company.

     "Charter" shall mean the Certificate of Incorporation of the Company, as
restated and amended from time to time.
<PAGE>

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

     "Common Shares" shall mean the issued and outstanding shares of the
Company's Common Stock, $0.001 par value.

     "Common Stock" shall mean the Company's authorized Common Stock, $0.001 par
value.

     "Common Stockholder" shall mean each Person who has purchased Common Stock
from the Company or who acquires Common Stock by Transfer or otherwise and who
becomes a party to this Agreement, other than Preferred Stockholders who acquire
Common Stock on conversion of the Preferred Shares.

     "Defaulting Stockholder" shall have the meaning set forth in Section
4.2(h).

     "Equity Securities" shall have the meaning set forth in Rule 3a 11-1 under
the Exchange Act.

     "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended,
and any successor statute and the rules and regulations thereunder, as shall be
in effect from time to time.

     "Excluded Stock" shall mean: (a) the Option Shares; (b) Common Stock
issuable upon conversion of the Preferred Shares; (c) shares of Common Stock
issued to lenders, lessors or otherwise for purposes other than raising capital;
(d) Common Stock issued or issuable pursuant to Restricted Stock Option
Agreements between the Company and the Founders and between the Company and Dr.
William Haseltine; (e) Equity Securities issued by the Company pursuant to
license agreements for the Company's technology or the acquisition of another
corporation, partnership, joint venture, trust or other entity by merger,
consolidation, stock acquisition, reorganization, or otherwise, whereby the
Company, or its stockholders of record immediately prior to the effectiveness of
such transaction, directly or indirectly, own at least the majority of the
voting power of such other entity or the resulting or surviving corporation
immediately after such transaction; (f) Capital Stock issuable or issued upon
the exercise of outstanding options or warrants.

     "Family" shall include any spouse, lineal ancestor or descendant, or
sibling.

     "Five Percent Preferred Stockholder" shall mean: (a) the Investors, for so
long as they continue to own Preferred Shares, (b) Shaw Venture Partners, L.P.
("Shaw"), for so long as they continue to own Shares, (c) any transferee of
Preferred Stock of an Investor who is a member of such Investor's Group, and any
transferee of Shaw who is a member of Shaw's Group, for so long as such
transferee continues to own Preferred Shares, (d) Kummell Investments Limited
("Kummell"), for so long as it continues to own Shares, and (e) any other
Preferred Stockholder owning (either of record or beneficially), at any time, a
number of Preferred Shares equal to or exceeding five percent of the aggregate
number of Preferred Shares then outstanding.

                                       2
<PAGE>

     "Founders" shall mean Dr. Sam Strober and Dr. Edgar G. Engleman and each of
the persons to whom the shares subscribed for by such persons has been assigned
pursuant to certain Restricted Stock Agreements dated as of September 30, 1992.

     "Founders Shares" shall mean the Common Shares issued to the Founders.

     "Group" shall mean as to: (a) a Stockholder that is a limited partnership,
any and all of the venture capital limited partnerships now existing or
hereafter arising that are "affiliates" (as defined by Rule 405 promulgated
under the Securities Act), in whole or in part, of one or more general partners
or of one or more general partners of a general partner of such Stockholder and
any predecessor or successor partnership any limited and general partners of any
such partnership; (b) a Stockholder that is a trust, any of the beneficiaries,
settlors or grantors now existing or hereafter arising of, or any Person under
common control with, such trust; (c) a Stockholder that is a general
partnership, any of its partners or former partners in accordance with their
partnership interests; (d) a Stockholder that is a corporation, its stockholders
in accordance with their interests in the corporation; and (e) a Stockholder
that is a limited liability company, its members or former members in accordance
with their interests in the limited liability company .

     "Investors" shall mean HealthCare Ventures III, L.P. ("HCV III") HealthCare
Ventures IV, L.P. ("HCV IV"), Everest Trust and Hudson Trust and members of
their respective Groups.

     "New Securities" shall mean any Equity Securities of the Company,
including, but not limited to, shares of Common Stock of the Company, any
security that is convertible into or exercisable or exchangeable for Common
Stock, or any right, option or warrant to acquire any Common Stock of the
Company.

     "Offer" shall have the meaning set forth in Section 4.2(a) hereof.

     "Offered Shares" shall have the meaning set forth in Section 4.2(a) hereof.

     "Option Shares" shall mean shares of Common Stock issued, available for
issuance or subject to options, warrants or rights granted or authorized to be
granted to employees and others who provide services to the Company pursuant to
any Stock Plan to the extent permitted under Section 6 hereof.

     "Person" shall mean and include a natural person, a corporation, a
partnership, a trust, an unincorporated organization and a government or any
department, agency or political subdivision thereof.

     "Preferred Shares" shall mean shares of the Company's Series A, Series B,
Series C, Series D and Series E Preferred Stock, $0.001 par value.

     "Preferred Stockholder" shall mean: (a) any Person who purchased Series A,
Series B, Series C or Series D Preferred Stock in the Company pursuant to a
Stock Purchase Agreement dated October 27, 1992, March 25, 1994, December 8,
1994, January 26, 1996, June 20, 1997 or July 10, 1998, (b) each of the
Stockholders purchasing Preferred Shares from the Company

                                       3
<PAGE>

pursuant to the Stock Purchase Agreement, and (c) any Person to whom there has
been a Transfer of Preferred Shares or Restricted Shares.

     "Pro Rata Fraction" shall have the meaning set forth in Section 4.2(c).

     "Proposed Transferee" shall have the meaning set forth in Section 4.2 (a).

     "Public Offering" shall mean a distribution of New Securities in an
underwritten public offering to the general public pursuant to a registration
statement filed with and declared effective by the Commission pursuant to the
Securities Act.

     "Purchasing Stockholder" shall have the meaning set forth in Section 4.2
(e).

     "Qualified Public Offering" shall mean the Company's distribution of New
Securities in an underwritten public offering to the general public pursuant to
a registration statement filed with and declared effective by the Commission
pursuant to the Securities Act providing minimum gross proceeds (without
deduction for underwriting commissions or discounts or expenses of the offering)
to the Company of $15,000,000 and resulting in a market capitalization for the
outstanding shares of Common Stock immediately after the public offering (based
on the public offering price) of not less than $40,000,000.

     "Registrable Securities" shall mean shares of Common Stock issued or
issuable upon conversion of: (a) the Preferred Shares; (b) up to 7,500 shares of
Common Stock, subject to adjustment for certain dilutive issuances, which may be
issued to MMC\GATX or its permitted assignees pursuant to the Warrant issued in
conjunction with the Company's Master Equipment Lease Agreement with MMC\GATX;
and (c) Common Stock issued or issuable to the Investors upon the exercise of
certain warrants held by each of them as a result of Loan Agreements entered
into by the Company, dated April 11, 1995, October 5, 1995, November 3, 1995 and
December 8, 1995; provided, however, that shares of Common Stock or other
securities shall only be treated as Registrable Securities if and so long as (i)
they have not been sold to or through a broker or dealer or underwriter in a
public distribution or a public securities transaction and (ii) they could not
be sold pursuant to Rule 144 in any two consecutive three-month periods.

     "Restricted Shares" shall mean the shares of Common Stock issued or
issuable upon the conversion of Preferred Shares.

     "Restricted Stock Agreements" shall mean the Restricted Stock Agreements
between the Company and the Founders and between the Company and William
Haseltine.

     "Restricted Stock Option Agreements" shall mean the Restricted Stock Option
Agreements between the Company and the Founders and between the Company and
William Haseltine.

     "Securities Act" shall mean the Securities Act of 1933, as amended, and any
successor statute and the rules and regulations of the Commission thereunder, as
shall be in effect at the applicable time.

     "Selling Stockholder" shall have the meaning set forth in Section 4.2(a).

                                       4
<PAGE>

     "Shares" shall mean and include all shares of voting capital stock of the
Company now owned or hereafter acquired by any Stockholder or transferee of such
Stockholder.

     "Stockholder" shall mean each Person who has purchased Shares from the
Company or who acquires Shares upon conversion of the Preferred Stock, the
exercise of options, Transfer or otherwise and who is a party to this Agreement.

     "Stock Plan" shall mean any equity incentive plan for officers, directors,
employees and others who render services to Company.

     "Transfer" shall include any direct or indirect sale, assignment, transfer,
pledge, hypothecation or other disposition of any Shares or of any legal or
beneficial interest therein.

SECTION 2.  Representations.

     2.1  By the Company. The Company represents to each stockholder that:

          (a)  The execution, delivery and performance by the Company of this
Agreement and all transactions contemplated in this Agreement have been duly
authorized by all action required by law, its Charter, its By-laws or otherwise.

          (b)  This Agreement has been duly executed and delivered by the
Company and constitutes the legal, valid and binding obligation of the Company
enforceable against it in accordance with its term, except as enforcement may be
limited by bankruptcy, insolvency, reorganization, moratorium or other similar
laws now or hereafter in effect relating to creditors' rights generally.

     2.2  By the Stockholders. Each Stockholder, as to itself, himself or
herself, represents to the Company and the other Stockholders that:

          (a)  The execution, delivery and performance by such Stockholder of
this Agreement and all transactions contemplated in this Agreement have been
duly authorized by all action required by law, and by the certificate of
incorporation and by-laws, partnership agreement or other governing instrument
of such Stockholder.

          (b)  This Agreement has been duly executed and delivered by such
Stockholder and constitutes the legal, valid and binding obligation of such
Stockholder enforceable against it, him or her in accordance with its terms,
except: (i) as enforcement may be limited by bankruptcy, insolvency,
reorganization, moratorium or other similar laws now or hereafter in effect
relating to creditors' rights generally, (ii) general principles of equity that
restrict the availability of equitable remedies, and (iii) to the extent that
the enforceability of the indemnification provisions of Section 9.8 of this
Agreement may be limited by applicable laws.

SECTION 3. Legend on Shares and Notice of Transfer.

     3.1  Restrictive Legends.

                                       5
<PAGE>

     Each certificate evidencing Shares, and each certificate evidencing Shares
held by subsequent direct or indirect transferees of any such certificate, shall
(unless otherwise permitted by the provisions of Section 3.2 hereof) be stamped
or otherwise imprinted with legends in substantially the following form (in
addition to any legends which may be required by Regulation S of the Securities
Act and state blue sky laws):

     THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
     INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
     AS AMENDED, OR ANY STATE SECURITIES LAW. THESE SECURITIES MAY NOT BE SOLD
     OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR ANY EXEMPTION
     THEREFROM UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY APPLICABLE
     STATE SECURITIES LAW.

     THE TRANSFER AND VOTING OF THESE SECURITIES IS SUBJECT TO THE TERMS AND
     CONDITIONS OF AN AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT DATED AS OF
     SEPTEMBER 3, 1999 AMONG DENDREON CORPORATION, THE HOLDER OF RECORD OF THIS
     CERTIFICATE AND CERTAIN OTHER SIGNATORIES THERETO, AS THE SAME MAY BE
     AMENDED FROM TIME TO TIME, AND NO SALE, ASSIGNMENT, TRANSFER, PLEDGE,
     HYPOTHECATION OR OTHER DISPOSITION OF SUCH SECURITIES SHALL BE VALID OR
     EFFECTIVE EXCEPT IN ACCORDANCE WITH SUCH AGREEMENT AND UNTIL SUCH TERMS AND
     CONDITIONS HAVE BEEN FULFILLED. COPIES OF SUCH AGREEMENT MAY BE OBTAINED AT
     NO COST BY WRITTEN REQUEST MADE BY THE HOLDER OF RECORD OF THIS CERTIFICATE
     TO THE SECRETARY OF DENDREON CORPORATION.

     3.2  Notice of Transfer.

          (a)  Each of the Stockholders, and any other holder of any Shares by
acceptance thereof, agrees that, prior to any Transfer of any Shares, such
holder will give written notice to the Company of such holder's intention to
effect such Transfer and to comply in all other respects with the provisions of
this Section 3.2. Each such notice shall contain (i) a statement setting forth
the intention of said holder's prospective transferee with respect to its
retention or disposition of said Shares, and (ii) unless waived by the Company,
an opinion, reasonably acceptable to the Company, of counsel for said holder
(who may be the inside or staff counsel employed by said holder), as to the
necessity or non-necessity for registration under the Securities Act and
applicable state securities laws in connection with such Transfer and stating
the factual and statutory bases relied upon by counsel. The following provisions
shall then apply:

               (i)  If the proposed Transfer of Shares may be effected without
registration or qualification under the Securities Act and any applicable state
securities laws, then the registered holder of such Shares shall be entitled to
Transfer such Shares in accordance with Section 4 hereof and the intended method
of disposition specified in the statement delivered by said holder to the
Company.

               (ii) If the proposed Transfer of such Shares may not be effected
without registration under the Securities Act or registration or qualification
under any applicable

                                       6
<PAGE>

state securities laws, the registered holder of such Shares shall not be
entitled to transfer such Shares until such registration or qualification is
effective or until an exception from the requirement for such registration or
qualification is available.

          (b)  Each certificate evidencing the Shares issued upon such transfer
(and each certificate evidencing any untransferred balance of such Shares) shall
bear the applicable legends set forth in Section 3.1 hereof unless: (i) the
Company shall have waived the requirement of such legend; or (ii) in the
reasonable opinion of counsel to the Company or of counsel reasonably acceptable
to the Company, such Transfer shall have been made in connection with an
effective registration statement filed pursuant to the Securities Act or in
compliance with the requirements of Rule 144 or Rule 144A (or any similar or
successor rule) promulgated under the Securities Act, and in compliance with
applicable state securities laws.

          (c)  Notwithstanding the provisions of subsections (a) and (b) above,
no such registration statement or opinion of counsel shall be necessary for a
transfer by a Stockholder to a member of such Stockholder's Group.

SECTION 4. Covenants of Stockholders.

     4.1  Prohibited Transfers.

          (a)  Each Common Stockholder agrees that he, she or it shall not
Transfer any Common Shares without the prior written consent of the holders of a
majority of the outstanding Shares other than the Shares held by the
Transferring Stockholder, except as provided for in Section 4.2.

          (b)  Notwithstanding anything to the contrary contained herein, a
Common Stockholder may Transfer all or any of his Common Shares: (i) to any
member of his Family or to any trust for the exclusive benefit of the Common
Stockholder or any member of the Family of the Common Stockholder; provided that
any such transferee shall agree in writing with the Company, prior to and as a
condition precedent to such Transfer, to be bound by all of the provisions of
this Agreement and provided, further, that, in the case of a trust, the
interests in any such trusts shall be non-transferable, except in compliance
with this Agreement, (ii) in the case of Dr. William Haseltine, to a foundation
qualified pursuant to Section 501(C)(3) of the Internal Revenue Code of 1986, as
amended, which was organized by Dr. Haseltine or to which Dr. Haseltine is the
sole donor, provided, that any such foundation shall agree in writing with the
Company, prior to and as a condition precedent to such transfer, to be bound by
all of the provisions of this Agreement and provided, further, that the
interests in such foundation shall be non-transferable, and (iii) by will or the
laws of descent and distribution, in which event each such transferee shall be
bound by all of the provisions of this Agreement to the same extent as if such
transferee were the deceased Stockholder.

          (c)  If requested in writing by the managing underwriters, if any, of
the initial Public Offering of the Company's Common Stock, each Stockholder
agrees not to offer, sell, contract to sell or otherwise dispose of any Shares
except as part of such Public Offering within 30 days before or 180 days after
the effective date of the registration statement filed with respect

                                       7
<PAGE>

to said Public Offering, and the Company hereby also so agrees; provided,
however, that this restriction will not apply to transfers permitted under
Section 4.1(b).

     4.2  Right of First Refusal on Dispositions.

          (a)  If a Common Stockholder (for purposes of this Section 4, the
"Selling Stockholder") desires to sell all or any part of his shares pursuant to
a bona fide, arm's length offer from a creditworthy third party (the "Proposed
Transferee"), the Selling Stockholder shall submit a written offer (the "Offer")
to sell such Shares (the "Offered Shares") to the Five Percent Preferred
Stockholders and the Company, on terms and conditions, including price, not less
favorable to the Five Percent Preferred Stockholders and the Company than those
on which the Selling Stockholder proposes to sell the Offered Shares to the
Proposed Transferee. The Offer shall disclose the identity of the Proposed
Transferee, the number of Offered Shares proposed to be sold, the total number
of Shares owned by the Selling Stockholder, the terms and conditions, including
price, of the proposed sale, the address of the Selling Stockholder and any
other material facts relating to the proposed sale.

          (b)  If the Company desires to purchase all of the Offered Shares, the
Company shall communicate in writing its acceptance of its election to purchase
(an "Acceptance") to the Selling Stockholder and the Five Percent Preferred
Stockholders, which Acceptance shall be delivered in person or mailed to the
Selling Stockholder and the Five Percent Preferred Stockholders within 20 days
of the date the Offer was made.

          (c)  Subject to and in accordance with the priorities of rights
established in subsection (d) below, if the Company does not accept the Offer in
accordance with Section 4.2(b), each Five Percent Preferred Stockholder shall
have the right to purchase that number of Offered Shares as shall be equal to
the number of Offered Shares multiplied by a fraction, the numerator of which
shall be the number of Shares then owned by such Preferred Stockholder and the
denominator of which shall be the aggregate number of Shares then owned by all
of the Five Percent Preferred Stockholders (the "Pro Rata Fraction"). For the
purpose of calculating the Pro Rata Fraction, each Preferred Share shall be
deemed to represent the number of Common Shares into which the Preferred Share
is then convertible.

          (d)  Five Percent Preferred Stockholders shall have a right of
oversubscription such that if any Five Percent Preferred Stockholder fails to
accept the Offer as to its or his full Pro Rata Fraction, the other Five Percent
Preferred Stockholders shall, among them, have the right to purchase up to the
balance of the Offered Shares not so purchased. Such right of oversubscription
may be exercised by a Five Percent Preferred Stockholder by accepting the Offer
as to more than its or his Pro Rata Fraction. If, as a result thereof, such
oversubscriptions exceed the total number of Offered Shares available in respect
of such oversubscription privilege, the oversubscribing Five Percent Preferred
Stockholders shall be cut back with respect to their oversubscriptions so as to
sell the Offered Shares as nearly as possible in accordance with their
respective Pro Rata Fractions or as they may otherwise agree among themselves.
In all instances, the Five Percent Preferred Stockholders shall have the right
to purchase only such Offered Shares as are not purchased by the Company.

                                       8
<PAGE>

          (e)  If a Five Percent Preferred Stockholder desires to purchase all
or any part of the Offered Shares, such Five Percent Preferred Stockholder (a
"Purchasing Stockholder") shall communicate in writing its or his Acceptance to
the Selling Stockholder, which Acceptance shall state the number of Offered
shares the Purchasing Stockholder desires to purchase and shall be delivered in
person or mailed to the Selling Stockholder at the address set forth in the
Offer, with a copy to the Company and the other Five Percent Preferred
Stockholders, within 30 days of the date the Offer was made.

          (f)  Sale of the Offered Shares pursuant to this Section 4.2 shall be
made at the offices of the Company no later than the thirtieth day following the
expiration of the 30-day period after the Offer is made (or if such thirtieth
day is not a Business Day, then on the next succeeding Business Day). Such sales
shall be effected by the Selling Stockholder's delivery to each Purchasing
Stockholder or the Company, as the case may be, of a certificate or certificates
evidencing the Offered Shares to be purchased by it or him, duly endorsed for
transfer to the Purchasing Stockholder or the Company, as the case may be, which
Offered Shares shall be delivered free and clear of all liens, charges, claims
and encumbrances of any nature whatsoever, against payment to the Selling
Stockholder of the purchase price therefor by the Company or such Purchasing
Stockholder, as the case may be. Payment for the Offered Shares shall be made as
provided in the Offer or by wire transfer or certified check.

          (g)  If the Purchasing Stockholders and the Company do not agree to
purchase all of the Offered Shares, then the Offered Shares may be sold by the
Selling Stockholder at any time within 120 days after the date the Offer was
made. Any such sale shall be to the Proposed Transferee, at not less than the
price and upon other terms and conditions, if any, not more favorable to the
Proposed Transferee than those specified in the Offer. Any Offered Shares not
sold within such 120-day period shall continue to be subject to the requirements
of a prior offer pursuant to this Section 4.2.

          (h)  If any Selling Stockholder becomes obligated to sell any shares
to any Purchasing Stockholder under this Agreement and fails to deliver such
Shares in accordance with the terms of this Agreement (a "Defaulting
Stockholder"), the Purchasing Stockholder may, at its or his option, in addition
to all other remedies it or he may have, send to the Defaulting Stockholder the
purchase price for such Shares as is herein specified. Thereupon, the Company,
upon written notice to the Defaulting Stockholder, shall: (i) cancel on its
books the certificate or certificates representing the Shares to be sold and
(ii) issue, in lieu thereof, in the name of the Purchasing Stockholder, a new
certificate or certificates representing such Shares, and thereupon all of the
Defaulting Stockholder's rights in and to such Shares shall terminate, except
for the right to receive payment of the purchase price therefor.

          (i)  Notwithstanding anything herein to the contrary, the Selling
Stockholder shall not be obligated to sell any Shares to the Company or the Five
Percent Preferred Stockholders, and will be free to sell all of the Shares to
the Proposed Transferee, if the Company and the Five Percent Preferred
Stockholders do not elect to buy all of the Shares specified in the Offer.

     4.3  Right of Participation in Sales of Common Shares and Preferred Shares.

                                       9
<PAGE>

          (a)  If a Selling Stockholder wishes to sell Common Shares to any
Person (including any sale to a Proposed Transferee or to the Company or a
Purchasing Stockholder pursuant to Section 4.2 but excluding Transfers pursuant
to Section 4.1(b)), then the Selling Stockholder shall, as a condition precedent
to such sale, permit each of the Five Percent Preferred Stockholders to sell
either to the Proposed Transferee or to other financially responsible purchasers
identified by the Selling Stockholder and reasonably acceptable to the Five
Percent Preferred Stockholders electing to be brought along pursuant to this
Section 4.3 (a), at the same price, on the same date and otherwise on the same
terms and conditions as those actually applicable to the Selling Stockholder in
such sale, a number of Common Shares (or Preferred Shares convertible into a
number of Common Shares) that bears the same proportion to the number of Common
Shares proposed to be sold by the Selling Stockholder (or Preferred Shares
convertible into a number of Common Shares) as the number of Common Shares held
by each Five Percent Preferred Stockholder (or Preferred Shares convertible into
a number of Common Shares) bears to the number of Common Shares then held (or
Preferred Shares convertible into a number of Common Shares) by all Five Percent
Preferred Stockholders immediately prior to such sale (excluding Transfers
pursuant to Section 4.1(b)).

          (b)  In the event any Five Percent Preferred Stockholder exercises his
rights to participate in a sale pursuant to Section 4.3, and the total number of
Shares to be sold by the Selling Stockholder who initiated the sale of shares of
Common Stock and those Five Percent Preferred Stockholders electing to
participate in such sale exceeds the number of shares of Common Stock the
prospective purchaser or purchasers thereof desire to acquire, the number of
shares of Common Stock that the Selling Stockholder and each such Five Percent
Preferred Stockholder shall be permitted to sell to such prospective purchaser
or purchasers shall be reduced, on a pro rata basis, based on the number of
shares of Common Stock each is permitted to offer to such purchaser or
purchasers pursuant to section 4.3(a).

          (c)  The obligations of Selling Stockholders to afford the Five
Percent Preferred Stockholders the rights referred to in this Section 4.3 will
be discharged if the Five Percent Preferred Stockholders are given written
notice simultaneously with the giving of the Offer required by Section 4.2(a),
and if such notice provides that each Five Percent Preferred Stockholder may
elect to avail himself of the rights set forth either in Section 4.2 or in
Section 4.3 by a written reply addressed to the Selling Stockholder and the
Company.

          (d)  Anything in this Agreement to the contrary notwithstanding, the
rights and obligations of Five Percent Preferred Stockholders under Section 4.3
shall not be assignable.

          (e)  Each Five Percent Preferred Stockholder wishing to participate in
any sale under Section 4.3 shall: (i) notify the Selling Stockholder and the
Company in writing of such intention within 20 days after the date the Offer was
made and (ii) not have the right to purchase Common Shares from the Selling
Stockholder pursuant to such Offer.

     4.4  Election of Directors.

          (a)  Each of the Stockholders agrees to take such action, including
the voting of the Shares owned or controlled by such Stockholder: (x) at any
annual or special meeting of Stockholders of the Company called for the purpose
of voting on the election or removal of

                                      10
<PAGE>

Directors; or (y) by consensual action of Stockholders with respect to the
election or removal of Directors, as may be necessary to cause the following:

               (i)   the Company shall be managed by a Board of Directors
consisting of no fewer than nine and no more than eleven members.

               (ii)  (A) two directors shall be persons selected by the holders
of a majority of the Common Stock held by Common Stockholders by vote or written
consent (the "Common Directors"). (B) The number of directors to be chosen by a
vote of a majority of the Preferred Stock held by the Investors (the "Preferred
Directors") shall be: (1) three for so long as the Investors, in the aggregate,
own no fewer than 750,000 Shares (on an as-converted to Common Stock basis); (2)
two for so long as the Investors, in the aggregate, own between 500,000 and
749,999 Shares (on an as-converted to Common Stock basis); (3) one for so long
as the Investors, in the aggregate, own between 250,000 and 499,999 Shares (on
an as-converted to Common Stock basis); and (4) none if the Investors, in the
aggregate, own fewer than 250,000 Shares (on an as-converted to Common Stock
basis). (C) One director shall be a person selected by Shaw for so long as Shaw
owns at least 50,000 Shares (on an as-converted to Common Stock basis). (D) In
addition to the directors chosen by Investors pursuant to Section B hereof, one
director shall be a person selected by Kummell for so long as Kummell owns at
least 300,000 Shares (on an as-converted to Common Stock basis), such director
to be reasonably acceptable to the Company. The foregoing numbers of Shares
shall be subject to adjustment for stock splits, adjustments, recapitalizations
and the like.

               (iii) the removal, with or without cause, of any Preferred
Directors designated for removal by a vote of the majority of the Investors, if
the Investors shall be Stockholders, or by a majority of the Preferred
Stockholders if the Investors are not Stockholders, and the removal, with or
without cause of any Common Director designated for removal by a majority of the
Common Stockholders.

               (iv)  the removal, with or without cause, of the director
designated by Shaw if such director is designated for removal by a vote of the
majority of Shares held by Shaw, if Shaw shall be a Stockholder, or by a
majority of the Preferred Stockholders if Shaw is not a Stockholder.

               (v)   the removal, with or without cause, of the director
designated by Kummell if such director is designated for removal by a vote of
the majority of Shares held by Kummell.

          (b)  If at any time a vacancy is created on the Board of Directors by
reason of the death, removal or resignation of any director, such vacancy shall
be filled pursuant to Article IV, Section 18 of the By-laws of the Company.

          (c)  The Company agrees to take all action necessary to ensure that,
except as otherwise unanimously approved by the Company's Board of Directors,
(i) the directors of the Company's subsidiaries are identical to the directors
of the Company and (ii) the by-laws of all of the Company's subsidiaries are
substantially identical to the Company's By-laws.

                                       11
<PAGE>

          (d)  No Stockholder shall grant any proxy or enter into or agree to be
bound by any voting agreement or voting trust with respect to voting any Shares,
except as provided herein. No Stockholder shall enter into any stockholder
agreement or arrangement of any kind with any Person with respect to any Shares
inconsistent with the provisions of this Agreement (whether or not such
agreements and arrangements are with other Stockholders or holders of Capital
Stock that are not bound by this agreement), including, but not limited to,
agreements or arrangements with respect to the acquisition, disposition or
voting of Shares, or act, for any reason, as a member of a Group or in concert
with any other Persons in connection with the acquisition, disposition or voting
of Shares in any manner which is inconsistent with the provisions of this
Agreement.

          (e)  Should the provisions of this Section 4.4 be construed to
constitute the granting of proxies, such proxies shall be deemed coupled with an
interest and, to the extent permitted by law, are irrevocable for the term of
this Agreement.

SECTION 5. Drag Along.

     Anything in this Agreement to the contrary notwithstanding, in the event
that the Board of Directors by unanimous vote or unanimous written consent
approves a transaction pursuant to which any Person or Persons will acquire all
of the Shares of the Company, each of the Stockholders agrees to vote in favor
of the transaction and to offer to sell all of his or its Shares, and to sell
all of his or its shares, to such Person or Persons, upon the terms and
conditions for the transaction approved by the Board of Directors.

SECTION 6. Option Shares.

     The Company may adopt a Stock Plan or Stock Plans providing for Option
Shares, or amend any such plan, on terms which are satisfactory to all of the
Preferred Directors.

SECTION 7. Rights to Purchase Additional Stock.

          (a)  Each of the Founders and each Five Percent Preferred Stockholder
shall have the right to subscribe to any and all issuances of Capital Stock of
the Company, other than issuances of Excluded Stock, in an amount equal to his
or its Proportional Share of the issuances to the extent set forth in this
Section 7. For purposes of determining the "Proportional Share" of the
securities to be issued to which each Founder and each Five Percent Preferred
Stockholder, as the case may be, is entitled to subscribe, the number of shares
of Common Stock or Common Stock then issuable upon conversion of Preferred
Shares (without giving effect to the proposed issuance of securities) held by
the subscribing Founder or the subscribing Five Percent Preferred Stockholder
shall be divided by the total number of shares of Common Stock outstanding prior
to the offering on a fully diluted basis (i.e., assuming that all the
outstanding Preferred Stock and any other outstanding security of the Company
convertible into or exercisable or exchangeable for shares of Common Stock
without the payment of consideration shall have been converted into or exercised
or exchanged for shares of Common Stock).

          (b)  In the event the Company shall propose to issue Capital Stock,
except for Excluded Stock, the Company shall give written notice (the "Notice")
to each Founder and each Five Percent Preferred Stockholder, which shall set
forth the number and kind or class of shares

                                      12
<PAGE>

of Capital Stock proposed to be issued (the "Offered Securities") the terms and
conditions thereof and the price therefor. Such notice shall be given at least
20 days prior to the issuance of such Capital Stock. The Offer by its term shall
remain open and irrevocable for a period of 20 days from the date of its
delivery to such Founder or Five Percent Preferred Stockholder (the "20-Day
Period").

          (c)  The Founder or Five Percent Preferred Stockholder shall evidence
its acceptance of the Notice by delivering a written notice ("Notice of
Acceptance"), signed by the Founder or Five Percent Preferred Stockholder,
setting forth the number of Offered Securities which the Founder or Five Percent
Preferred Stockholder elects to purchase. The Notice of Acceptance must be
delivered to the Company prior to the end of the 20-Day Period.

          (d)  The Company shall have 90 days from the expiration of the 20-Day
Period to sell all or any part of the Offered Securities refused by the Founders
or the Five Percent Preferred Stockholders to any person(s), but only upon terms
and conditions which are in all material respects no more favorable to such
other person(s) than those set forth in the Notice.

          (e)  Upon the closing of the sale of Offered Securities to any third
party, each Founder or Five Percent Preferred Stockholder shall purchase from
the Company, and the Company shall issue and sell to such Founder or Five
Percent Preferred Stockholder, any Offered Securities for which such Founder or
Five Percent Preferred Stockholder tendered a Notice of Acceptance upon the
terms specified in the Notice.

          (f)  In each case, any Offered Securities not purchased either by the
Founders or the Five Percent Preferred Stockholders or by any other person in
accordance with this Section 7 may not be sold or otherwise disposed of until
they are again offered to the Founders and the Five Percent Preferred
Stockholders under the procedures specified in this Section 7.

          (g)  If the Capital Stock to be issued by the Company is to be issued
pursuant to a Public Offering, the Company may require that the Founders and the
Five Percent Preferred Stockholders make an election to either: (i) commit to
purchase shares of Capital Stock from the Company at the public offering price
at the closing of the Public Offering or (ii) waive their rights to subscribe
for additional shares of Common Stock to be issued in the Public Offering. Such
election shall be effective if made by a majority of Five Percent Preferred
Stockholders and shall be made sufficiently in advance of the filing of the
registration statement relating to the Public Offering as shall be reasonably
requested by the Company.

          (h)  The rights provided by this Section 7 may be assigned by any Five
Percent Preferred Stockholder which is a limited partnership or a trust to any
and all members of its Group, provided that all rights of any assignee to
purchase Offered Securities will be subject to receipt of appropriate
representations from such assignee as requested by the Company to ensure
compliance with all applicable securities laws.

          (i)  In order to complete the offering and sale of the Series E
Preferred Stock in a timely manner, the Founders and Five Percent Preferred
Stockholders hereby waive compliance by the Company with the notice requirements
as set forth in this Section 7 with

                                       13
<PAGE>

respect to the sale and issuance of Series E Preferred Stock pursuant to the
Stock Purchase Agreement.

SECTION 8. Reporting of Public Information Under Rule 144.

          (a)  With a view to making available the benefits of Rule 144 under
the Securities Act (or any similar or successor rule which may at any time
permit the sale of Common Shares to the public without registration), at all
times after 90 days after any registration statement covering an offering of
securities of the Company under the Securities Act shall have become effective,
the Company agrees to:

               (i)   make and keep public information available, as those terms
are defined in Rule 144 under the Securities Act;

               (ii)  use its best efforts to 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; and

               (iii) furnish to each Preferred Stockholder promptly upon request
a written statement by the Company as to its compliance with the reporting
requirements of the Exchange Act, a copy of the most recent annual or quarterly
report of the Company, and such other reports and documents so filed by the
Company as such holder may reasonably request in availing itself of Rule 144 (or
any similar or successor rule).

          (b)  With a view to making available the benefits of Rule 144A (or any
similar or successor rule), the Company shall, upon request of a Stockholder,
make and keep available such information as is required pursuant to that rule.

SECTION 9. Registration Rights.

     9.1  Demand Registration Rights. Upon written request by holders of
Registrable Securities representing in the aggregate at least 25 percent of the
total number of Registrable Securities that have not been registered under the
Securities Act, the Company shall use its best efforts to effect the
registration under the Securities Act and registration or qualification under
all applicable state securities laws of the Registrable Securities, as requested
by the holders of Registrable Securities, all as provided in the following
provisions of this Section 9. Holders of Registrable Securities may require the
Company to effect no more than two registrations under the Securities Act, in
the aggregate, upon the request of the holders of Registrable Securities
pursuant to this Section 9.1. Except as set forth in Section 9.6 below, any
registration which is not declared effective pursuant to the Securities Act or
which does not remain effective as required by Section 9.5(a) below shall not
constitute one of the two registrations which the Company is obligated to effect
pursuant to this Section 9.1. The Company shall not be obligated to take any
action to effect any such registration, qualification or compliance pursuant to
this Section 9.1, (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: (i) January 31, 2002 or (ii) twelve months
after the effective date of the registration statement pertaining to the first
Public Offering

                                      14
<PAGE>

of securities of the Company for its own account (the "Initial Public Offering")
(other than a registration relating solely to a Rule 145 transaction or a
registration relating solely to employee benefit plans), (c) during the 180 day
period commencing with the effective date of a Public Offering, (d) if at the
time of the request to register Registrable Securities the Company gives notice
within 30 days of such request that it is engaged or has fixed plans to engage
within 30 days of the time of the request in a Public Offering as to which the
Holders may include Registrable Securities pursuant to Section 9.3 hereof, (e)
where aggregate gross proceeds would be less than $10,000,000, or (f) for so
long as the Company has not previously registered its shares of Common Stock
pursuant to Section 13 or Section 15(d) of the Exchange Act, if the Company
shall furnish to the initiating holders a certificate signed by the President of
the Company stating that, in the good faith judgment of the Board of Directors
of the Company, it would not be in the best interests of the Company and its
stockholders for such registration statement to be filed and it is therefore
appropriate to defer the filing of such registration statement, in which case
the Company may direct that such request for registration be delayed for a
period not in excess of 120 days, such right to delay a request to be exercised
by the Company not more than once in any 12-month period.

     9.2  Registration Requested by Holders. Whenever the Company shall be
requested, pursuant to Section 9.1 hereof, to effect the registration of any of
the Registrable Securities under the Securities Act (a "Request for
Registration"), the Company shall give notice of such proposed registration to
all holders of Registrable Securities at least 20 days before the Company files
a registration statement and thereupon shall, as expeditiously as possible after
such 20-day notice period, use its best efforts to effect the registration under
the Securities Act and under all applicable state securities laws of:

          (a)  all Registrable Securities which the Company has been requested
to register pursuant to the Request for Registration; and

          (b)  all other Registrable Securities which holders of Registrable
Securities have, within 20 days after the Company has given such notice,
requested the Company to register, all to the extent requisite to permit the
sale or other disposition by the holders of the Registrable Securities so to be
registered. If the holders of Registrable Securities who requested the
registration of Registrable Securities engage one or more underwriters to
distribute such Registrable Securities, the Company shall permit the managing
underwriter(s) and counsel to the underwriter(s) at the Company's expense to
visit and inspect any of the properties of the Company, examine its books, take
copies and extracts therefrom and discuss the affairs, finances and accounts of
the Company with its officers, employees and public accountants (and by this
provision the Company hereby authorizes said accountants to discuss with such
underwriter(s) and such counsel its affairs, finances and accounts), at
reasonable times and upon reasonable notice, with or without a representative of
the Company being present. The Company shall have the right to include in any
registration of Registrable Securities required pursuant to this Section 9.2
additional shares of its Common Stock ("Third Party Registrable Securities"),
provided that if any Registrable Securities to be so registered for sale are to
be distributed by or through underwriters, then all Registrable Securities to be
so registered for sale, and Third Party Registrable Securities, if any, shall be
included in such underwriting on the same terms and provided, further that if,
in the written opinion of the managing underwriter(s), the total amount of such
securities to be registered will exceed the maximum amount of the Company's
securities

                                      15
<PAGE>

which can be marketed (i) at a price reasonably related to their then current
market value, or (ii) without otherwise materially and adversely affecting the
entire offering, then the Company shall exclude from such underwriting: (x)
first, the maximum number of Third Party Registrable Securities as is necessary
in the opinion of the managing underwriter(s) to reduce the size of the
offering; and (y) then, the minimum number of Registrable Securities, pro rata
to the extent practicable, on the basis of the number of Registrable Securities
requested to be registered, among the participating holders of Registrable
Securities, as is necessary to reduce the size of the offering.

     9.3  "Piggyback" Registrations.

          (a)  If the Company at any time proposes other than in accordance with
a Request for Registration to register any of its securities under the
Securities Act on Form S-1, S-2 or S-3 or on any other form upon which the
Registrable Securities may be registered for sale to the general public, whether
for its own account or for the account of others, the Company will at each such
time give notice to all holders of Registrable Securities of such proposal at
least 20 days before the Company files a registration statement. Upon the
request of any holder of Registrable Securities given within 20 days after the
Company has given such notice, the Company will cause the Registrable Securities
which the Company has been requested to register by such holder of Registrable
Securities to be registered under the Securities Act, all to the extent
requisite to permit the sale or other disposition by such holder of the
Registrable Securities so registered.

          (b)  If securities are to be registered for sale under a registration
not initiated by a Request for Registration and are to be distributed for the
account of the Company by or through a firm of underwriters then any Registrable
Securities which the Company has been requested to register pursuant to clause
(a) of this section 9.3 shall also be included in such underwriting on the same
terms as other securities of the same class as the Registrable Securities
included in such underwriting, provided that if, in the written opinion of the
managing underwriter(s), the total amount of such securities to be so
registered, when added to the Registrable Securities and the securities held by
holders of securities other than the Registrable Securities, if any, will exceed
the maximum amount of the Company's securities which can be marketed at a price
reasonably related to their then current market value or without otherwise
materially and adversely affecting the entire offering, then (subject to clause
(e) of this Section 9.3) the Company shall exclude from such underwriting: (i)
first, the maximum number of securities, if any, other than Registrable
Securities, being sold for the account of persons other than the Company as is
necessary to reduce the size of the offering; and (ii) then, the minimum number
of Registrable Securities, pro rata to the extent practicable; on the basis of
the number of Registrable Securities requested to be registered among the
participating holders of Registrable Securities, as is necessary in the opinion
of the managing underwriter(s) to reduce the size of the offering.

          (c)  If securities are to be registered for sale under a registration
not initiated by a Request for Registration and are to be distributed for the
account of holders of Third Party Registrable Securities or holders (other than
the Company) of other securities of the Company other than Registrable
Securities by or through a firm of underwriters of recognized standing

                                      16
<PAGE>

under underwriting terms appropriate for such transaction, then any Registrable
Securities which the Company has been requested to register pursuant to clause
(a) of this Section 9.3 shall also be included in such underwriting on the same
terms as other securities included in such underwriting, provided that if, in
the written opinion of the managing underwriter or underwriters, the total
amount of such securities to be so registered, when added to such Registrable
Securities, will exceed the maximum amount of the Company's securities which can
be marketed at a price reasonably related to their then current market value or
without otherwise materially and adversely affecting the entire offering, then
the Company shall exclude from such underwriting the number of Registrable
Securities and other securities, pro rata to the extent practicable, on the
basis of the number of securities requested to be registered, as is necessary to
in the opinion of the managing underwriter(s) to reduce the size of the
offering.

          (d)  The Company may exclude all Registrable Securities from
registration in connection with the Company's Initial Public Offering in its
sole discretion, whether or not such exclusion is required in the opinion of the
managing underwriter(s).

     9.4  Registration on S-3.  In case the Company shall receive from any
holder or holders of Registrable Securities a written request or requests that
the Company effect a registration on Form S-3 (or any successor to Form S-3) or
any similar short-form registration statement and any related qualification or
compliance with respect to all or a part of the Registrable Securities owned by
such holder or holders, the Company will:

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

          (b)  as soon as practicable, effect such registration and all such
qualifications and compliances as may be so requested and as would permit or
facilitate the sale and distribution of all or such portion of such holder's or
holders' Registrable Securities as are specified in such request, together with
all or such portion of the Registrable Securities of any other holder or holders
joining in such request as are specified in a written request given within
fifteen (15) days after receipt of such written notice from the Company;
provided, however, that the Company shall not be obligated to effect any such
registration, qualification or compliance pursuant to this Section 9.4:

               (i)   if Form S-3 (or any successor or similar form) is not
available for such offering by the holders; or

               (ii)  if the holders, together with the holders of any other
securities of the Company entitled to inclusion in such registration, propose to
sell Registrable Securities and such other securities (if any) at an aggregate
price to the public of less than one million dollars ($1,000,000); or

               (iii) if within thirty (30) days of receipt of a written request
to register Registrable Securities pursuant to this Section 9.4, the Company
gives notice to the holders of the Company's intention to make a public offering
within ninety (90) days;

               (iv)  if the Company shall furnish to the holders a certificate
signed by the Chairman of the Board of Directors of the Company stating that in
the good faith judgment

                                      17

<PAGE>

of the Board of Directors of the Company, it would be seriously detrimental to
the Company and its stockholders for such Form S-3 registration to be effected
at such time, in which event the Company shall have the right to defer the
filing of the Form S-3 registration statement for a period of not more than
ninety (90) days after receipt of the request of the holder or holders under
this Section 9.4; provided, that such right to delay a request shall be
exercised by the Company not more than once in any twelve (12) month period;

               (v)    during the period starting with the date thirty (30) days
prior to the Company's estimated date of filing of, and ending on the date four
(4) months immediately following, the effective date of any registration
statement pertaining to securities of the Company (other than a registration of
securities in a Rule 145 transaction or with respect to any employee benefit
plan);

               (vi)   if the Company has already effected two (2) registrations
on Form S-3 for the holders pursuant to this Section 9.4; or

               (vii)  in any particular jurisdiction in which the Company would
be required to qualify to do business or to execute a general consent to service
of process in effecting such registration, qualification or compliance.

Subject to the foregoing, the Company shall file a Form S-3 registration
statement covering the Registrable Securities and other securities so requested
to be registered as soon as practicable after receipt of the request or requests
of the holders.  Registrations effected pursuant to this Section 9.4 shall not
be counted as demands for registration or registrations effected pursuant to
Sections 9.1 or 9.3, respectively.

     9.5  Company's Obligations in Registration.  Whenever the Company is
obligated to effect the registration of any Registrable Securities under the
Securities Act, as expeditiously as possible the Company will use its best
efforts to:

          (a)  prepare and file with the Commission, a registration statement
with respect to such Registrable Securities and cause such registration
statement to become and remain effective, provided, that the Company shall not
be required to keep such registration statement effective, or to prepare and
file any amendments or supplements thereto, later than 150 days following the
date on which such registration statement becomes effective under the Securities
Act;

          (b)  subject to Section 9.5(a) hereof, prepare and file with the
Commission such amendments and supplements to such registration statement and
the prospectus used in connection therewith as may be necessary to keep such
registration statement effective and to comply with provisions of the Securities
Act with respect to the disposition of all Registrable Securities covered by
such registration statement whenever the holders of Registrable Securities
covered by such registration statement shall desire to dispose of the same;

          (c)  furnish to the holders of Registrable Securities for whom such
Registrable Securities are registered or are to be registered such numbers of
copies of a printed prospectus, including a preliminary prospectus and any
amendments or supplements thereto, in conformity with the requirements of the
Securities Act, and such other documents as such holders of

                                       18
<PAGE>

Registrable Securities may reasonably request in order to facilitate the
disposition of such Registrable Securities;

          (d)  notify each holder of Registrable Securities, at any time when a
prospectus relating to the Registrable Securities covered by such registration
statement is required to be delivered under the Securities Act, of the Company's
becoming aware that the prospectus in such registration statement, as then in
effect, includes an untrue statement of a material fact or omits to state any
material fact required to be stated therein or necessary to make the statements
therein not misleading, and at the request of any holder of Registrable
Securities, prepare and furnish to such holder any reasonable number of copies
of any supplement to or amendment of such prospectus necessary so that, as
thereafter delivered to any purchaser of the Registrable Securities, such
prospectus shall not include an untrue statement of a material fact or omit to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading;

          (e)  register or qualify the Registrable Securities covered by such
registration statement under such securities or blue sky laws of such
jurisdictions as the holders of Registrable Securities for whom such Registrable
Securities are registered or are to be registered shall reasonably request, and
do any and all other reasonable acts and things which may be necessary or
advisable to enable such holders of Registrable Securities to consummate the
disposition in such jurisdictions of such Registrable Securities;

          (f)  in the case of a Public Offering, use its reasonable best efforts
to furnish to the holders of Registrable Securities for whom such Registrable
Securities are registered or are to be registered an agreement satisfactory in
form and substance to them by the Company and each of its officers, directors
and holders of five percent or more of any class of capital stock, that during
the 30 days before and the 180 days after the effective date of any Public
Offering, the Company and such officers, directors and five percent security
holders shall not offer, sell, contract to sell or otherwise dispose of any
shares of capital stock or securities convertible into capital stock, except as
part of such Public Offering and except that gifts may be made to relatives or
their legal representatives upon the condition that the donees agree in writing
to be bound by the restrictions contained in this clause (f) of Section 9.5;

          (g)  in the case of a Public Offering, furnish to the holders of
Registrable Securities for whom such Registrable Securities are registered or
are to be registered at the closing of the sale of such Registrable Securities
by such holders of Registrable Securities a signed copy of: (i) an opinion or
opinions of counsel for the Company acceptable to such holders of Registrable
Securities in form and substance as is customarily given to underwriters in
public offerings, and (ii) a "cold comfort" letter from the independent
certified public accountants of the Company, in form and substance as is
customarily given by independent certified public accounts to underwriters in an
underwritten public offering, to the extent that such "cold comfort" letters are
then available to selling stockholders;

         (h)  otherwise use its efforts to comply with all applicable rules and
regulations of the Commission, and, if required, make available to its security
holders, as soon as reasonably practicable, an earning statement covering the
period of at least 12 months, but not more than eighteen months, beginning with
the first day of the Company's first calendar quarter

                                       19
<PAGE>

after the effective date of the registration statement, which earning statement
shall satisfy the provisions of Section 11(a) of the Securities Act and Rule 158
thereunder;

               (i)  use its best efforts to cause all Registrable Securities
covered by such registration statement to be listed on the principal securities
exchange on which similar equity securities issued by the Company are then
listed, if the listing of such Registrable Securities is then permitted under
the rules of such exchange or, if similar equity securities are not listed, to
include the Registrable Securities on the National Association of Securities
Dealers Automated Quotation System;

               (j)  in connection with any Public Offering, enter into an
underwriting agreement with the underwriter(s) of such offering in the form
customary for such underwriter(s) for similar offerings, including such
representations and warranties by the Company, provisions regarding the delivery
of opinions of counsel for the Company and accountants' letters, provisions
regarding indemnification and contribution, and such other terms and conditions
as are at the time customarily contained in such underwriter's underwriting
agreements for similar offerings (and, at the request of any holder of
Registrable Securities that are to be distributed by such underwriter(s), any or
all (as requested by such holder) of the representations and warranties by, and
the other agreements on the part of, the Company to and for the benefit of such
underwriter(s) shall also be made to and for the benefit of such holder); and

               (k)  permit any holder of Registrable Securities who, in the sole
judgment, exercised in good faith, of such holder, might be deemed to be a
controlling person of the Company, to participate in the preparation of such
registration statement and to require the insertion therein of material,
furnished to the Company in writing, that in the judgment of such holder, as
aforesaid, should be included.

     9.6  Payment of Registration Expenses.  The costs and expenses of all
registrations and qualifications under the Securities Act and of all other
actions which the Company is required to take or effect pursuant to this Section
9, shall be paid by the Company or holders of Third Party Registrable Securities
or other securities of the Company other than Registrable Securities, if any
(including, without limitation, all registration and filing fees, printing
expenses, auditing costs and expenses, and the reasonable fees and disbursements
of counsel for the Company and one special counsel for the holders of
Registrable Securities) and the holders of Registrable Securities shall pay only
the underwriting discounts and commissions and transfer taxes, if any, relating
to the Registrable Securities sold by them. The Company shall not be required to
pay for any expenses of any registration begun pursuant to Section 9.1 if the
registration request is subsequently withdrawn at the request of the holders of
a majority of the Registrable Securities to be registered (in which case all
participating holders shall bear such expenses), unless the holders of a
majority of the Preferred Shares agree to forfeit their right to one demand
registration pursuant to Section 9.1.

     9.7  Information from Holders of Registrable Securities.  Notices and
requests delivered by holders of Registrable Securities to the Company pursuant
to this Section 9 shall contain such information regarding the Registrable
Securities to be so registered and the intended method of disposition thereof as
shall reasonably be required in connection with the action to be taken. Each
holder of Registrable Securities hereby agrees to provide the Company, or its
agents

                                       20
<PAGE>

or designees, with all information reasonably required in connection with the
registration under the Securities Act or any applicable state securities law of
any Registrable Securities.

     9.8  Indemnification.

          (a)  In the event of any registration under the Securities Act of any
Registrable Securities pursuant to this Section 9, the Company shall indemnify
and hold harmless each holder of Registrable Securities disposing of such
Registrable Securities and each other person, if any, which controls (within the
meaning of the Securities Act) such holder of Registrable Securities and each
other person (including underwriters) who participates in the offering of such
Registrable Securities, against any losses, claims, damages or liabilities,
joint or several, to which such holder of Registrable Securities or controlling
person or participating person may become subject under the Securities Act or
otherwise, to the extent that such losses, claims, damages or liabilities (or
proceedings in respect thereof) arise out of or are based upon any untrue
statement or alleged untrue statement of any material fact contained, on the
effective date thereof, in any registration statement under which such
Registrable Securities were registered under the Securities Act, in any
preliminary prospectus or final prospectus contained therein, or in any
amendment or supplement thereto, or arise out of or are based upon the omission
or alleged omission to state therein a material fact required to be stated
therein (in the case of a prospectus, in the light of the circumstances under
which they were made) or necessary to make the statements therein not
misleading, and will reimburse such holder of Registrable Securities and each
such controlling person or participating person for any legal or any other
expenses reasonably incurred by such holder of Registrable Securities or such
controlling person or participating person in connection with investigating or
defending any such loss, claim, damage, liability or proceeding; provided, that
the Company will not be liable in any such case to the extent that any such
loss, claim, damage or liability arises out of or is based upon an untrue
statement or alleged untrue statement or omission or alleged omission made in
such registration statement, preliminary or final prospectus or amendment or
supplement in reliance upon and in conformity with written information furnished
to the Company by an instrument duly executed by such holder of Registrable
Securities or such controlling or participating person, as the case may be,
specifically for use in the preparation thereof. Each holder of Registrable
Securities will, if Registrable Securities held by such holder are included in a
registration under the Securities Act pursuant to this Section 9, severally but
not jointly, indemnify and hold harmless the Company and each person which
controls (within the meaning of the Securities Act) the Company and each other
person (including underwriters) who participates in the offering of such
Registrable Securities and each person which controls such person, against all
losses, claims, damages and liabilities to which the Company or such controlling
person or participating person may become subject under the Securities Act or
otherwise, insofar as such losses, claims, damages or liabilities arise out of
or are based upon any untrue statement of any material fact contained, on the
effective date thereof, in any registration statement under which such
Registrable Securities were registered under the Securities Act, or in any
preliminary prospectus or final prospectus contained therein, or in any
amendment or supplement thereto, or arise out of or are based upon the omission
or alleged omission to state therein a material fact required to be stated
therein or necessary to make the statements therein (in the case of a
prospectus, in the light of the circumstances under which they were made) not
misleading, to the extent that any such loss, claim, damage or liability arises
out of or is based upon any such statement or omission made in such registration
statement, preliminary or final prospectus or amendment or supplement

                                       21
<PAGE>

in reliance upon and in conformity with written information furnished to the
Company by an instrument duly executed by such holder of Registrable Securities
and stated to be specifically for use in connection with such registration. Each
indemnified party shall cooperate with each indemnifying party in defending any
loss, claim, damage, liability or proceeding.

          (b)  Indemnification similar to that specified in the preceding clause
of this Section 9.8 (with appropriate modifications) shall be given by the
Company and each holder of Registrable Securities with respect to any
registration or other qualification of securities under any state securities
laws.

          (c)  If the indemnification provided for in clause (a) and (b) of this
Section 9.8 is unavailable or insufficient to hold harmless an indemnified
party, then each indemnifying party shall contribute to the amount paid or
payable by such indemnified party referred to in clause (a) and (b) of this
Section 9.8 in such proportion as is appropriate to reflect the relative fault
of the indemnifying party on the one hand and the indemnified party on the other
hand in connection with statements or omissions which resulted in losses,
claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative fault shall be determined by reference to, among
other things, whether the untrue or alleged untrue statement of a material fact
or the omission or alleged omission to state a material fact relates to
information supplied by the indemnifying party or the indemnified party and the
parties' relative intent, knowledge, access to information and opportunity to
correct or prevent such untrue statements or omissions. The parties agree that
it would not be just and equitable if contributions pursuant to this clause were
to be determined by pro rata allocation or by any other method of allocation
which does not take account of the equitable considerations referred to in the
first sentence of this clause. The amount paid by an indemnified party as a
result of the losses, claims, damages or liabilities referred to in the first
sentence of this clause shall be deemed to include any legal or other expenses
reasonably incurred by such indemnified party in connection with investigating
or defending any loss, claim, damage, liability or proceeding which is the
subject of this clause. No person guilty of fraudulent misrepresentation (within
the meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation.

          (d)  Each indemnified party shall notify the indemnifying party in
writing within 10 days after its receipt of notice of the commencement of any
action against it in respect of which indemnity may be sought from the
indemnifying party pursuant to this Section 9.8. In case any such action shall
be brought against an indemnified party and it shall notify the indemnifying
party, the indemnifying party will be entitled to participate in the defense
with counsel reasonably satisfactory to such indemnified party. Each indemnified
party shall cooperate with each indemnifying party in defending any loss, claim,
damage, liability or proceeding.

          (e)  Notwithstanding clauses (a) through (c) of this Section 9.8, the
aggregate amount which may be recovered by the Company, controlling persons of
the Company or underwriters from each holder of Registrable Securities pursuant
to the indemnification and contribution provided for in this Section 9.8 shall
be limited to the total net proceeds for which the Registrable Securities were
sold by such holder of Registrable Securities.

                                       22
<PAGE>

          (f)  Notwithstanding any of the foregoing, if, in connection with a
Public Offering of Registrable Securities, the Company, the selling stockholders
and the underwriter(s) enter into an underwriting agreement relating to such
Public Offering which contains provisions covering indemnification and
contribution among the parties, the indemnification and contribution provisions
of this Section 9.8 shall be deemed inoperative for purposes of such offering.

          (g)  The obligations of the Company and Stockholders under this
Section 9.8 shall survive completion of any offering of Registrable Securities
in a registration statement. 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 which does not
include as an unconditional term thereof the giving by the claimant or plaintiff
to such indemnified party of a release of all liability in respect to such claim
or litigation.

     9.9  Deliveries to Holders of Registrable Securities.  The Company will use
its best efforts to promptly send by first-class mail, postage prepaid, to each
holder of Registrable Securities, at such holder's address appearing on the
Company's records, a copy of: (i) each registration statement filed by the
Company under the Securities Act and each amendment thereof and each exhibit and
schedule thereto and (ii) each order of the Securities and Exchange Commission
declaring any such registration statement to be effective.

SECTION 10.  Duration of Agreement.

     The rights and obligations of each Stockholder, except the rights and
obligations contained in Sections 3, 4.1(c), 8 and 9 hereof, and the covenants
hereunder to that Stockholder shall terminate as to each Stockholder upon the
closing of a Qualified Public Offering by the Company.  The obligations
contained in Sections 3, 4.1(c), 8 and 9 shall survive indefinitely until, by
their respective terms, they are no longer applicable.

SECTION 11.  Remedies.

     In case any one or more of the covenants and/or agreements set forth in
this Agreement shall have been breached by any party hereto, the party or
parties entitled to the benefit of such covenants or agreements may proceed to
protect and enforce their rights either by suit in equity and/or by action at
law, including, but not limited to, an action for damages as a result of any
such breach; and/or an action for specific performance of any such covenant or
agreement contained in this Agreement and/or a temporary or permanent
injunction, in any case without showing any actual damage. The rights, powers
and remedies of the parties under this Agreement are cumulative and not
exclusive of any other right, power or remedy which such parties may have under
any other agreement or law. No single or partial assertion or exercise of any
right, power or remedy of a party, hereunder shall preclude any other or further
assertion or exercise thereof. Any purported Transfer in violation of the
provisions of this Agreement shall be void.

SECTION 12.  Successors and Assigns.

     Except as otherwise expressly provided herein, this Agreement shall bind
and inure to the benefit of the Company, each of the Stockholders and the
respective successors or heirs and

                                       23
<PAGE>

personal representatives and permitted assigns of the Company and each of the
Stockholders. Each Stockholder agrees further that, except in connection with
Transfers by Stockholders pursuant to Section 4.2, such stockholder shall not
sell any Shares to any Person not a party to this Agreement unless such Person
becomes a party to this Agreement contemporaneously with such sale by executing
and delivering to the Company an agreement to be bound hereby, whereupon such
Person shall be deemed a Stockholder and shall have the same rights and
obligations as other Stockholders.

SECTION 13.  Additional Stockholders.

     The parties hereto acknowledge and agree that the Company may issue and
sell shares of Common Stock to Persons not presently a party to this Agreement,
provided, however, that: (i) all such sales are not made in violation of this
Agreement; and (ii) no shares of Common Stock shall be issued and sold to a
Person not presently a party to this Agreement unless and until such Person
becomes a party to this Agreement contemporaneously with such issuance and sale
by executing and delivering to the Company an agreement to be bound hereby,
whereupon such Person shall be deemed a Stockholder and shall have the same
rights and obligations as other Stockholders or agrees to be bound by and to
comply with provisions substantially identical to the provisions of Sections 3,
4.1, 4.2 and 5 hereof; provided, however, that the provisions of this section 13
shall not apply to Common Stock that is Excluded Stock.

SECTION 14.  Entire Agreement.

     This Agreement contains the entire agreement among the parties with respect
to the subject matter hereof and supersedes all prior Stockholders' agreements
and other prior and contemporaneous arrangements or understandings with respect
thereto. To the extent the provisions of this Agreement conflict with the
provisions contained in the Restricted Stock Agreements and the Restricted Stock
Option Agreements, provisions of this Agreement supersede such provisions of the
Restricted Stock Agreements and the Restricted Stock Option Agreements.

SECTION 15.  Notices.

     All notices, consents, and other communications under this Agreement shall
be in writing and shall be deemed to have been duly given: (a) when delivered by
hand, (b) one business day after the business day of transmission if sent by
telecopier (with receipt confirmed), provided that a copy is mailed by
registered mail, return receipt requested, or (c) one business day after the
business day of deposit with the carrier, if sent Express Mail, Federal Express
or other express delivery service (receipt requested), in each case to the
appropriate address telex numbers and telecopier numbers set forth below or in
Exhibit A hereto (or to such other addresses or telecopier numbers as a party
may designate by notice to the other parties):

If to the Company:    Dendreon Corporation
                      3005 First Avenue
                      Seattle, Washington  98121
                      Facsimile No.: (206) 256-0571
                      Attention: Chief Executive Officer

                                       24
<PAGE>

with a copy to:    Cooley Godward LLP
                   3000 El Camino Real
                   Five Palo Alto Square
                   Palo Alto, CA 94306
                   Facsimile No: (650) 849-7400
                   Attention: Julie M. Robinson, Esq.

SECTION 16.  Changes.

     The terms and provisions of this Agreement may not be modified or amended,
or any of the provisions hereof waived, temporarily or permanently, except
pursuant to the written consent of: (i) the Company; (ii) the holders of a
majority of the voting power of the Shares; (iii) the holders of a majority of
the voting power of the Preferred Shares; (iv) the holders of a majority of the
voting power of the Founders Shares; and (v) with respect to Sections
4.4(a)(ii)(D) and 4.4(a)(v), of Kummell. Notwithstanding the foregoing, this
Agreement may be amended solely with the written consent of the Company to
include as additional parties hereto any purchasers of the Company's Series E
Preferred Stock in any subsequent Closing.

SECTION 17.  Counterparts.

     This Agreement may be executed in any number of counterparts and each such
counterpart shall be deemed to be an original instrument, but all such
counterparts together shall constitute but one agreement.

SECTION 18.  Headings.

     The headings of the various sections of this Agreement have been inserted
for convenience of reference only and shall not be deemed to be part of this
Agreement.

SECTION 19.  Nouns and Pronouns.

     Whenever the context may require, any pronouns used herein shall include
the corresponding masculine, feminine or neuter forms, and the singular form of
names and pronouns shall include the plural and vice-versa.

SECTION 20.  Severability.

     Any provision of this Agreement that is prohibited or unenforceable in any
jurisdiction shall, as to such jurisdiction, be ineffective to the extent of
such prohibition or unenforceability.  Such prohibition or unenforceability in
any one jurisdiction shall not invalidate or render unenforceable such provision
in any other jurisdiction.

SECTION 21.  Governing Law; Jurisdiction.

     This Agreement shall be governed by and construed under the laws of the
State of Delaware as applied to agreements among Delaware residents entered into
and to be performed entirely within Delaware.

                                       25
<PAGE>

SECTION 22.  Delays or Omissions.

     It is agreed that no delay or omission to exercise any right, power, or
remedy accruing to any Stockholder upon any breach, default or noncompliance of
the Company under this Agreement shall impair any such right, power or remedy,
nor shall it be construed to be a waiver of any such breach, default or
noncompliance, or any acquiescence therein, or of any similar breach, default or
noncompliance thereafter occurring. It is further agreed that any waiver,
permit, consent or approval of any kind or character on any Stockholder's part
of any breach, default or noncompliance under this Agreement or any waiver on
such Stockholder's part of any provisions or conditions of this Agreement must
be in writing and shall be effective only to the extent specifically set forth
in such writing. All remedies, either under this Agreement, by law, or otherwise
afforded to Stockholders, shall be cumulative and not alternative.

                                       26
<PAGE>

     In Witness Whereof, the parties have executed this Agreement on the date
first above written, in the case of corporations, by their respective officers
thereunto duly authorized.

                                       Dendreon Corporation



                                       By: /s/ Martin A. Simonetti
                                           -------------------------
                                           Martin A. Simonetti
                                           Chief Financial Officer


                                       STOCKHOLDERS:

                                       /s/ * set forth below is a schedule of
                                       persons and entities who have signed and
                                       are parties to this agreement

                                       *
                                       HealthCare Ventures III, L.P.

                                       HealthCare Ventures IV, L.P.

                                       Dr. Sam Strober

                                       Dr. Edgar Engleman

                                       Dr. William Haseltine

                                       Erik N. Engleman

                                       Jason Engleman

                                       Edgar G. Engleman and Judith L. Engleman,
                                       as Trustees for the Engleman Family Trust

                                       Jason M. Strober

                                       Elizabeth E. Strober

                                       Everest Trust

                                       Hudson Trust

                                       Shaw Venture Partners III, L.P.

                                       Vulcan Ventures Incorporated

                                       Sanderling Venture Partners III, L.P.



             [FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT]
<PAGE>

                                       Sanderling III Limited Partnership

                                       Singapore Bio-Innovations Pte Ltd.

                                       Refco Group, Ltd.

                                       Sanderling III Biomedical, L.P.

                                       Sanderling Ventures Management

                                       Sanderling Venture Partners IV Co-
                                       Investment Fund, L.P.

                                       Soloman and Matilda Amon

                                       Amon Investments, L.P.

                                       HCA Enterprise Worldwide, LLC

                                       Colin H. Owen

                                       William F. Hamilton

                                       Sears Living Trust DTD 3/11/91

                                       GC&H Investments

                                       Peter S. Garcia

                                       Prithipal Singh

                                       Bobby H. Singh

                                       Clyde W. Shores

                                       Jane R. Shores

                                       Universite Libre De Bruxelles

                                       Jacques Urbain

                                       Kristianan Thielemans

                                       Shipley Raidy Capital Partners, L.P.

                                       Sanderling IV Biomedical
                                       Co-investment Fund, L.P.


             [FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT]
<PAGE>

                                       Health Care Ventures V, L.P.

                                       Richard J. Bastiani UTA
                                       Charles Schwab & Co. Inc.
                                       IRA contributory DTD 01/08/86

                                       Martin A. and Mary Ann Simonetti

                                       Kummell Investments Limited

                                       Kirin Brewery Company, Limited

                                       Jerry Weisbach

                                       Alexandria Real Estate Equities, L.P.

                                       New York Life Insurance Co.


             [FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT]
<PAGE>

                              EXHIBIT A
         List of Stockholders (Parties to the Stockholders' Agreement)


Investors

HealthCare Ventures III, L.P. ("HCV III")
c/o HealthCare Investment Corporation
44 Nassau Street
Princeton, New Jersey  08542-4506
Telecopier No.: (609) 430-9525
Attention: William Crouse

HealthCare Ventures IV, L.P. ("HCV IV")
c/o HealthCare Investment Corporation
44 Nassau Street
Princeton, New Jersey  08542-4506
Telecopier No.: (609) 430-9525
Attention: William Crouse

(copies of notices to HCV III or HCV IV should be sent to:

Bachner, Tally, Polevoy & Misher
380 Madison Avenue
New York, New York 10017
Telecopier No.: (212) 682-5729
Attention: Steven A. Fishman, Esq.)

Everest Trust:
c/o Rho Management Company, Inc.
767 Fifth Avenue -- 43rd Floor
New York, New York 10153
Telecopier No.: (212) 751-3613
Attention: Joshua Ruch

Hudson Trust:
c/o Summit Asset Management Co., Inc.
47 Hulfish Street, Suite 420
Princeton, New Jersey 08542
Telecopier No.: (609) 279-1892
Attention: Scott Ciccone



(copies of notices to Everest Trust or Hudson Trust should be sent to:
<PAGE>

Gregory F.W. Todd, Esq.
900 Third Avenue
New York, New York 10022
Telecopier No.: (212) 355-7674)


Founders

Dr. Samuel Strober, Jason Strober, or Elizabeth Strober
405 Minoca Drive
Portola Valley, CA 94611

Edgar, Jason or Eric Engleman or to the Engleman Family Trust:
c/o Dr. Ed Engleman
60 Lane Place
Atherton, CA 94027


Shaw

Shaw Venture Partners, L.P.
400 Southwest Sixth Avenue, Suite 1100
Portland, Oregon 97204-1636
Telecopier No.: (503) 227-2471
Attention: Ralph Shaw

Additional Stockholders

Dr. Haseltine:
c/o Human Genome Sciences, Inc.
6410 Key West Avenue
Rockville, MD 20850
<PAGE>

Kummell Investments Limited
Suite 835A, Europort
Gibraltar (via London)

Copies of all notices and correspondence to:

  Alice Li
  Springfield Financial Advisory Limited
  22nd Floor, Hang Lung Centre
  2-20 Paterson Street
  Causeway Bay, Hong Kong
  Tel: (852) 2576-6800
  Telecopier No.: (852) 2881-5741

  Stephanie Monaghan O'Brien
  Springfield & Company
  1188 Centre Street
  Newton Centre, MA  02159
  Tel: (617) 244-2800
  Telecopier No.: (617) 244-2889

Universite Libre De Bruxelles
Department Recherche
Avenue F.D. Roosevelt 50
CP 161
B-1050 Bruxelles
Belgium
Telecopier No. 32-2-650 35 2-850 32 04
Attn:  Jean-Louis Vanherweghem

Jacques Urbain
Universite Libre De Bruxelles
Department Recherche
Avenue F.D. Roosevelt 50
CP 161
B-1050 Bruxelles
Belgium
Telecopier No. 32-2-650 35 2-850 32 04
<PAGE>

Kristiaan Thielemans
Universite Libre De Bruxelles
Department Recherche
Avenue F.D. Roosevelt 50
CP 161
B-1050 Bruxelles
Belgium
Telecopier No.: 32-2-650 35 2-850 32 04

Shipley Raidy Capital Partners, L.P.
One Tower Bridge
Suite 1370
West Conshohocken, PA  19428
Telecopier No. (610) 828-4131
Attn:  Kevin J. Raidy

Sanderling IV Biomedical Co-Investment Fund, L.P.
2730 Sand Hill Road, Suite 200
Menlo Park, CA  94025
Telecopier No. (415) 854-3648
Attn:  Fred A. Middleton

Sanderling Venture Partners IV Co-Investment Fund, L.P.
2730 Sand Hill Road, Suite 200
Menlo Park, CA  94025
Telecopier No.: (415) 854-3648
Attn:  Fred A. Middleton

Health Care Ventures V, L.P ("HCV V")
c/o HealthCare Investment Corporation
44 Nassau Street
Princeton, New Jersey  08542-4506
Telecopier No.: (609) 430-9525
Attn: William Crouse

Copies of notices to HCV V should be sent to:

Bachner, Tally, Polevoy & Misher
380 Madison Avenue
New York, New York  10017
Telecopier No.: (212) 682-5729


Richard J. Bastiani UTA Charles Schwab & Co Inc
IRA Contributory DTD 01/08/86
18700 Serramonte Drive
Los Gatos, California 95030
Telecopier No. (408) 395-0214
Attn:  Richard J. Bastiani
<PAGE>

Shaw Venture Partners, L.P.
400 Southwest Sixth Avenue, Suite 1100
Portland, Oregon 97204-1636
Telecopier No.: (503) 227-2471
Attention: Ralph Shaw

Kirin Brewery Company, Limited
26-1 Jingumae 6-chome
Shibuya-ku, Tokyo
150-8011
Telecopier No.: +81-3-3499-6152
Attention: Akihiro Shimosaka

Vulcan Ventures Incorporated
110 - 110th Avenue NE, Suite 550
Bellevue, WA  98004
Telecopier No.: (425) 453-1985
Attention: William D. Savoy

New York Life Insurance Company
51 Madison Avenue
New York, NY  10010
Telecopier No.: (212) 576-8080
Attention: Richard F. Drake

Alexandria Real Estate Equities, L.P.
135 North Los Robles Avenue, Suite 250
Pasadena, California  91101
Telecopier No.: (626) 578-0770
Attention: Joel S. Marcus

GC&H Investments
One Maritime Plaza, 20th Floor
San Francisco, CA  94111
Telecopier No.: (415) 951-3699
Attention: John L. Cardoza

Martin A. and Mary Ann Simonetti
4553 84th Avenue SE
Mercer Island, WA  98040

Sears Living Trust DTD 3/11/91
70 Cheyenne Point
Portola Valley, CA  94028-7623
Telecopier No.: (650) 851-5052
<PAGE>

Jerry A. Weisbach
1351 Glendaloch Circle
Ann Arbor, Michigan  48104
Telecopier No.: (734) 668-4160
Attention: Jerry A. Weisbach
<PAGE>

                                   EXHIBIT B

              Purchasers of the Company's Series E Preferred Stock




FIRST CLOSING:
Vulcan Ventures Inc.
GC&H Investments
Sears Living Trust Dtd. 3/11/91
Martin A. and Mary Ann Simonetti

SECOND CLOSING:
HealthCare Ventures V, L.P.
Sanderling Venture Partners IV Co-Investment Fund, L.P.

THIRD CLOSING:
Kummell Investments Limited
New York Life Insurance Company

FOURTH CLOSING:
Shaw Venture Partners III, L.P.

FIFTH CLOSING:
Leavitt Investments, LP