<PAGE>

                            ROSETTA BIOSYSTEMS, INC.

                         COMMON STOCK PURCHASE AGREEMENT

         This Common Stock Purchase Agreement (the "AGREEMENT") is made as of
May 15, 1997 by and between Rosetta Biosystems, Inc., a Delaware corporation
(the "COMPANY"), and John King ("PURCHASER").

         1. SALE OF STOCK. Subject to the terms and conditions of this
Agreement, on the Purchase Date (as defined below) the Company will issue and
sell to Purchaser, and Purchaser agrees to purchase from the Company, 164,415
shares of the Company's Common Stock (the "SHARES") at a purchase price of $.01
per Share for a total purchase price of $1,644.15. The term "Shares" refers to
the purchased Shares and all securities received in replacement of or in
connection with the Shares pursuant to stock dividends or splits, all securities
received in replacement of the Shares in a recapitalization, merger,
reorganization, exchange or the like, and all new, substituted or additional
securities or other properties to which Purchaser is entitled by reason of
Purchaser's ownership of the Shares.

         2. PURCHASE. The purchase and sale of the Shares under this Agreement
shall occur at the principal office of the Company simultaneously with the
execution of this Agreement by the parties, or on such other date as the Company
and Purchaser shall agree (the "PURCHASE DATE"). On the Purchase Date, the
Company will deliver to Purchaser a certificate representing the Shares to be
purchased by Purchaser (which shall be issued in Purchaser's name) against
payment of the purchase price therefor by Purchaser by delivery of a check in
the amount of $1,644.15.

         3. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY. The
Company hereby represents, warrants and covenants to Purchaser that the
authorized capital of the Company consists, as of the Purchase Date, of
5,000,000 shares of Preferred Stock, none of which are issued and outstanding,
and 10,000,000 shares of Common Stock, 8,547,207 shares of which will be issued
and outstanding as of the Purchase Date. All of the outstanding shares of Common
Stock have been duly authorized, fully paid and are nonassessable and issued in
compliance with all applicable federal and state securities laws.

         4. LIMITATIONS ON TRANSFER. In addition to any other limitation on
transfer created by applicable securities laws, Purchaser shall not assign,
encumber or dispose of any interest in the Shares while the Shares are subject
to the Company's Repurchase Option (as defined below), except as provided below.
After any Shares have been released from the Repurchase Option, Purchaser shall
not assign, encumber or dispose of any interest in such Shares except in
compliance with the provisions below and applicable securities laws.

                  (a)      REPURCHASE OPTION.

                           (i)      In the event of the voluntary or
involuntary termination of Purchaser's employment or consulting relationship
with the Company for any reason (including death or disability), with or without
cause, the Company shall upon the date of such termination (the "TERMINATION
DATE") have an irrevocable, exclusive option (the

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"REPURCHASE OPTION") for a period of 60 days from such date to repurchase all or
any portion of the Shares held by Purchaser as of the Termination Date which
have not yet been released from the Company's Repurchase Option at the original
purchase price per Share specified in Section 1 (adjusted for any stock splits,
stock dividends and the like); PROVIDED, HOWEVER, that the Repurchase Option
shall continue for a period of up to one year from the Termination Date to the
extent that the Company reasonably determines that such an extension of time is
necessary to prevent the repurchase of Purchaser's Shares from causing other
capital stock of the Company to not qualify as "small business stock" under
Section 1202 of the Internal Revenue Code of 1986, as amended.

                           (ii) The Repurchase Option shall be exercised by the
Company by written notice to Purchaser or Purchaser's executor and, at the
Company's option, (A) by delivery to Purchaser or Purchaser's executor with such
notice of a check in the amount of the purchase price for the Shares being
purchased, or (B) in the event Purchaser is indebted to the Company, by
cancellation by the Company of an amount of such indebtedness equal to the
purchase price for the Shares being repurchased, or (C) by a combination of (A)
and (B) so that the combined payment and cancellation of indebtedness equals
such purchase price. Upon delivery of such notice and payment of the purchase
price in any of the ways described above, the Company shall become the legal and
beneficial owner of the Shares being repurchased and all rights and interest
therein or related thereto, and the Company shall have the right to transfer to
its own name the number of Shares being repurchased by the Company, without
further action by Purchaser.

                           (iii) One hundred percent (100%) of the Shares shall
initially be subject to the Repurchase Option. 1/12 of the Shares shall be
released from the Repurchase Option on the date that is one (1) month after the
Vesting Commencement Date (as set forth on the signature page of this
Agreement), and 1/12 of the total number of Shares shall be released from the
Repurchase Option at the end of each month thereafter, until all Shares are
released from the Repurchase Option (provided in each case that Purchaser's
employment or consulting relationship with the Company has not been terminated
prior to the date of any such release). Fractional shares shall be rounded to
the nearest whole share.

                           (iv)     Notwithstanding the above, in the event
of Purchaser's death or disability at such time as more than fifty percent (50%)
of the Shares remain subject to the Repurchase Option, all Shares in excess of
such fifty percent (50%) that remain subject to the Repurchase Option shall be
deemed to be released from the Repurchase Option as of the time of such death or
disability.

                           (v)      Notwithstanding the above, in the event
Purchaser's employment or consulting relationship with the Company is
involuntarily terminated without cause (excluding Purchaser's death or
disability), fifty percent (50%) of the Shares held by Purchaser which are still
subject to the Company's Repurchase Option as of the Termination Date shall be
deemed to have been released from the Repurchase Option immediately prior to the
Termination Date.

                  (b) RIGHT OF FIRST REFUSAL. Before any Shares held by
Purchaser or any transferee of Purchaser (either being sometimes referred to
herein as the "HOLDER") may be

                                      -2-

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sold or otherwise transferred (including transfer by gift or operation of law),
the Company or its assignee(s) shall have a right of first refusal to purchase
the Shares on the terms and conditions set forth in this Section 4(b) (the
"RIGHT OF FIRST REFUSAL").

                           (i)  NOTICE OF PROPOSED TRANSFER.  The Holder of the
Shares shall deliver to the Company a written notice (the "NOTICE") stating: (A)
the Holder's bona fide intention to sell or otherwise transfer such Shares; (B)
the name of each proposed purchaser or other transferee ("PROPOSED TRANSFEREE");
(C) the number of Shares to be transferred to each Proposed Transferee; and (D)
the terms and conditions of each proposed sale or transfer. The Holder shall
offer the Shares at the same price (the "OFFERED PRICE") and upon the same terms
(or terms as similar as reasonably possible) to the Company or its assignee(s).

                           (ii)     EXERCISE OF RIGHT OF FIRST REFUSAL.  At any
time within thirty (30) days after receipt of the Notice, the Company and/or its
assignee(s) may, by giving written notice to the Holder, elect to purchase all,
but not less than all, of the Shares proposed to be transferred to any one or
more of the Proposed Transferees, at the purchase price determined in accordance
with subsection (iii) below.

                           (iii)    PURCHASE PRICE.  The purchase price
("PURCHASE PRICE") for the Shares purchased by the Company or its assignee(s)
under this Section 4(b) shall be the Offered Price. If the Offered Price
includes consideration other than cash, the cash equivalent value of the
non-cash consideration shall be determined by the Board of Directors of the
Company in good faith. In the event the transfer occurs by gift or operation of
law, the Offered Price shall be equal to the fair market value as determined by
the Board of Directors of the Company in its reasonable judgment.

                           (iv)     PAYMENT.  Payment of the Purchase Price
shall be made, at the option of the Company or its assignee(s), in cash (by
check), by cancellation of all or a portion of any outstanding indebtedness of
the Holder to the Company (or, in the case of repurchase by an assignee, to the
assignee), or by any combination thereof within 30 days after receipt of the
Notice or in the manner and at the times set forth in the Notice.

                           (v)      HOLDER'S RIGHT TO TRANSFER.  If all of the
Shares proposed in the Notice to be transferred to a given Proposed Transferee
are not purchased by the Company and/or its assignee(s) as provided in this
Section 4(b), then the Holder may sell or otherwise transfer such Shares to that
Proposed Transferee at the Offered Price or at a higher price, provided that
such sale or other transfer is consummated within 60 days after the date of the
Notice and provided further that any such sale or other transfer is effected in
accordance with any applicable securities laws and the Proposed Transferee
agrees in writing that the provisions of this Section 4 shall continue to apply
to the Shares in the hands of such Proposed Transferee. If the Shares described
in the Notice are not transferred to the Proposed Transferee within such period,
or if the Holder proposes to change the price or other terms to make them more
favorable to the Proposed Transferee, a new Notice shall be given to the
Company, and the Company and/or its assignees shall again be offered the Right
of First Refusal before any Shares held by the Holder may be sold or otherwise
transferred.

                                      -3-

<PAGE>

                           (vi)     EXCEPTION FOR CERTAIN FAMILY TRANSFERS.
Anything to the contrary contained in this Section 4(b) notwithstanding, the
transfer of any or all of the Shares during Purchaser's lifetime or on
Purchaser's death by will or intestacy to Purchaser's Immediate Family or a
trust for the benefit of Purchaser's Immediate Family shall be exempt from the
provisions of this Section 4(b). "IMMEDIATE FAMILY" as used herein shall mean
spouse, lineal descendant or antecedent, father, mother, brother or sister. In
such case, the transferee or other recipient shall receive and hold the Shares
so transferred subject to the provisions of this Section, and there shall be no
further transfer of such Shares except in accordance with the terms of this
Section 4.

                  (c)      INVOLUNTARY TRANSFER.

                           (i)      COMPANY'S RIGHT TO PURCHASE UPON
INVOLUNTARY TRANSFER. In the event, at any time after the date of this
Agreement, of any transfer by operation of law or other involuntary transfer
(including death or divorce, but excluding a transfer to Immediate Family as set
forth in Section 4(b)(vi) above) of all or a portion of the Shares by the record
holder thereof, the Company shall have an option to purchase all of the Shares
transferred at the greater of the purchase price paid by Purchaser pursuant to
this Agreement or the fair market value of the Shares on the date of transfer.
Upon such a transfer, the person acquiring the Shares shall promptly notify the
Secretary of the Company of such transfer. The right to purchase such Shares
shall be provided to the Company for a period of thirty (30) days following
receipt by the Company of written notice by the person acquiring the Shares.

                           (ii)     PRICE FOR INVOLUNTARY TRANSFER.  With
respect to any stock to be transferred pursuant to Section 4(c)(i), the price
per Share shall be a price set by the Board of Directors of the Company that
will reflect the current value of the stock in terms of present earnings and
future prospects of the Company. The Company shall notify Purchaser or his or
her executor of the price so determined within thirty (30) days after receipt by
it of written notice of the transfer or proposed transfer of Shares. However, if
the Purchaser does not agree with the valuation as determined by the Board of
Directors of the Company, the Purchaser shall be entitled to have the valuation
determined by an independent appraiser to be mutually agreed upon by the Company
and the Purchaser and whose fees shall be borne equally by the Company and the
Purchaser.

                  (d) ASSIGNMENT. The right of the Company to purchase any part
of the Shares may be assigned in whole or in part to any stockholder or
stockholders of the Company or other persons or organizations; provided,
however, that an assignee, other than a corporation that is the parent or a 100%
owned subsidiary of the Company, must pay the Company, upon assignment of such
right, cash equal to the difference between the original purchase price and fair
market value, if the original purchase price is less than the fair market value
of the Shares subject to the assignment.

                  (e) RESTRICTIONS BINDING ON TRANSFEREES. All transferees of
Shares or any interest therein will receive and hold such Shares or interest
subject to the provisions of this Agreement, including, insofar as applicable,
the Repurchase Option. Any sale or transfer of the Company's Shares shall be
void unless the provisions of this Agreement are met.

                                      -4-

<PAGE>

                  (f) TERMINATION OF RIGHTS. The right of first refusal granted
the Company by Section 4(b) above and the option to repurchase the Shares in the
event of an involuntary transfer granted the Company by Section 4(c) above shall
terminate upon the first sale of Common Stock of the Company to the general
public pursuant to a registration statement filed with and declared effective by
the Securities and Exchange Commission under the Securities Act. Upon
termination of the right of first refusal described in Section 4(b) and the
expiration or exercise of the Repurchase Option, a new certificate or
certificates representing the Shares not repurchased shall be issued, on
request, without the legend referred to in Section 7(a)(ii) below and delivered
to Purchaser.

         5. ESCROW OF UNVESTED SHARES. For purposes of facilitating the
enforcement of the provisions of Section 4 above, Purchaser agrees, immediately
upon receipt of the certificate(s) for the Shares subject to the Repurchase
Option, to deliver such certificate(s), together with an Assignment Separate
from Certificate in the form attached to this Agreement as EXHIBIT C executed by
Purchaser and by Purchaser's spouse (if required for transfer), in blank, to the
Secretary of the Company, or the Secretary's designee, to hold such
certificate(s) and Assignment Separate from Certificate in escrow and to take
all such actions and to effectuate all such transfers and/or releases as are in
accordance with the terms of this Agreement. Purchaser hereby acknowledges that
the Secretary of the Company, or the Secretary's designee, is so appointed as
the escrow holder with the foregoing authorities as a material inducement to
make this Agreement and that said appointment is coupled with an interest and is
accordingly irrevocable. Purchaser agrees that said escrow holder shall not be
liable when acting in good faith to any party hereof (or to any other party).
The escrow holder may rely upon any letter, notice or other document executed by
any signature purported to be genuine and may resign at any time. Purchaser
agrees that if the Secretary of the Company, or the Secretary's designee,
resigns as escrow holder for any or no reason, the Board of Directors of the
Company shall have the power to appoint a successor to serve as escrow holder
pursuant to the terms of this Agreement.

         6. INVESTMENT AND TAXATION REPRESENTATIONS. In connection with the
purchase of the Shares, Purchaser represents to the Company the following:

                  (a) Purchaser is aware of the Company's business affairs and
financial condition and has acquired sufficient information about the Company to
reach an informed and knowledgeable decision to acquire the Shares. Purchaser is
purchasing the Shares for investment for his or her own account only and not
with a view to, or for resale in connection with, any "distribution" thereof
within the meaning of the Securities Act.

                  (b) Purchaser understands that the Shares have not been
registered under the Securities Act by reason of a specific exemption therefrom,
which exemption depends upon, among other things, the bona fide nature of
Purchaser's investment intent as expressed herein.

                  (c) Purchaser understands that the Shares are "restricted
securities" under applicable U.S. federal and state securities laws and that,
pursuant to these laws, Purchaser must hold the Shares indefinitely unless they
are registered with the Securities and Exchange Commission and qualified by
state authorities, or an exemption from such registration and

                                      -5-

<PAGE>

qualification requirements is available. Purchaser acknowledges that the Company
has no obligation to register or qualify the Shares for resale. Purchaser
further acknowledges that if an exemption from registration or qualification is
available, it may be conditioned on various requirements including, but not
limited to, the time and manner of sale, the holding period for the Shares, and
requirements relating to the Company which are outside of the Purchaser's
control, and which the Company is under no obligation and may not be able to
satisfy.

                  (d) Purchaser understands that Purchaser may suffer adverse
tax consequences as a result of Purchaser's purchase or disposition of the
Shares. Purchaser represents that Purchaser has consulted any tax consultants
Purchaser deems advisable in connection the purchase or disposition of the
Shares and that Purchaser is not relying on the Company for any tax advice.

         7.       RESTRICTIVE LEGENDS AND STOP-TRANSFER ORDERS.

                  (a) LEGENDS. The certificate or certificates representing the
Shares shall bear the following legends (as well as any legends required by
applicable state and federal corporate and securities laws):

                           (i)      THE SHARES REPRESENTED BY THIS CERTIFICATE
                                    HAVE NOT BEEN REGISTERED UNDER THE
                                    SECURITIES ACT OF 1933 OR UNDER THE
                                    SECURITIES LAW OF ANY PARTICULAR STATE, AND
                                    HAVE BEEN ACQUIRED FOR INVESTMENT AND NOT
                                    WITH A VIEW TO, OR IN CONNECTION WITH, THE
                                    SALE OR DISTRIBUTION THEREOF. NO SUCH SALE
                                    OR DISPOSITION MAY BE EFFECTED WITHOUT AN
                                    EFFECTIVE REGISTRATION STATEMENT RELATED
                                    THERETO OR AN OPINION OF COUNSEL FOR THE
                                    COMPANY THAT SUCH REGISTRATION IS NOT
                                    REQUIRED UNDER THE SECURITIES ACT OF 1933.

                           (ii)     THE SHARES REPRESENTED BY THIS CERTIFICATE
                                    MAY BE TRANSFERRED ONLY IN ACCORDANCE WITH
                                    THE TERMS OF AN AGREEMENT BETWEEN THE
                                    COMPANY AND THE STOCKHOLDER, A COPY OF WHICH
                                    IS ON FILE WITH THE SECRETARY OF THE
                                    COMPANY.

                           (iii)    Any legend required to be placed thereon by
                                    the Washington Secretary of State or other
                                    applicable state securities law.

                  (b) STOP-TRANSFER NOTICES. Purchaser agrees that, in order to
ensure compliance with the restrictions referred to herein, the Company may
issue appropriate "stop

                                      -6-

<PAGE>

transfer" instructions to its transfer agent, if any, and that, if the Company
transfers its own securities, it may make appropriate notations to the same
effect in its own records.

                  (c) REFUSAL TO TRANSFER. The Company shall not be required (i)
to transfer on its books any Shares that have been sold or otherwise transferred
in violation of any of the provisions of this Agreement or (ii) to treat as
owner of such Shares or to accord the right to vote or pay dividends to any
purchaser or other transferee to whom such Shares shall have been so
transferred.

         8. NO EMPLOYMENT RIGHTS. Nothing in this Agreement shall affect in any
manner whatsoever the right or power of the Company, or a parent or subsidiary
of the Company, to terminate Purchaser's employment, for any reason, with or
without cause.

         9. SECTION 83(b) ELECTION. Purchaser understands that Section 83(a) of
the Internal Revenue Code of 1986, as amended (the "CODE"), taxes as ordinary
income the difference between the amount paid for the Shares and the fair market
value of the Shares as of the date any restrictions on the Shares lapse. In this
context, "RESTRICTION" means the right of the Company to buy back the Shares
pursuant to the Repurchase Option set forth in Section 4(a) of this Agreement.
Purchaser understands that Purchaser may elect to be taxed at the time the
Shares are purchased, rather than when and as the Repurchase Option expires, by
filing an election under Section 83(b) (an "83(b) ELECTION") of the Code with
the Internal Revenue Service within 30 days from the date of purchase. Even if
the fair market value of the Shares at the time of the execution of this
Agreement equals the amount paid for the Shares, the election must be made to
avoid income under Section 83(a) in the future. Purchaser understands that
failure to file such an election in a timely manner may result in adverse tax
consequences for Purchaser. Purchaser further understands that an additional
copy of such election form should be filed with his or her federal income tax
return for the calendar year in which the date of this Agreement falls.
Purchaser acknowledges that the foregoing is only a summary of the effect of
United States federal income taxation with respect to purchase of the Shares
hereunder, and does not purport to be complete. Purchaser further acknowledges
that the Company has directed Purchaser to seek independent advice regarding the
applicable provisions of the Code, the income tax laws of any municipality,
state or foreign country in which Purchaser may reside, and the tax consequences
of Purchaser's death.

                  Purchaser agrees that he will execute and deliver to the
Company with this executed Agreement a copy of the Acknowledgment and Statement
of Decision Regarding Section 83(b) Election (the "ACKNOWLEDGMENT"), attached
hereto as EXHIBIT D. Purchaser further agrees that Purchaser will execute and
submit with the Acknowledgment a copy of the 83(b) Election, attached hereto as
EXHIBIT E, if Purchaser has indicated in the Acknowledgment his or her decision
to make such an election.

                                      -7-

<PAGE>

         10. MARKET STANDOFF AGREEMENT. In connection with the initial public
offering of the Company's securities and upon request of the Company or the
underwriters managing any underwritten offering of the Company's securities,
Purchaser agrees not to sell, make any short sale of, loan, grant any option for
the purchase of, or otherwise dispose of any Shares (other than those included
in the registration) without the prior written consent of the Company or such
underwriters, as the case may be, for such period of time (not to exceed 180
days) from the effective date of such registration as may be requested by the
Company or such managing underwriters and to execute an agreement reflecting the
foregoing as may be requested by the underwriters at the time of the public
offering.

         11. REGISTRATION RIGHTS. The Company will use best efforts to ensure
that Purchaser shall be entitled to the same or substantially similar piggy-back
registration rights that the Company grants to investors in the Company.

         12.      MISCELLANEOUS.

                  (a) GOVERNING LAW. This Agreement and all acts and
transactions pursuant hereto and the rights and obligations of the parties
hereto shall be governed, construed and interpreted in accordance with the laws
of the State of Washington, without giving effect to principles of conflicts of
law.

                  (b) ENTIRE AGREEMENT; ENFORCEMENT OF RIGHTS. This Agreement
sets forth the entire agreement and understanding of the parties relating to the
subject matter herein and merges all prior discussions between them. No
modification of or amendment to this Agreement, nor any waiver of any rights
under this Agreement, shall be effective unless in writing signed by the parties
to this Agreement. The failure by either party to enforce any rights under this
Agreement shall not be construed as a waiver of any rights of such party.

                  (c) SEVERABILITY. If one or more provisions of this Agreement
are held to be unenforceable under applicable law, the parties agree to
renegotiate such provision in good faith. In the event that the parties cannot
reach a mutually agreeable and enforceable replacement for such provision, then
(i) such provision shall be excluded from this Agreement, (ii) the balance of
the Agreement shall be interpreted as if such provision were so excluded and
(iii) the balance of the Agreement shall be enforceable in accordance with its
terms.

                  (d) CONSTRUCTION. This Agreement is the result of negotiations
between and has been reviewed by each of the parties hereto and their respective
counsel, if any; accordingly, this Agreement shall be deemed to be the product
of all of the parties hereto, and no ambiguity shall be construed in favor of or
against any one of the parties hereto.

                  (e) NOTICES. Any notice required or permitted by this
Agreement shall be in writing and shall be deemed sufficient when delivered
personally or sent by telegram or fax or forty-eight (48) hours after being
deposited in the U.S. mail, as certified or registered mail, with postage
prepaid, and addressed to the party to be notified at such party's address as
set forth below or as subsequently modified by written notice.

                                      -8-

<PAGE>

                  (f) COUNTERPARTS. This Agreement may be executed in two or
more counterparts, each of which shall be deemed an original and all of which
together shall constitute one instrument.

                  (g) SUCCESSORS AND ASSIGNS. The rights and benefits of this
Agreement shall inure to the benefit of, and be enforceable by the Company's
successors and assigns. The rights and obligations of Purchaser under this
Agreement may only be assigned with the prior written consent of the Company.

                            [Signature Page Follows]


                                      -9-

<PAGE>

         The parties have executed this Agreement as of the date first set forth
above.

                                        ROSETTA BIOSYSTEMS, INC.

                                        By: /s/ Stephen H. Friend
                                           -----------------------------------


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


                                        Address:
                                        c/o Venture Law Group
                                        4750 Carillon Point
                                        Kirkland, WA  98033

         PURCHASER ACKNOWLEDGES AND AGREES THAT THE VESTING OF SHARES PURSUANT
TO SECTION 4 HEREOF IS EARNED ONLY BY CONTINUING SERVICE AS AN EMPLOYEE OR
CONSULTANT AT THE WILL OF THE COMPANY OR BY THE PROVISIONS OF SECTION 4(A)(IV)
AND SECTION 4(A)(V). PURCHASER FURTHER ACKNOWLEDGES AND AGREES THAT NOTHING IN
THIS AGREEMENT SHALL CONFER UPON PURCHASER ANY RIGHT WITH RESPECT TO
CONTINUATION OF SUCH EMPLOYMENT OR CONSULTING RELATIONSHIP WITH THE COMPANY, NOR
SHALL IT INTERFERE IN ANY WAY WITH PURCHASER'S RIGHT OR THE COMPANY'S RIGHT TO
TERMINATE PURCHASER'S EMPLOYMENT OR CONSULTING RELATIONSHIP AT ANY TIME, WITH OR
WITHOUT CAUSE.

                                        PURCHASER:

                                        JOHN KING

                                        /s/ John J. King
                                        --------------------------------------
                                        (Signature)

                                        Address:

                                        --------------------------------------
                                        --------------------------------------

Vesting Commencement
Date:  April 15, 1997

I, __________, spouse of John King, have read and hereby approve the foregoing
Agreement. In consideration of the Company's granting my spouse the right to
purchase the Shares as set forth in the Agreement, I hereby agree to be
irrevocably bound by the Agreement and further agree that any community property
or other such interest shall be similarly bound by the Agreement. I hereby
appoint my spouse as my attorney-in-fact with respect to any amendment or
exercise of any rights under the Agreement.

                                        ______________________________________

                                      -10-

<PAGE>

                                    EXHIBIT A

                      ASSIGNMENT SEPARATE FROM CERTIFICATE

         FOR VALUE RECEIVED and pursuant to that certain Common Stock Purchase
Agreement between the undersigned ("PURCHASER") and Rosetta Biosystems, Inc.
(the "COMPANY") dated May __, 1997 (the "AGREEMENT"), Purchaser hereby sells,
assigns and transfers unto the Company _________________________________
(________) shares of the Common Stock of the Company standing in Purchaser's
name on the Company's books and represented by Certificate No. ______, and does
hereby irrevocably constitute and appoint _____________________________ to
transfer said stock on the books of the Company with full power of substitution
in the premises. THIS ASSIGNMENT MAY ONLY BE USED AS AUTHORIZED BY THE AGREEMENT
AND THE EXHIBITS THERETO.

Dated: ______________________

                                           Signature:

                                           /s/ John J. King
                                           ------------------------------
                                           John King

                                           /s/ Pamela S. King
                                           ------------------------------
                                           Spouse of John King (if applicable)

Instruction: Please do not fill in any blanks other than the signature line. The
purpose of this assignment is to enable the Company to exercise its repurchase
option set forth in the Agreement without requiring additional signatures on the
part of Purchaser.


                                      -11-

<PAGE>

                                    EXHIBIT B

                    ACKNOWLEDGMENT AND STATEMENT OF DECISION
                        REGARDING SECTION 83(b) ELECTION

         The undersigned has entered a stock purchase agreement with Rosetta
Biosystems, Inc., a Delaware corporation (the "COMPANY"), pursuant to which the
undersigned is purchasing 102,450 shares of Common Stock of the Company (the
"SHARES"). In connection with the purchase of the Shares, the undersigned hereby
represents as follows:

         1.       The undersigned has carefully reviewed the stock purchase
agreement pursuant to which the undersigned is purchasing the Shares.

         2.       The undersigned either [check and complete as applicable]:

         (a) ____ has consulted, and has been fully advised by, the
                  undersigned's own tax advisor, __________________________,
                  whose business address is _____________________________,
                  regarding the federal, state and local tax consequences of
                  purchasing the Shares, and particularly regarding the
                  advisability of making elections pursuant to Section 83(b) of
                  the Internal Revenue Code of 1986, as amended (the "CODE")
                  and pursuant to the corresponding provisions, if any, of
                  applicable state law; or

         (b) ____ has knowingly chosen not to consult such a tax advisor.

         3.       The undersigned hereby states that the undersigned has
decided [check as applicable]:

         (a) ____ to make an election pursuant to Section 83(b) of the
                  Code, and is submitting to the Company, together with the
                  undersigned's executed Common Stock Purchase Agreement, an
                  executed form entitled "Election Under Section 83(b) of the
                  Internal Revenue Code of 1986;" or

         (b) ____ not to make an election pursuant to Section 83(b) of the Code.

<PAGE>

         4._______Neither the Company nor any subsidiary or representative of
the Company has made any warranty or representation to the undersigned with
respect to the tax consequences of the undersigned's purchase of the Shares or
of the making or failure to make an election pursuant to Section 83(b) of the
Code or the corresponding provisions, if any, of applicable state law.


Date:       5/15/97                               /s/ John J. King
      ------------------------                    --------------------------
                                                  John King

Date:
      ------------------------                    --------------------------
                                                  Spouse of John King

<PAGE>

                                    EXHIBIT E

                          ELECTION UNDER SECTION 83(b)
                      OF THE INTERNAL REVENUE CODE OF 1986

         The undersigned taxpayer hereby elects, pursuant to Section 83(b) of
the Internal Revenue Code, to include in taxpayer's gross income for the current
taxable year, the amount of any compensation taxable to taxpayer in connection
with taxpayer's receipt of the property described below:

1.       The name, address, taxpayer identification number and taxable year of
         the undersigned are as follows:

         NAME OF TAXPAYER:  John King

         NAME OF SPOUSE:  _______________________

         ADDRESS:

         IDENTIFICATION NO. OF TAXPAYER:

         IDENTIFICATION NO. OF SPOUSE:

         TAXABLE YEAR:  1997

2.       The property with respect to which the election is made is described as
         follows:

         164,415 shares of the Common Stock (the "Shares"), $.001 par value, of
         Rosetta Biosystems, Inc., a Delaware corporation (the "Company").

3.       The date on which the property was transferred is: May __, 1997

4.       The property is subject to the following restrictions:

         Repurchase option at cost in favor of the Company upon termination of
         taxpayer's employment or consulting relationship.

5.       The fair market value at the time of transfer, determined without
         regard to any restriction other than a restriction which by its terms
         will never lapse, of such property is: $1,644.15

6.       The amount (if any) paid for such property: $1,644.15

The undersigned has submitted a copy of this statement to the person for whom
the services were performed in connection with the undersigned's receipt of the
above-described property. The transferee of such property is the person
performing the services in connection with the transfer of said property.

THE UNDERSIGNED UNDERSTANDS THAT THE FOREGOING ELECTION MAY NOT BE REVOKED
EXCEPT WITH THE CONSENT OF THE COMMISSIONER.

Dated:                                            /s/ John J. King
      ------------------------                    --------------------------
                                                  Taxpayer

Dated:                                            /s/ Pamela S. King
      ------------------------                    --------------------------
                                                  Spouse of Taxpayer

<PAGE>

                                     RECEIPT

         Rosetta Biosystems, Inc. hereby acknowledges receipt of a check in the
amount of $1,644.15 given by John King as consideration for Certificate No. __
for 164,415 shares of Common Stock of Rosetta Biosystems, Inc.

Dated:  ________________

                                         Rosetta Biosystems, Inc.

                                         By: /s/ Stephen H. Friend
                                             ---------------------------------

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

<PAGE>

                               RECEIPT AND CONSENT

         The undersigned hereby acknowledges receipt of a photocopy of
Certificate No. ___ for 164,415 shares of Common Stock of Rosetta Biosystems,
Inc. (the "COMPANY").

         The undersigned further acknowledges that the Secretary of the Company,
or his or her designee, is acting as escrow holder pursuant to the Common Stock
Purchase Agreement Purchaser has previously entered into with the Company. As
escrow holder, the Secretary of the Company, or his or her designee, holds the
original of the aforementioned certificate issued in the undersigned's name.

Dated:  _________________________


                                               /s/ John J. King
                                               -------------------------------
                                               John King