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                                                                EXHIBIT 10.12(a)

                           INVESTOR'S RIGHTS AGREEMENT

                           INVESTORS' RIGHTS AGREEMENT

        THIS INVESTORS' RIGHTS AGREEMENT is made as of the ____ day of November,
2000 (the "Effective Date"), by and among Toxco, a ___________ corporation (the
"Company"), CombiMatrix Corporation, a Delaware corporation ("CombiMatrix") and
Acacia Research Corporation, a Delaware corporation ("Acacia"; together with
CombiMatrix, the "Investors" each of which is herein referred to as an
"Investor".)

                                    RECITALS

        WHEREAS, in order to induce the Company to approve the issuance of the
Common Stock and to induce the Investors to invest funds in the Company, the
Investors and the Company hereby agree that this Agreement shall govern the
rights of the Investors to cause the Company to register shares of Common Stock
issued or issuable to them and certain other matters as set forth herein;

        NOW, THEREFORE, the parties hereby agree as follows:

1.      Registration Rights. The Company covenants and agrees as follows:

        1.1    Definitions. For purposes of this Section 1:

               (a)    The term "Act" means the Securities Act of 1933, as
                      amended.

               (b)    The term "Common Stock" means the Common Stock of the
                      Company, par value $0.001 per share.

               (c)    The term "Common Stock Equivalents" means and includes all
                      shares of Common Stock issued and outstanding at the
                      relevant time plus (i) all shares of Common Stock that may
                      be issued upon exercise of any options, warrants and other
                      rights of any kind that are then exercisable, and (ii) all
                      shares of Common Stock that may be issued upon conversion
                      or exchange of any convertible securities which are by
                      their terms then convertible into or exchangeable for
                      Common Stock, including without limitation, preferred
                      stock.

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               (d)    The term "Form S-3" means such form under the Act as in
                      effect on the date hereof or any registration form under
                      the Act subsequently adopted by the SEC that permits
                      inclusion or incorporation of substantial information by
                      reference to other documents filed by the Company with the
                      SEC.

               (e)    The term "Holder" means any person owning or having the
                      right to acquire Registrable Securities or any assignee
                      thereof in accordance with Section 1.11 hereof.

               (f)    The term "1934 Act" means the Securities Exchange Act of
                      1934, as amended.

               (g)    The term "register," "registered," and "registration"
                      refer to a registration effected by preparing and filing a
                      registration statement or similar document in compliance
                      with the Act, and the declaration or ordering of
                      effectiveness of such registration statement or document.

               (h)    "Qualified IPO" means the closing of a registered
                      underwritten public offering of shares of Common Stock of
                      the Company.

               (i)    The term "Registrable Securities" means (i) any Common
                      Stock or Common Stock Equivalents owned by Combimatrix and
                      Acacia and (ii) any Common Stock or Common Stock
                      Equivalents of the Company issued as (or issuable upon the
                      conversion or exercise of any warrant, right or other
                      security that is issued as) a dividend or other
                      distribution with respect to, or in exchange for, or in
                      replacement of, the shares referenced in (i) above,
                      excluding in all cases, however, any Registrable
                      Securities sold by a person in a transaction in which his
                      rights under this Section 1 are not assigned.

               (j)    The number of shares of "Registrable Securities"
                      outstanding shall be determined by the number of shares of
                      Common Stock outstanding that are, and the number of
                      shares of Common Stock issuable pursuant to then
                      exercisable or convertible Common Stock Equivalents that
                      are, Registrable Securities.

               (k)    The term "SEC" shall mean the Securities and Exchange
                      Commission.

        1.2 Request for Registration.

               (a)    Subject to the conditions of this Section 1.2, if the
                      Company shall receive at any time after the earlier of (i)
                      three (3) years after the date of this Agreement or (ii)
                      six (6) months after the effective date

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                      of the Qualified IPO, a written request from either or
                      both of CombiMatrix and Acacia (the "Initiating Holders")
                      that the Company file a registration statement under the
                      Act covering the registration of Registrable Securities
                      with an anticipated aggregate offering price of at least
                      $7,500,000, then the Company shall, within twenty (20)
                      days of the receipt thereof, give written notice of such
                      request to all Holders, and subject to the limitations of
                      this Section 1.2, use all reasonable efforts to effect, as
                      soon as practicable, the registration under the Act of all
                      Registrable Securities that the Holders request to be
                      registered in a written request received by the Company
                      within twenty (20) days of the mailing of the Company's
                      notice pursuant to this Section 1.2(a).

               (b)    If the Initiating Holders intend to distribute the
                      Registrable Securities covered by their request by means
                      of an underwriting, they shall so advise the Company as a
                      part of their request made pursuant to this Section 1.2
                      and the Company shall include such information in the
                      written notice referred to in Section 1.2(a). In such
                      event the right of any Holder to include its Registrable
                      Securities in such registration shall be conditioned upon
                      such Holder's participation in such underwriting and the
                      inclusion of such Holder's Registrable Securities in the
                      underwriting (unless otherwise mutually agreed by a
                      majority in interest of the Initiating Holders and such
                      Holder) to the extent provided herein. All Holders
                      proposing to distribute their securities through such
                      underwriting shall enter into an underwriting agreement in
                      customary form with the underwriter or underwriters
                      selected for such underwriting by the Company (which
                      underwriter or underwriters shall be reasonably acceptable
                      to CombiMatrix and Acacia). Notwithstanding any other
                      provision of this Section 1.2, if the underwriter advises
                      the Company that marketing factors require a limitation of
                      the number of securities underwritten (including
                      Registrable Securities), then the Company shall so advise
                      all Holders of Registrable Securities that would otherwise
                      be underwritten pursuant hereto, and the number of shares
                      that may be included in the underwriting shall be
                      allocated to the Holders of such Registrable Securities on
                      a pro rata basis based on the number of Registrable
                      Securities held by all such Holders (including the
                      Initiating Holders); provided, however, that the number of
                      shares of Registrable Securities to be included in such
                      underwriting shall not be reduced unless all other
                      securities proposed to be included in such underwriting
                      are first excluded from the underwriting. To facilitate
                      the allocation of shares in accordance with these
                      promises, the Company may round the number of shares
                      allocated to any holder to the nearest 100 shares. Any
                      Registrable Securities

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                      excluded or withdrawn from such underwriting shall be
                      withdrawn from the registration.

               (c)    The Company shall not be required to effect a registration
                      pursuant to this Section 1.2:

                      (i)    after the Company has effected three (3)
                             registrations pursuant to this Section 1.2, and
                             such registration has been declared or ordered
                             effective excluding any registration in which the
                             number of Registrable Securities sold by the
                             Initiating Holders is reduced pursuant to
                             Section1.2(b) to less than 60% of the number of
                             Registrable Securities originally requested by
                             initiating Holders to be registered in such
                             offering; or

                      (ii)   during the period starting with the date sixty (60)
                             days prior to the Company's good faith estimate of
                             the date of the filing of, and ending on a date one
                             hundred eighty (180) days following the effective
                             date of, a Company-initiated registration subject
                             to Section 1.3, provided that the Company is
                             actively employing in good faith all reasonable
                             efforts to cause such registration statement to
                             become effective; or

                      (iii)  if the Initiating Holders propose to dispose of
                             Registrable Securities that may be registered on
                             Form S-3 pursuant to Section 1.4 hereof; or

                      (iv)   if the Company shall furnish to Holders requesting
                             a registration statement pursuant to this Section
                             1.2, a certificate signed by the Company's Chief
                             Executive Officer or Chairman of the Board stating
                             that in the good faith judgment of the Board of
                             Directors of the Company, it would be detrimental
                             to the Company and its stockholders for such
                             registration statement to be effected at such time,
                             in which event the Company shall have the right to
                             defer such filing for a period of not more than one
                             hundred twenty (120) days after receipt of the
                             request of the Initiating Holders, provided that
                             such right to delay a request shall be exercised by
                             the Company not more than once in any twelve
                             (12)-month period.

        1.3 Company Registration.

               (a)    Notice of Registration. If (but without any obligation to
                      do so) the Company proposes to register (including for
                      this purpose a

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                      registration effected by the Company for stockholders
                      other than the Holders) any of its stock or other
                      securities under the Act in connection with the public
                      offering of such securities (other than a registration
                      relating solely to the sale of securities to participants
                      in a Company stock or other employee benefit plan, a
                      registration relating to a corporate reorganization or
                      other transaction under Rule 145 of the Act, a
                      registration on any form that does not include
                      substantially the same information as would be required to
                      be included in a registration statement covering the sale
                      of the Registrable Securities, or a registration in which
                      the only Common Stock being registered is Common Stock
                      issuable upon conversion of debt securities that are also
                      being registered), the Company shall, at such time,
                      promptly give each Holder written notice of such
                      registration. Upon the written request of each Holder
                      given within twenty (20) days after mailing of such notice
                      by the Company in accordance with Section 3.5, the Company
                      shall, subject to the provisions of Section 1.3(c), use
                      all reasonable efforts to cause to be registered under the
                      Act all of the Registrable Securities that each such
                      Holder has requested to be registered.

               (b)    Right to Terminate Registration. The Company shall have
                      the right to terminate or withdraw any registration
                      initiated by it under this Section 1.3 prior to the
                      effectiveness of such registration whether or not any
                      Holder has elected to include securities in such
                      registration. The expenses of such withdrawn registration
                      shall be borne by the Company in accordance with Section
                      1.7 hereof.

               (c)    Underwriting Requirements. In connection with any offering
                      involving an underwriting of shares of the Company's
                      capital stock, the Company shall not be required under
                      this Section 1.3 to include any of the Holders' securities
                      in such underwriting unless they accept the terms of the
                      underwriting as agreed upon between the Company and the
                      underwriters selected by it (or by other persons entitled
                      to select the underwriters) and enter into an underwriting
                      agreement in customary form with an underwriter or
                      underwriters selected by the Company, and then only in
                      such quantity as the underwriters determine in their sole
                      discretion will not jeopardize the success of the offering
                      by the Company. If the total amount of securities,
                      including Registrable Securities, requested by
                      stockholders to be included in such offering exceeds the
                      amount of securities sold other than by the Company that
                      the underwriters determine in their sole discretion is
                      compatible with the success of the offering, then the
                      Company shall be required to include in the offering only
                      that number of such securities, including Registrable
                      Securities, that the underwriters determine in

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                      their sole discretion will not jeopardize the success of
                      the offering (the securities so included to be first on
                      behalf of the Company and then to be apportioned pro rata
                      among the selling stockholders (including the Holders)
                      according to the total amount of securities proposed to be
                      included therein by each selling stockholder or in such
                      other proportions as shall mutually be agreed to by such
                      selling stockholders), and if advised by the underwriters,
                      the selling Holders may be excluded entirely if no other
                      stockholder's securities are included. To facilitate the
                      allocation of shares in accordance with these promises,
                      the Company may round the number of shares allocated to
                      any stockholder to the nearest 100 shares. For purposes of
                      the preceding parenthetical concerning apportionment, for
                      any selling stockholder that is a Holder of Registrable
                      Securities and that is a partnership or corporation, the
                      partners, retired partners and stockholders of such
                      Holder, or the estates and family members of any such
                      partners and retired partners and any trusts for the
                      benefit of any of the foregoing persons shall be deemed to
                      be a single "selling Holder," and any pro rata reduction
                      with respect to such "selling Holder" shall be based upon
                      the aggregate amount of Registrable Securities owned by
                      all such related entities and individuals.

        1.4    Form S-3 Registration. In case the Company shall receive from the
               Holders of at least ten percent (10%) of the Registrable
               Securities a written request or requests that the Company effect
               a registration on Form S-3 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 shall:

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

               (b)    use all reasonable efforts to effect, as soon as
                      practicable, 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 Holders' Registrable Securities as are
                      specified in such request, together with all or such
                      portion of the Registrable Securities of any other 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 1.4:

                      (i)    if Form S-3 is not available for such offering by
                             the Holders;

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                      (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 (net
                             of any underwriters' discounts or commissions) of
                             less than $7,500,000;

                      (iii)  if the Company shall furnish to the Holders a
                             certificate signed by the Chief Executive Officer
                             or Chairman of the Board of the Company stating
                             that in the good faith judgment of the Board of
                             Directors of the Company, it would be 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 one hundred
                             twenty (120) days after receipt of the request of
                             the Holder or Holders under this Section 1.4;
                             provided, however, that the Company shall not
                             utilize this right more than twice in any twelve
                             month period;

                      (iv)   if the Company has, within the twelve (12) month
                             period preceding the date of such request, already
                             effected two registrations on Form S-3 for the
                             Holders pursuant to this Section 1.4; or

                      (v)    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.

               (c)    Subject to the foregoing, the Company shall file a
                      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 1.4 shall not be counted as requests for
                      registration effected pursuant to Sections 1.2.

        1.5    Obligations of the Company. Whenever required under this Section
               1 to effect the registration of any Registrable Securities, the
               Company shall, as expeditiously as reasonably possible:

               (a)    prepare and file with the SEC a registration statement
                      with respect to such Registrable Securities and use all
                      reasonable efforts to cause such registration statement to
                      become effective, and, upon the request of the Holders of
                      a majority of the Registrable Securities

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                      registered thereunder, keep such registration statement
                      effective for a period of up to one hundred twenty (120)
                      days or, if earlier, until the distribution contemplated
                      in the Registration Statement has been completed;

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

               (c)    furnish to the Holders such numbers of copies of a
                      prospectus, including a preliminary prospectus, in
                      conformity with the requirements of the Act, and such
                      other documents as they may reasonably request in order to
                      facilitate the disposition of Registrable Securities owned
                      by them;

               (d)    use all reasonable efforts to register and qualify the
                      securities covered by such registration statement under
                      such other securities or Blue Sky laws of such
                      jurisdictions as shall be reasonably requested by the
                      Holders, provided that the Company shall not be required
                      in connection therewith or as a condition thereto to
                      qualify to do business or to file a general consent to
                      service of process in any such states or jurisdictions;

               (e)    in the event of any underwritten public offering, enter
                      into and perform its obligations under an underwriting
                      agreement, in usual and customary form, with the managing
                      underwriter of such offering;

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

               (g)    cause all such Registrable Securities registered pursuant
                      hereunder to be listed on each securities exchange on
                      which similar securities issued by the Company are then
                      listed;

               (h)    provide a transfer agent and registrar for all Registrable
                      Securities registered pursuant hereunder and a CUSIP
                      number for all such

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                      Registrable Securities, in each case not later than the
                      effective date of such registration;

               (i)    furnish, if required by the underwriters or pursuant to an
                      underwriting agreement, on the date that such Registrable
                      Securities are delivered to the underwriters for sale in
                      connection with a registration pursuant to this Section 1,
                      if such securities are being sold through underwriters,
                      (i) an opinion, dated such date, of the counsel
                      representing the Company for the purposes of such
                      registration, in form and substance as is customarily
                      given to underwriters in an underwritten public offering,
                      addressed to the underwriters and (ii) a letter dated such
                      date, from the independent certified public accountants of
                      the Company, in form and substance as is customarily given
                      by independent certified public accountants to
                      underwriters in an underwritten public offering, addressed
                      to the underwriters; and

               (j)    if required by the underwriters or pursuant to an
                      underwriting agreement, the Company shall make itself
                      available for road shows and due diligence in such a
                      manner as is customarily given to underwriters in an
                      underwritten public offering.

        1.6    Information From Holder. It shall be a condition precedent to the
               obligations of the Company to take any action pursuant to this
               Section 1 with respect to the Registrable Securities of any
               selling Holder that such Holder shall furnish to the Company such
               information regarding itself, the Registrable Securities held by
               it, and the intended method of disposition of such securities as
               shall be required to effect the registration of such Holder's
               Registrable Securities.

        1.7    Expenses of Registration. All expenses (other than any selling
               Holder's pro rata share of underwriting discounts, commissions,
               out-of-pocket expenses of selling Holder or Holders and fees and
               costs of counsel for the selling Holder or Holders) incurred in
               connection with registrations, filings or qualifications pursuant
               to Sections 1.2, 1.3 and 1,4, including (without limitation) all
               registration, filing and qualification fees, printers' and
               accounting fees, fees and disbursements of counsel for the
               Company shall be borne by the Company. Notwithstanding the
               foregoing, the Company shall not be required to pay for any
               expenses of any registration proceeding begun pursuant to Section
               1.2 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 pro rata based upon the number
               of Registrable Securities that were to be requested in the
               withdrawn registration), unless, in the case of a registration
               requested under Section 1.2, the Holders of a majority of the
               Registrable Securities agree to forfeit

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               their right to one demand registration pursuant to Section 1.2,
               provided, however, that if at the time of such withdrawal, the
               Holders have learned of a material adverse change in the
               condition, business, or prospects of the Company from that known
               to the Holders at the time of their request and have withdrawn
               the request with reasonable promptness following disclosure by
               the Company of such material adverse change, then the Holders
               shall not be required to pay any of such expenses and shall
               retain their rights pursuant to Section 1.2.

        1.8    Delay of Registration. No Holder shall have any right to obtain
               or seek an injunction restraining or otherwise delaying any such
               registration as the result of any controversy that might arise
               with respect to the interpretation or implementation of this
               Section 1.

        1.9    Indemnification. In the event any Registrable Securities are
               included in a registration statement under this Section 1:

               (a)    To the extent permitted by law, the Company will indemnify
                      and hold harmless each Holder, the partners or officers,
                      directors and stockholders of each Holder, legal counsel
                      and accountants for each Holder, any underwriter (as
                      defined in the Act) for such Holder and each person, if
                      any, who controls such Holder or underwriter within the
                      meaning of the Act or the 1934 Act, against any losses,
                      claims, damages or liabilities (joint or several) to which
                      they may become subject under the Act, the 1934 Act or any
                      state securities laws, insofar as such losses, claims,
                      damages, or liabilities (or actions in respect thereof)
                      arise out of or are based upon any of the following
                      statements, omissions or violations (collectively a
                      "Violation"): (i) any untrue statement or alleged untrue
                      statement of a material fact contained in such
                      registration statement, including any preliminary
                      prospectus or final prospectus contained therein or any
                      amendments or supplements thereto, (ii) the omission or
                      alleged omission to state therein a material fact required
                      to be stated therein, or necessary to make the statements
                      therein not misleading, or (iii) any violation or alleged
                      violation by the Company of the Act, the 1934 Act, any
                      state securities laws or any rule or regulation
                      promulgated under the Act, the 1934 Act or any state
                      securities laws; and the Company will reimburse each such
                      Holder, underwriter or controlling person for any legal or
                      other expenses reasonably incurred by them in connection
                      with investigating or defending any such loss, claim,
                      damage, liability or action; provided, however, that the
                      indemnity agreement contained in this subsection l.9(a)
                      shall not apply to amounts paid in settlement of any such
                      loss, claim, damage, liability or action if such
                      settlement is effected without the consent of the Company
                      (which consent shall not be unreasonably withheld), nor
                      shall the

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                      Company be liable in any such case for any such loss,
                      claim, damage, liability or action to the extent that it
                      arises out of or is based upon a Violation that occurs in
                      reliance upon and in conformity with written information
                      furnished expressly for use in connection with such
                      registration by any such Holder, underwriter or
                      controlling person; provided further, however, that the
                      foregoing indemnity agreement with respect to any
                      preliminary prospectus shall not inure to the benefit of
                      any Holder or underwriter, or any person controlling such
                      Holder or underwriter, from whom the person asserting any
                      such losses, claims, damages or liabilities purchased
                      shares in the offering, if a copy of the prospectus (as
                      then amended or supplemented if the Company shall have
                      furnished any amendments or supplements thereto) was not
                      sent or given by or on behalf of such Holder or
                      underwriter to such person, if required by law so to have
                      been delivered, at or prior to the written confirmation of
                      the sale of the shares to such person, and if the
                      prospectus (as so amended or supplemented) would have
                      cured the defect giving rise to such loss, claim, damage
                      or liability.

               (b)    To the extent permitted by law, each selling Holder will
                      indemnify and hold harmless the Company, each of its
                      directors, each of its officers who has signed the
                      registration statement, each person, if any, who controls
                      the Company within the meaning of the Act, legal counsel
                      and accountants for the Company, any underwriter, any
                      other Holder selling securities in such registration
                      statement and any controlling person of any such
                      underwriter or other Holder, against any losses, claims,
                      damages or liabilities (joint or several) to which any of
                      the foregoing persons may become subject, under the Act,
                      the 1934 Act or any state securities laws, insofar as such
                      losses, claims, damages or liabilities (or actions in
                      respect thereto) arise out of or are based upon any
                      Violation, in each case to the extent (and only to the
                      extent) that such Violation occurs in reliance upon and in
                      conformity with written information furnished by such
                      Holder expressly for use in connection with such
                      registration; and each such Holder will reimburse any
                      person intended to be indemnified pursuant to this
                      subsection l.9(b), for any legal or other expenses
                      reasonably incurred by such person in connection with
                      investigating or defending any such loss, claim, damage,
                      liability or action; provided, however, that the indemnity
                      agreement contained in this subsection l.9(b) shall not
                      apply to amounts paid in settlement of any such loss,
                      claim, damage, liability or action if such settlement is
                      effected without the consent of the Holder (which consent
                      shall not be unreasonably withheld), provided that in no
                      event shall any indemnity under this subsection

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                      l.9(b) exceed the gross proceeds from the offering
                      received by such Holder.

               (c)    Promptly after receipt by an indemnified party under this
                      Section 1.9 of notice of the commencement of any action
                      (including any governmental action), such indemnified
                      party will, if a claim in respect thereof is to be made
                      against any indemnifying party under this Section 1.9,
                      deliver to the indemnifying party a written notice of the
                      commencement thereof and the indemnifying party shall have
                      the right to participate in, and, to the extent the
                      indemnifying party so desires, jointly with any other
                      indemnifying party similarly noticed, to assume the
                      defense thereof with counsel mutually satisfactory to the
                      parties; provided, however, that an indemnified party
                      (together with all other indemnified parties that may be
                      represented without conflict by one counsel) shall have
                      the right to retain one separate counsel, with the fees
                      and expenses to be paid by the indemnifying party, if
                      representation of such indemnified party by the counsel
                      retained by the indemnifying party would be inappropriate
                      due to actual or potential differing interests between
                      such indemnified party and any other party represented by
                      such counsel in such proceeding. The failure to deliver
                      written notice to the indemnifying party within a
                      reasonable time of the commencement of any such action, if
                      prejudicial to its ability to defend such action, shall
                      relieve such indemnifying party of any liability to the
                      indemnified party under this Section 1.9, but the omission
                      so to deliver written notice to the indemnifying party
                      will not relieve it of any liability that it may have to
                      any indemnified party otherwise than under this Section
                      1.9.

               (d)    If the indemnification provided for in this Section 1.9 is
                      held by a court of competent jurisdiction to be
                      unavailable to an indemnified party with respect to any
                      loss, liability, claim, damage or expense referred to
                      herein, then the indemnifying party, in lieu of
                      indemnifying such indemnified party hereunder, shall
                      contribute to the amount paid or payable by such
                      indemnified party as a result of such loss, liability,
                      claim, damage or expense in such proportion as is
                      appropriate to reflect the relative fault of the
                      indemnifying party on the one hand and of the indemnified
                      party on the other in connection with the statements or
                      omissions that resulted in such loss, liability, claim,
                      damage or expense, as well as any other relevant equitable
                      considerations. The relative fault of the indemnifying
                      party and of the indemnified party shall be determined by
                      reference to, among other things, whether the untrue or
                      alleged untrue statement of a material fact or the
                      omission to

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                      state a material fact relates to information supplied by
                      the indemnifying party or by the indemnified party and the
                      parties' relative intent, knowledge, access to
                      information, and opportunity to correct or prevent such
                      statement or omission.

               (e)    Notwithstanding the foregoing, to the extent that the
                      provisions on indemnification and contribution contained
                      in the underwriting agreement entered into in connection
                      with the underwritten public offering are in conflict with
                      the foregoing provisions, the provisions in the
                      underwriting agreement shall control.

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

        1.10   Reports Under Securities Exchange Act of 1934. With a view to
               making available to the Holders the benefits of Rule 144
               promulgated under the Act and any other rule or regulation of the
               SEC that may at any time permit a Holder to sell securities of
               the Company to the public without registration or pursuant to a
               registration on Form S-3, the Company agrees to:

               (a)    make and keep public information available, as those terms
                      are understood and defined in SEC Rule 144, at all times
                      after ninety (90) days after the effective date of the
                      Qualified IPO;

               (b)    file with the SEC in a timely manner all reports and other
                      documents required of the Company under the Act and the
                      1934 Act; and

               (c)    furnish to any Holder, so long as the Holder owns any
                      Registrable Securities, forthwith upon request (i) a
                      written statement by the Company that it has complied with
                      the reporting requirements of SEC Rule 144 (at any time
                      after ninety (90) days after the effective date of the
                      first registration statement filed by the Company), the
                      Act and the 1934 Act (at any time after it has become
                      subject to such reporting requirements), or that it
                      qualifies as a registrant whose securities may be resold
                      pursuant to Form S-3 (at any time after it so qualifies),
                      (ii) a copy of the most recent annual or quarterly report
                      of the Company and such other reports and documents so
                      filed by the Company, and (iii) such other information as
                      may be reasonably requested in availing any Holder of any
                      rule or regulation of the SEC that permits the selling of
                      any such securities without registration or pursuant to
                      such form.

                                       13
<PAGE>   14

        1.11   Assignment of Registration Rights. The rights to cause the
               Company to register Registrable Securities pursuant to this
               Section 1 may be assigned (but only with all related obligations)
               by a Holder to a transferee or assignee of such securities,
               provided: (a) the transferee acquires 100% of the Registrable
               Securities owned or held by the transferring Holder on the day of
               transfer; (b) the Transferee is not eligible to sell the shares
               pursuant to any provisions of Rule 144(k) of the Act; (c) the
               Company is, within a reasonable time after such transfer,
               furnished with written notice of the name and address of such
               transferee or assignee and the securities with respect to which
               such registration rights are being assigned; (d) such transferee
               or assignee agrees in writing to be bound by and subject to the
               terms and conditions of this Agreement, including without
               limitation the provisions of Section 1.13 below; and (e) such
               assignment shall be effective only if immediately following such
               transfer the further disposition of such securities by the
               transferee or assignee is restricted under the Act.

        1.12   "Market Stand-Off" Agreement. Each Holder hereby agrees that,
               during the period of duration specified by the Company and an
               underwriter of common stock or other securities of the Company,
               following the effective date of a registration statement of the
               Company filed under the Act in connection with an underwritten
               offering, it shall not, to the extent requested by the Company
               and such underwriter, directly or indirectly sell, offer to sell,
               contract to sell (including, without limitation, any short sale),
               grant any option to purchase or otherwise transfer or dispose of
               (other than to donees who agree to be similarly bound) any
               securities of the Company held by it at any time during such
               period except Common Stock included in such registration;
               provided, however, that:

               (a)    all officers and directors of the Company and all other
                      persons with registration rights (whether or not pursuant
                      to this Agreement) enter into similar agreements; and

               (b)    such market stand-off time period shall not exceed 180
                      days.

        In order to enforce the foregoing covenant, the Company may impose
stop-transfer instructions with respect to the Registrable Securities of each
Holder (and the shares or securities of every other person subject to the
foregoing restriction) until the end of such period.

        Notwithstanding the foregoing, the obligations described in this Section
1.12 shall not apply to a registration relating solely to employee benefit plans
on Form S-l or Form S-8 or similar forms which may be promulgated in the future,
or a registration relating

                                       14
<PAGE>   15

solely to a Commission Rule 145 transaction on Form S-14 or Form S-15 or similar
forms which may be promulgated in the future.

        1.13   Damages. The Company recognizes and agrees that the holder of
               Registrable Securities will not have an adequate remedy if the
               Company fails to comply with this Section 1 and that damages may
               not be readily ascertainable, and the Company expressly agrees
               that, in the event of such failure, it shall not oppose an
               application by the holder of Registrable Securities or any other
               person entitled to the benefits of this Section 1 requiring
               specific performance of any and all provisions hereof or
               enjoining the Company from continuing to commit any such breach
               of this Section 1.

        1.14   Conflict. In the event of a conflict between the provisions of
               this Section 1 and an underwriting agreement entered into by the
               Company and the holder(s) of Registrable Securities included in a
               registration pursuant to this Section 1, the provisions of such
               underwriting agreement shall control.

        1.15   Termination of Registration Rights. No Holder shall be entitled
               to exercise any right provided for in this Section 1 after five
               (5) years following the consummation of the Qualified IPO or, as
               to any Holder, such earlier time at which all Registrable
               Securities held by such Holder (and any affiliate of the Holder
               with whom such Holder must aggregate its sales under Rule 144)
               can be sold in any three (3)-month period without registration in
               compliance with Rule 144 of the Act.

2.      Covenants of the Company.

        2.1    Delivery of Financial Statements. The Company shall deliver to
               CombiMatrix, Acacia, and each Investor owning shares constituting
               at least 20% of the Registrable Securities:

               (a)    as soon as practicable, but in any event within ninety
                      (90) days after the end of each fiscal year of the
                      Company, an income statement for such fiscal year, a
                      balance sheet of the Company and statement of
                      stockholder's equity as of the end of such year, and a
                      statement of cash flows for such year, such year-end
                      financial reports to be in reasonable detail, prepared in
                      accordance with generally accepted accounting principles
                      ("GAAP"), and audited and certified by independent public
                      accountants of nationally recognized standing selected by
                      the Company;

                                       15
<PAGE>   16

               (b)    as soon as practicable, but in any event within forty-five
                      (45) days after the end of each of the first three (3)
                      quarters of each fiscal year of the Company, an unaudited
                      income statement, statement of cash flows for such fiscal
                      quarter and an unaudited balance sheet as of the end of
                      such fiscal quarter;

               (c)    with respect to the financial statements called for in
                      subsections (a) and (b) of this Section 2.1, an instrument
                      executed by the Chief Financial Officer or President of
                      the Company certifying that such financials were prepared
                      in accordance with GAAP consistently applied with prior
                      practice for earlier periods (with the exception of
                      footnotes that may be required by GAAP) and fairly present
                      the financial condition of the Company and its results of
                      operation for the period specified, subject to year-end
                      audit adjustment; and

               (d)    budgets of the Company prior to the commencement of a new
                      fiscal year.

        2.2    Termination of Information Covenants. The covenants set forth in
               Section 2.1 shall terminate as to Investors and be of no further
               force or effect when the sale of securities pursuant to a
               registration statement filed by the Company under the Act in
               connection with the firm commitment underwritten offering of its
               securities to the general public is consummated or when the
               Company first becomes subject to the periodic reporting
               requirements of Sections 13 or 15(d) of the 1934 Act, whichever
               event shall first occur.

        2.3    Right of First Offer. Subject to the terms and conditions
               specified in this Section 2.3, the Company hereby grants to each
               Investor a right of first offer with respect to future sales by
               the Company of its Shares (as hereinafter defined).

        Subject to the exclusions identified in (i) and (ii) above, each time
the Company proposes to offer any shares of, or securities convertible into or
exercisable for any shares of, any class of its capital stock ("Shares"), the
Company shall first make an offering of Shares to each Investor in accordance
with the following provisions:

               (a)    The Company shall mail a notice by certified mail (a "Sale
                      Notice") to the Investors stating (i) its bona fide
                      intention to offer such Shares, (ii) the number of such
                      Shares to be offered, and (iii) the price and terms, if
                      any, upon which it proposes to offer such Shares.

                                       16
<PAGE>   17

               (b)    By written notification received by the Company within 15
                      calendar days of the mailing of the Sale Notice, the
                      Investor may elect to purchase or obtain, at the price and
                      on the terms specified in the Sale Notice, up to that
                      portion of such Shares which equals the proportion that
                      the number of shares of Common Stock issued and held, or
                      Common Stock Equivalents then held, by such Investor bears
                      to the total number of shares of Common Stock of the
                      Company then outstanding (assuming full conversion and
                      exercise of all convertible or exercisable securities).
                      The Company shall promptly mail a written notice by
                      certified mail to each Investor which purchases all the
                      shares available to it ("Fully-Exercising Investor") of
                      any other Investor's failure to do likewise. During the
                      ten-day period commencing after such notice is mailed,
                      each Fully-Exercising Investor shall be entitled to obtain
                      that portion of the Shares for which Investors were
                      entitled to subscribe but which were not subscribed for by
                      the Investors which is equal to the proportion that the
                      number of shares of Common Stock issued and held, or
                      issuable upon conversion of Common Stock Equivalents then
                      held, by such Fully-Exercising Investor bears to the total
                      number of shares of Common Stock or Common Stock
                      Equivalents issued and held, by all Fully-Exercising
                      Investors who wish to purchase some of the unsubscribed
                      shares.

               (c)    If all Shares which Investors are entitled to obtain
                      pursuant to subsection 2.3(b) are not elected to be
                      obtained as provided in subsection 2.3(b) hereof, the
                      Company may, during the 45 day period following the
                      expiration of the period provided in subsection 2.3(b)
                      hereof, offer the remaining unsubscribed portion of such
                      Shares to any person or persons at a price not less than,
                      and upon terms no more favorable to the offeree than those
                      specified in the Sale Notice. If the Company does not
                      enter into an agreement for the sale of the Shares within
                      such period, or if such agreement is not consummated
                      within 45 days of the execution thereof, the right
                      provided hereunder shall be deemed to be revived and such
                      Shares shall not be offered unless first reoffered to the
                      Investors in accordance herewith.

               (d)    The right of first offer in this Section 2.3 shall not be
                      applicable to Shares of Common Stock issued or issuable:

                      (i)    upon conversion of any preferred stock of the
                             Company;

                      (ii)   pursuant to the acquisition of another corporation
                             or other entity by the Company by merger, purchase
                             of substantially all of the assets, or other
                             reorganization approved by the Board of Directors;

                                       17
<PAGE>   18

                      (iii)  to employees, officers, directors, consultants, or
                             other persons performing services for the Company
                             pursuant to any stock offering, plan, or
                             arrangement approved by the Board of Directors;

                      (iv)   to financial institutions or equipment lessors in
                             connection with the extension of credit to the
                             Company or in connection with the lease of
                             equipment approved by the Board of Directors but
                             not in the aggregate exceeding 5% of the Company's
                             fully diluted equity provided that in no event
                             issued for purposes of equity financing;

                      (v)    to unaffiliated third parties in connection with
                             participation in strategic bona fide alliances or
                             other corporate partner transactions with the
                             Company approved by the Board of Directors; or

                      (vi)   pursuant to the acquisition or licenses of
                             technology, software, or other intellectual
                             property rights approved by the Board of Directors.

               (e)    The right of first offer set forth in this Section 2.3 may
                      not be assigned or transferred.

               (f)    The right of first offer set forth in this Section 2.3
                      shall terminate upon the completion of a Qualified IPO as
                      defined in Section 1.

        2.4    Vesting of Employee Stock and Options. Unless the Board of
               Directors of the Company determines otherwise, all Common Stock
               and options to acquire Common Stock granted to employees of the
               Company after the date of this Agreement shall have a vesting
               schedule as follows: 12.5% shall vest at the end of the six month
               period following full-time employment or the date of the grant,
               which ever is later, and the remaining 87.5% shall vest at a rate
               of 1/10th per quarter thereafter during the employee's full-time
               employment.

3.      Voting.

        3.1    Board Composition. During the term of this Agreement and subject
               to Section 3.6 hereof and the Company's Certificate of
               Incorporation (the "Certificate of Incorporation") and the
               relevant sections of the Company's Bylaws (if any), each Investor
               agrees to vote all Common Stock Equivalents now or hereafter
               directly or indirectly acquired (of record or beneficially) by
               the Investors, in such manner as may be necessary to elect (and
               maintain in office) as members of the Company's Board of
               Directors, the following individuals:

                                       18
<PAGE>   19

               (a)    three (3) directors designated by Acacia (each such
                      director, an "Acacia Designee"); and

               (b)    two (2) directors designated by CombiMatrix (each such
                      director, a "CombiMatrix Designee").

        For purposes of this Agreement: (i) any individual who is designated for
election to the Company's Board of Directors pursuant to the foregoing
provisions of this Section 3.1 is hereinafter referred to as a "Board Designee";
and (ii) any individual, entity, or group of individuals and/or entities who has
the right to designate one or more Board Designees for election the Company's
Board of Directors pursuant to the foregoing provisions of this Section 3.1 is
hereinafter referred to as a "Designator" or as "Designators", as applicable.

        3.2    Board Members. The initial Acacia Designees shall be
               [______________], [_____________] and [_____________], the
               initial CombiMatrix Designees shall be [______________] and
               [______________].

        3.3    Changes in Board Designees. From time to time during the term of
               this Agreement, a Designator or Designators shall, in their sole
               discretion, have the sole right to:

               (a)    elect to remove from the Company's Board of Directors any
                      incumbent Board Designee who occupies a Board seat for
                      which such Designator or Designators are entitled to
                      designate the Board Designee under Section 3.1; and/or

               (b)    designate a new Board Designee for election to a Board
                      seat for which such Designator or Designators are entitled
                      to designate the Board Designee under Section 3.1 (whether
                      to replace a prior Board Designee or to fill a vacancy in
                      such Board seat); provided, that, such removal and/or
                      designation of a Board Designee is approved in a writing
                      signed by Designator or Designators who are entitled to
                      designate such Board Designee under Section 3.1, in which
                      case such election to remove a Board Designee and/or elect
                      a new Board Designee will be binding on all such
                      Designators who are entitled to designate such Board
                      Designee.

                                       19
<PAGE>   20

        3.4    Further Assurances. The Company agrees not to vote any shares of
               Company capital stock, or to take any other actions, that would
               in any manner defeat, impair, be inconsistent with or adversely
               affect the stated intentions of the parties under Section 3 of
               this Agreement.

        3.5    Term. The provisions of this Section 3 shall commence upon the
               Effective Date and shall terminate upon the first to occur of the
               following:

               (a)    Written agreement of the Investors; or

               (b)    Immediately prior to the closing of a Qualified IPO.

4.      Miscellaneous.

        4.1    Successors and Assigns. Except as otherwise provided herein, the
               terms and conditions of this Agreement shall inure to the benefit
               of and be binding upon the respective successors and assigns of
               the parties (including transferees of any shares of Registrable
               Securities). Nothing in this Agreement, express or implied, is
               intended to confer upon any party other than the parties hereto
               or their respective successors and assigns any rights, remedies,
               obligations, or liabilities under or by reason of this Agreement,
               except as expressly provided in this Agreement.

        4.2    Governing Law. 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.

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

        4.4    Titles and Subtitles. The titles and subtitles used in this
               Agreement are used for convenience only and are not to be
               considered in construing or interpreting this Agreement.

        4.5    Notices. Unless otherwise provided, any notice required or
               permitted under this Agreement shall be given in writing and
               shall be deemed effectively given upon personal delivery to the
               party to be notified or upon delivery by confirmed facsimile
               transmission, nationally recognized overnight courier service, or
               upon deposit with the United States Post Office, by registered or
               certified mail, postage prepaid and addressed to the party to be
               notified at the address indicated for such party on the signature
               page hereof, or at such other address as such party may designate
               by ten (10) days' advance written notice to the other parties.

                                       20
<PAGE>   21

        4.6    Expenses. If any action at law or in equity is necessary to
               enforce or interpret the terms of this Agreement, the prevailing
               party shall be entitled to reasonable attorneys' fees, costs and
               necessary disbursements in addition to any other relief to which
               such party may be entitled.

        4.7    Entire Agreement: Amendments and Waivers. This Agreement
               (including schedules or exhibits hereto, if any) constitutes the
               full and entire understanding and agreement among the parties
               with regard to the subjects hereof and thereof and supercedes any
               prior oral or written understanding. Any term of this Agreement
               may be amended and the observance of any term of this Agreement
               may be waived (either generally or in a particular instance and
               either retroactively or prospectively), only with the written
               consent of the Company and the holders of at least seventy five
               percent (75%) of the Registrable Securities. Any amendment or
               waiver effected in accordance with this paragraph shall be
               binding upon each holder of any Registrable Securities each
               future holder of all such Registrable Securities, and the
               Company.

        4.8    Severability. If one or more provisions of this Agreement are
               held to be unenforceable under applicable law, such provision
               shall be excluded from this Agreement and the balance of the
               Agreement shall be interpreted as if such provision were so
               excluded and shall be enforceable in accordance with its terms.

        4.9    Aggregation of Stock. All shares of Registrable Securities held
               or acquired by affiliated entities or persons shall be aggregated
               together for the purpose of determining the availability of any
               rights under this Agreement.

                  [remainder of page intentionally left blank]

                                       21
<PAGE>   22

        IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.

                                        TOXCO., a _________ corporation

                                        By:
                                            ------------------------------------
                                             Name:
                                             Title:

                                        INVESTORS:

                                        ACACIA RESEARCH CORPORATION, a Delaware
                                        corporation

                                        By:
                                            ------------------------------------
                                             Name:
                                             Title:

                                        COMBIMATRIX CORPORATION, a Delaware
                                        corporation

                                        By:
                                            ------------------------------------
                                             Name:
                                             Title:<PAGE>   1
                                                                EXHIBIT 10.12(b)

                                CO-SALE AGREEMENT

                                CO-SALE AGREEMENT

        THIS CO-SALE AGREEMENT (the "Co-Sale Agreement") is entered into as of
November __, 2000, by and among Toxco., a _______ corporation (the "Company"),
CombiMatrix Corporation, a Delaware corporation ("CombiMatrix") and Acacia
Research Corporation, a Delaware corporation ("Acacia"; together with
CombiMatrix, the "Investors" each of which is herein referred to as an
"Investor".)

                                    RECITALS

        WHEREAS, the Company and the Investors are acquiring the Common Stock of
the Company (the "Common Stock");

        WHEREAS, the rights and obligations granted in this Co-Sale Agreement
are a condition to the effectiveness of the acquisition of such Common Stock;

        NOW, THEREFORE, the parties agree as follows:

5. Definitions. For purposes of this Co-Sale Agreement, the following terms
shall have the following meanings.

        5.1    "Act" means the Securities Act of 1933, as amended.

        5.2    "Common Stock" means the common stock, par value $.001 per share,
               of the Company.

        5.3    "Common Stock Equivalents" means and includes all shares of
               Common Stock issued and outstanding at the relevant time plus (i)
               all shares of Common Stock that may be issued upon exercise of
               any options, warrants and other rights of any kind that are then
               exercisable, and (ii) all shares of Common Stock that may be
               issued upon conversion or exchange of any convertible securities
               which are by their terms then convertible into or exchangeable
               for Common Stock, including without limitation, preferred stock.

6. Investors' Right of First Offer. Before any Common Stock owned by any
Investor may be Transferred (as defined hereinafter), the other Investor shall
have a right of first offer with respect to such Common Stock, all on the terms
and conditions set forth in this Section 2.

               Each time an Investor proposes to sell, transfer or otherwise
dispose of Common Stock other than to any wholly owned subsidiary or parent of,
or to any corporation that is, within the meaning of the Act, controlling,
controlled by or under common control with, any such investor, in the case of a
limited liability company, to its members, or gift transfers to its family
members or one or more trusts for the benefit of the foregoing (in each case a
"Transfer"),

<PAGE>   2

that Investor (the "Transferring Investor") shall first make an offering of
Common Stock to the other Investor (the "Other Investor") in accordance with the
following provisions:

               (a) The Transferring Investor shall provide notice (an "Investor
Transfer Notice") to the Other Investor stating (i) its bona fide intention to
Transfer such Common Stock, (ii) the number of such Common Stock shares proposed
to be Transferred (the "Offered Stock"), (iii) the name of each proposed
transferee, and (iv) the cash price or other consideration (including the cash
equivalent value of any non-cash consideration) per share for which the
Transferring Investor proposes to transfer the Offered Stock (the "Offered
Price").

               (b) By written notification received by the Other Investor within
ten (10) calendar days of the mailing of the Investor Transfer Notice, the Other
Investor may elect to purchase or obtain, at the price and on the terms
specified in the Investor Transfer Notice, all of the Offered Stock.

               (c) If all Offered Stock which the Other Investor is entitled to
obtain pursuant to subsection 2(b) is not elected to be obtained, the
Transferring Investor may, during the sixty (60) day period following the
expiration of the period provided in subsection 2(b) hereof, offer the Offered
Stock to any person or persons, subject to Section 3 hereof, at a price not less
than, and upon terms no more favorable to the offeree than those specified in
the Investor Transfer Notice. If the Transferring Investor does not enter into
an agreement for the sale of the Common Stock within such period, or if such
agreement is not consummated within ninety (90) days of the execution thereof,
the right provided hereunder shall be deemed to be revived and such Offered
Stock shall not be transferred unless first reoffered to the Other Investor in
accordance herewith.

               (d) The right of first offer set forth in this Section 2 shall
terminate upon the completion of an underwritten public offering of shares of
Common Stock of the Company (a "Qualified Public Offering").

               (e) Any attempt by an Investor to transfer Common Stock in
violation of Section 2 hereof shall be void and the Company agrees it will not
effect such a transfer nor will it treat any alleged transferee as the holder of
such shares without the written consent of a majority in interest of the
Investors.

7.      Rights of Co-Sale

        7.1    Investors' Rights. If the Other Investor does not fully exercise
               its right of first offer pursuant to Section 2(b) hereof, such
               Other Investor shall have a right of co-sale with respect to the
               sale of any Common Stock owned by the Transferring Investor, all
               on the terms and conditions set forth in this Section 3.

        7.2    Notice of Proposed Transfer. Before a Transferring Investor may
               transfer its shares of Common Stock and after the Other Investor
               has elected not to fully exercise its right of first offer
               pursuant to Section 2, such Transferring Investor shall provide
               notice (a "Transferring Investor Notice") to the Other Investor
               stating (i) its bona

                                       2
<PAGE>   3

               fide intention to Transfer such Common Stock, (ii) the number of
               shares of Offered Stock, (iii) the name of each proposed
               transferee, and (iv) the Offered Price.

        7.3    Right of Co-Sale.

                      (a)    Exercise of Right of Co-Sale. The Other Investor
                             shall have the right to participate in the
                             Transferring Investor's transfer of the Offered
                             Stock to the proposed transferee pursuant to the
                             specified terms and conditions of the Transfer as
                             set forth in the Transferring Investor Notice and
                             in accordance with the terms and conditions of the
                             Transfer as set forth in this Section 3.3 (the
                             "Right of Co-Sale"). For purposes of the preceding
                             sentence, the participation of the Other Investor
                             shall be on the same terms as the Transferring
                             Investor. To the extent the Other Investor
                             exercises his, her or its Right of Co-Sale, the
                             number of shares of Offered Stock that the
                             Transferring Investor may transfer pursuant to the
                             Transferring Investor Notice shall be
                             correspondingly reduced. The Other Investor shall
                             be responsible for its pro rata share of the
                             reasonable fees and expenses of the Transferring
                             Investor relating to the negotiation of the
                             transaction. The Right of Co-Sale of the Other
                             Investor shall be subject to the following terms
                             and conditions:

                             (i)    The Other Investor may transfer all or any
                                    part of such Investor's Common Stock that is
                                    not in excess of the number obtained by
                                    multiplying the aggregate number of shares
                                    of Common Stock constituting the Offered
                                    Stock by a fraction (A) the numerator of
                                    which is the number of shares of such
                                    Investor's Common Stock, and (B) the
                                    denominator of which is the total number of
                                    Common Stock Equivalents then owned by the
                                    Investors and investors, if any, who are
                                    parties to any other co-sale agreement with
                                    the Company and the Investors which is in
                                    existence at the time of the Transfer.

                             (ii)   By written notice received by the Company
                                    within fifteen (15) calendar days of the
                                    mailing of the Transferring Investor Notice
                                    by the Transferring Investor to the Company,
                                    the Other Investor may effect its election
                                    to participate in the transfer subject to
                                    this Section 3.3. Such written notice shall

                                       3
<PAGE>   4

                                    contain the Other Investor's election to
                                    participate in the Transfer of the Offered
                                    Stock setting forth the number and type of
                                    shares that the Other Investor elects to
                                    include in the Transfer, accompanied by one
                                    or more certificates or other documentation,
                                    properly endorsed for transfer, representing
                                    those shares (if the Other Investor so
                                    elects, then such Other Investor shall be
                                    referred to in this Co-Sale Agreement as a
                                    "Participant"). The Company shall keep the
                                    Transferring Investor promptly informed at
                                    all times as to the Other Investor's
                                    election to participate.

                      (b)    Delivery of Stock Certificates and Proceeds. Upon
                             the closing of the Transfer subject to this Section
                             3.3, the stock certificate(s) or other
                             documentation representing the Common Stock to be
                             transferred shall be transferred and delivered to
                             the investor or transferee pursuant to the terms
                             and conditions specified in the Transferring
                             Investor Notice, and there shall be promptly
                             thereafter remitted to the Participant that portion
                             of the proceeds from the Transfer to which it is
                             entitled by reason of participating in the
                             Transfer.

        7.4    Offering Investor's Right to Transfer. The Transferring Investor
               may transfer that portion of the Offered Stock permitted to be
               transferred by the Transferring Investor, after application of
               the Right of Co-Sale contained in Section 3.3 hereof, to any
               person named as an investor or other transferee in the
               Transferring Investor Notice, at the Offered Price, provided that
               the transfer (i) is consummated within ninety (90) days after the
               date of the Transferring Investor Notice and (ii) is in
               accordance with all the terms of this Co-Sale Agreement. If the
               Offered Stock is transferred in accordance with the terms and
               conditions of this Co-Sale Agreement, then the transferee(s) of
               the Offered Stock shall thereafter hold the Offered Stock free of
               this Co-Sale Agreement and the Right of Co-Sale set forth herein.
               If the Offered Stock is not so transferred during such ninety
               (90) day period, then the Transferring Investor shall not
               transfer any of the Offered Stock without complying again in full
               with the provisions of this Co-Sale Agreement.

        7.5    Effect of Prohibited Transfer. In the event an Investor should
               sell any Common Stock in contravention of the co-sale rights of
               the other Investor under this Co-Sale Agreement (such an investor
               a "Violating Investor"; such a transaction a "Prohibited
               Transfer"), the other Investor, in addition to all other remedies
               available at law, in equity or hereunder, shall have the put
               option provided below, and the Violating Investor shall be bound
               by the applicable provisions of that option. Any attempt by a
               Violating Investor to transfer Common Stock in

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<PAGE>   5

               violation of Section 3 hereof shall be void and the Company
               agrees it will not effect such a transfer nor will it treat any
               alleged transferee as the holder of such shares without the
               written consent of a majority in interest of the Investors.

        7.6    Put Option. Notwithstanding Section 3.5 hereof, in the event of a
               Prohibited Transfer, the other Investor shall have the right to
               sell to the Violating Investor the type and number of shares of
               Common Stock equal to the number of shares such Investor would
               have been entitled to sell to the Violating Investor or
               transferee under Section 3.3 hereof had the Prohibited Transfer
               been effected pursuant to and in compliance with the terms
               hereof. This sale shall be made on the following terms and
               conditions:

                      (a)    The price per share at which the Common Stock is to
                             be sold to the Violating Investor shall be equal to
                             the price per share (on an as-converted basis) paid
                             by the transferee to the Violating Investor in the
                             Prohibited Transfer. The Violating Investor shall
                             also reimburse the other Investor for any and all
                             fees and expenses, including legal fees and
                             expenses, incurred pursuant to the exercise or the
                             attempted exercise of the Investor's rights under
                             Section 3.

                      (b)    Within ninety (90) days after the later of the
                             dates on which the other Investor (A) receives
                             notice of the Prohibited Transfer or (B) otherwise
                             becomes aware of the Prohibited Transfer, the other
                             Investor, if exercising the option created hereby,
                             shall deliver to the Violating Investor the
                             certificate or certificates representing shares to
                             be sold, each certificate to be properly endorsed
                             for transfer.

                      (c)    The Violating Investor shall, upon receipt of the
                             certificate or certificates for the shares to be
                             sold by the Investor pursuant to this Section 3.6,
                             pay the aggregate purchase price therefor and the
                             amount of reimbursable fees and expenses, as
                             specified in Section 3.6(a), in cash or by other
                             means acceptable to the other Investor.

8.  Exceptions, Termination.

        8.1    Exceptions. Notwithstanding anything in this Co-Sale Agreement to
               the contrary, the Right of Co-Sale set forth in this Co-Sale
               Agreement shall not apply to any Transfer or series of Transfers
               of Common Stock by an Investor (i) pursuant to a reorganization
               or merger of the Company with or into any other corporation or
               entity, or a sale of all or substantially all of the assets of
               the Company, in which the Investors in the aggregate own
               immediately after such transaction less than

                                       5
<PAGE>   6

               50% of the voting equity securities of the surviving entity, (ii)
               pursuant to the winding up and dissolution of the Company or
               (iii) in connection with a Qualified Public Offering.

        8.2    Termination of Rights. The Right of Co-Sale set forth herein
               shall terminate upon the earlier to occur of (i) the closing of a
               Qualified Public Offering, or (ii) the date on which this Co-Sale
               Agreement is terminated by a written agreement to such effect
               executed by the Company and holders of at least seventy five
               percent (75%) of the shares of Common Stock then held by the
               Investors.

9.      Restrictive Legend and Stop-Transfer Orders.

        9.1    Legend. The Investors understand and agree that the Company shall
               cause the legend set forth below, or a legend substantially
               equivalent thereto, to be placed upon any certificate(s)
               evidencing ownership of the Common Stock:

        THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO CERTAIN
        RESTRICTIONS ON TRANSFER SET FORTH IN A CO-SALE AGREEMENT ENTERED INTO
        BY THE ORIGINAL HOLDER OF THESE SHARES, THE COMPANY AND OTHER PERSONS, A
        COPY OF WHICH IS ON FILE AT THE PRINCIPAL OFFICE OF THE COMPANY. SUCH
        RESTRICTIONS ARE BINDING ON TRANSFEREES OF THESE SHARES UNDER SOME
        CIRCUMSTANCES.

        9.2    StopTransfer Instructions. The Investors agree, to ensure
               compliance with the restrictions referred to herein, that the
               Company may issue appropriate "stop transfer" certificates or
               instructions with respect to the Common Stock and that, the
               Company may make appropriate notations to the same effect in its
               records.

10.     Miscellaneous Provisions.

        10.1   Notices. All notices, requests, demands and other communications
               which are required to be or may be given under this Co-Sale
               Agreement to any party by any of the other parties shall be in
               writing and shall be deemed to have been duly given when (a)
               delivered in person, (b) the day following dispatch by a
               nationally recognized overnight courier service (such as Federal
               Express or UPS, etc.) for next day delivery, (c) five (5) days
               after dispatch by certified or registered first class mail,
               postage prepaid, return receipt requested, to the party to whom
               the same is so given or made or (d) in the case of notice sent by
               telecopy, on the date of dispatch, provided that the report
               generated by the sender's telecopy machine shows that all pages
               of such notice or other communication were properly transmitted
               to each recipient's telecopy number. Any notice or other
               communication given hereunder shall be addressed to the Company,
               at its principal offices as set forth below or to the Investors
               at their addresses indicated on the signature page hereto.

                                       6
<PAGE>   7

        10.2   Binding on Successors and Assigns. This Co-Sale Agreement, and
               the rights and obligations of the parties hereunder, shall inure
               to the benefit of, and be binding upon, their respective
               successors, assigns and legal representatives.

        10.3   Severability. If one or more of the provisions of this Co-Sale
               Agreement should, for any reason, be held to be invalid, illegal
               or unenforceable in any respect, such invalidity, illegality or
               unenforceability shall not affect any other provisions of this
               Co-Sale Agreement, and such invalid, illegal or unenforceable
               provision shall be enforced to the extent permissible.

        10.4   Amendment. Any amendment, modification or waiver of this Co-Sale
               Agreement shall be effective only with the written consent of (i)
               Investors holding more than seventy five percent (75%) of the
               then outstanding Common Stock, and (ii) the Company; provided,
               however, that any person may waive, reduce or release (in whole
               or in part) any of its rights hereunder without the consent of
               any other parties hereto. Any waiver by a party of its rights
               hereunder shall be effective only if evidenced by a written
               instrument executed by a duly authorized representative of such
               party.

        10.5   Governing Law. This Co-Sale Agreement shall be governed by and
               construed in accordance with the internal laws (and not the laws
               pertaining to choice of conflict of laws) of the State of
               Delaware. The parties hereby agree that any dispute which may
               arise between them arising out of or in connection with this
               Co-Sale Agreement shall be adjudicated before a court located in
               Wilmington, Delaware and they hereby submit to the exclusive
               jurisdiction of the courts of the State of Delaware located in
               Wilmington, Delaware and of the federal courts in the State of
               Delaware with respect to any action or legal proceeding commenced
               by any party, and irrevocably waive any objection they now or
               hereafter may have respecting the venue of any such action or
               proceeding brought in such a court or respecting the fact that
               such court is an inconvenient forum, relating to or arising out
               of this Co-Sale Agreement or any acts or omissions relating to
               the sale of the securities hereunder, and consent to the service
               of process in any such action or legal proceeding by means of
               registered or certified mail, return receipt requested, in care
               of the address set forth in Section 6.1 or such other address as
               each party shall furnish in writing to the other parties.

        10.6   Obligations of Company. The Company agrees to use its best
               efforts to enforce and abide by the terms of this Co-Sale
               Agreement, to inform the Investors of any breach hereof (to the
               extent the company has knowledge thereof) and to assist the
               Investors in the exercise of their rights hereunder.

        10.7   Expenses. If any action at law or in equity is necessary to
               enforce or interpret the terms of this Co-Sale Agreement, the
               prevailing party shall be entitled to reasonable attorneys' fees,
               costs and necessary disbursements in addition to any other relief
               to which such party may be entitled.

                                       7
<PAGE>   8

        10.8   Counterparts. This Co-Sale Agreement may be executed in any
               number of counterparts, each of which shall be deemed an original
               and all of which together shall constitute one and the same
               instrument.

        10.9   Facsimile Signatures. Any signature page delivered by a fax
               machine or telecopy machine shall be binding to the same extent
               as an original signature page, with regard to any agreement
               subject to the terms hereof or any amendment thereto. Any party
               who delivers such a signature page agrees to later deliver an
               original counterpart to any party which requests it.

        10.10  Entire Agreement. This Co-Sale Agreement, together with the
               exhibit hereto, constitutes the entire understanding and
               agreement of the parties with respect to the subject matter
               hereof and supersedes all prior understandings and agreements
               with respect to such subject matter.

                  [remainder of page intentionally left blank]

                                       8
<PAGE>   9

        IN WITNESS WHEREOF, the parties hereto have executed this Co-Sale
Agreement as of the date first written above.

                                        TOXCO, INC.,
                                        a _________ corporation

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

                                        Address:

                                        INVESTORS:

                                        ACACIA RESEARCH CORPORATION, a Delaware
                                        corporation

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

                                        Address:

                                        COMBIMATRIX CORPORATION, a Delaware
                                        corporation

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

                                        Address:

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