Document:

<PAGE>   1
                                           *** Text Omitted and Filed Separately
                                               Confidential Treatment Requested
                                               Under 17 C.F.R. Sections 200.80,
                                               200.83 and 230.406.

                                                                   EXHIBIT 10.29

                     [UNIVERSITY OF CALIFORNIA LETTERHEAD]

                                             February 2,1998

Carl F. Bobkoski
Executive Vice President
Signal Pharmaceuticals, Inc.
5555 Oberlin Drive
San Diego, CA 92121

RE:   Second Amendment to Exclusive License Agreement for:
      Methods for Production of Neuroblasts and Testing Drugs...
      UC Case No. 92-116 etc.: UC Agreement Control No. 93-04-0786

Dear Carl:

I write regarding a proposed designated sublicense agreement, which Signal
Pharmaceuticals, Inc. "Signal") is negotiating for the [***] Technologies (as
defined in the subject License Agreement) with a certain other party
("Designated Sublicensee"). Signal wishes to issue this sublicense with a
royalty base that differs from the royalty base defined in the License Agreement
and on other terms and conditions which vary from the License Agreement.
Accordingly:

The Regents agrees to waive certain provisions of the License Agreement solely
for this designated sublicense in consideration of Signal agreeing that
(capitalized terms should have the meaning set forth herein, if not specified
herein, the meaning defined in the above referenced License Agreement):

      (a)   The designated sublicense grant will be confined to a field allowing
      the Designated Sublicensee to use the [***] Technologies in connection
      with [***]

                      ***Confidential Treatment Requested

<PAGE>   2
Carl F. Bobkoski
February 2, 1998
Page 2

     (b)  Signal will pay The Regents an earned royalty on Net Sales of
     Licensed Product sold by the Designated Sublicensee, its affiliates and
     sublicensees as set forth in Appendix A hereto; accordingly, solely for
     purposes of the designated sublicense agreement. Paragraphs 5.1 through
     5.3 and 5.9 of the License Agreement are hereby superseded by the terms
     and conditions set forth in Appendix A.

     (c)  Upon termination of the License Agreement or reduction of Signal's
     license under the License Agreement to non-exclusive, for any reason, The
     Regents shall accept assignment of this designated sublicense, as provided
     for in the First Amendment to Exclusive License Agreement for [***]
     effective June 22, 1997. Notwithstanding the foregoing, The Regents shall
     accept assignment of this sublicense even if Signal was in breach of the
     License Agreement when entering into the sublicense.

     (d)  The designated sublicense agreement will require that Sublicensee
     (itself or through a designee) develop at least [***] as follows:

          - [***]

          - [***]

          - [***]

          - [***]

Notwithstanding the foregoing, the designated sublicense agreement may provide
for reasonable extensions of the foregoing diligence deadlines, if despite
diligent efforts such deadline is or may not be met. However, in the event that
The Regents accepts assignment of the sublicense, The Regents shall not be
required to provide any extension of the diligence deadlines and any extension
of the diligence deadlines shall be at the sole discretion of The Regents.

Apart from the above-specified changes, the subject sublicense agreement will
adhere to all the sublicensing provisions of the License Agreement. However, to
the extent this letter and the License Agreement conflict solely with respect
to the designated sublicense agreement this letter will control.

                      ***Confidential Treatment Requested
<PAGE>   3
Carl F. Bobkoski
February 2, 1998
Page 3

If you agree to the terms of this letter, please sign both enclosed originals
and return them to me. I will return a fully executed original to you for your
records.

                                        Sincerely,

                                        /s/ PATRICIA ANDERSON COTTON, PH.D.
                                        -----------------------------------
                                        Patricia Anderson Cotton, Ph.D.
                                        Licensing Officer

______________________________________________________________________________

SIGNAL PHARMACEUTICALS                 THE REGENTS OF THE UNIVERSITY
INC.:                                  OF CALIFORNIA:

By: /s/  CARL F. BOBKOSKI              By: /s/  TERENCE A. FEUERBORN
    -------------------------------        -------------------------------
              (Signature)                            (Signature)

Name: Carl F. Bobkoski                 Name: Terence A. Feuerborn

Title: Executive Vice President        Title: Executive Director
                                              Office of Research Administration
                                              and Technology Transfer

Date: Feb. 13, 98                      Date: 3-2-98
      -----------------------------          -----------------------------

                                       [ILLEGIBLE]
                                       /s/ P. Martin Simpson, Jr.     3-Jan 98
                                           -----------------------    --------
                                           P. Martin Simpson, Jr.       Date
                                           Resident Counsel
                                           Office of Technology Transfer
                                           University of California
<PAGE>   4
             APPENDIX A TO SECOND AMENDMENT DATED FEBRUARY 2, 1998

Signal and The Regents agree that solely for purposes of the designated
sublicense agreement:

1.   The following terms defined in Paragraphs 1.2, 1.3, 1.4 and 1.5 of the
License Agreement shall have the following meanings:

     1.2  "Licensed Product" means any product, composition or material (i) the
sale of which would, but for the licenses granted to the Designated
Sublicensee, infringe a Valid Claim within The Regents' Patent Rights in the
country for which such product, composition or material is sold or (ii) is
manufactured or administered to a patient by a Licensed Method.

     1.3  "Licensed Method" means any method (i) the practice of which would,
but for the license granted to Designated Sublicensee, infringe a Valid Claim
within Regents' Patent Rights or (ii) consisting of a method of [***]

1.4  "Net Sales" means the total amount invoiced to third parties on sales of
Licensed Products by Designated Sublicensee, its affiliates or sublicensees,
less the following reasonable and customary deductions to the extent applicable
to such invoiced amounts: (i) all trade, cash and quantity credits, discounts,
refunds or government rebates; (ii) amounts for claims, allowances or credits
for returns, retroactive price reductions, or chargebacks; (iii) packaging,
handling fees and prepaid freight, sales taxes, duties and other governmental
charges (including value added tax); and (iv) provisions for uncollectible
accounts determined in accordance with reasonable accounting practices,
consistently applied to all products of the selling party; provided, however,
that in the case of Patient-Specific Licensed Products, "Net Sales" shall equal
[***] of the foregoing amounts (after the deductions described in (i) through
(iv) above). For purposes of the foregoing, it is understood that Net Sales
shall include only the amount invoiced for materials consisting of Licensed
Products (less the foregoing deductions and adjustments) and shall not include
charges related to services [***] performed in connection with the sale of such
Licensed Products; accordingly, Net Sales shall not include, without limitation,
charges for [***] or the like. For the removal of doubt, Net Sales shall not
include sales by Sublicensee to its affiliates for resale, provided that if
Sublicensee sells a Licensed Product to an affiliate for resale, Net Sales shall
include the amounts invoiced by such affiliate to third parties on the resale of
such Licensed Product.

     1.5  "Affiliate" means any corporation or other business entity in which
the Licensee or Designated Sublicensee owns or controls, directly or
indirectly, at least fifty (50%) of the outstanding stock or other voting
rights entitled to elect directors; provided however, that any country where
the local law shall not permit foreign equity participation of at least 50%,
then an

                      ***Confidential Treatment Requested
<PAGE>   5
"Affiliate" shall include any company in which the Licensee or Sublicensee owns
or controls, directly or indirectly, the maximum percentage of such outstanding
stock or voting rights permitted by local law.

2.   The following definitions are added to Article I (Definitions) of the
License Agreement:

     1.9  [***]

     2.0  "Valid Claim" shall mean a claim of an issued and unexpired patent or
a claim of a pending patent application within Regents' Patent Rights which has
not been held unpatentable, invalid or unenforceable by a court or other
government agency of competent jurisdiction and has not been admitted to be
invalid or unenforceable through reissue, re-examination, disclaimer or
otherwise. Notwithstanding the foregoing, if a claim of a pending patent
application within Regents' Patent Rights has not issued as a claim of an
issued patent within Regent's Patent Rights, on or before [***]
such pending claim shall not be a Valid Claim for purposes of the sublicense
agreement unless and until such claim issues.

3.   Paragraphs 5.1, 5.2, 5.3 and 5.9 of the License Agreement shall be removed
and replaced with the following:

     5.1(a)    The Licensee shall pay The Regents an earned royalty of [***] of
the Net Sales of Licensed Product sold by the Designated Sublicensee, its
Affiliates or sublicensees; provided that the earned royalty shall be [***] of
the Net Sales of Licensed Products, if such Net Sales by Licensee, its
Affiliates and sublicensees (including without limitation, Designated
Sublicensee, its Affiliates and sublicensees) are less than [***] per year in
any given calendar year of sales.

     5.1(b)    The earned royalty due under Paragraph 5.1(a) above with respect
to a Licensed Product which is not covered by any Valid Claim but is
manufactured or administered to patients using a Licensed Method not covered by
a Valid Claim shall be [***].

     5.2  Royalties shall continue under Paragraph(s) 5.1(a) and 5.1(b) above
with respect to each Licensed Product, (i) on a country-by-country basis for so
long as the sale of such Licensed Product would, but for the license granted to
Designated Sublicensee, infringe a Valid Claim within the Regents' Patent rights
in such country or (ii) with respect to a Licensed Product(s) not covered by any
Valid Claim but that is manufactured or administered to patients using a
Licensed Method not covered by a Valid Claim for ten (10) years from the date of
first commercialization of the first such Licensed Product in any country.

                      ***Confidential Treatment Requested

<PAGE>   6
      5.3   Earned royalties due on Net Sales of Licensed Products by Designated
Sublicensee, its affiliates or sublicensees shall be deemed accrued in the
quarter in which Licensee receives payment from the Sublicensee with respect to
such Net Sales, but in no event later than the quarter following the quarter in
which Net Sales are made by the Sublicensee, its affiliates or sublicensee.<PAGE>   1
                          ***Text Omitted and Filed Separately
                             Confidential Treatment Requested
                             Under 17 C.F.R. Sections 200.80, 200.83 and 230.406

                                                                   EXHIBIT 10.30

                       RESTRICTED STOCK PURCHASE AGREEMENT

      Agreement made as of this 26th day of October, 1993 by and between Signal
Pharmaceuticals, Inc., a California corporation (the "Company"), and The Regents
of the University of California ("The Regents" or "Purchaser").

      Unless otherwise defined herein, all capitalized terms used herein shall
have the same meanings given to them in that certain Exclusive License Agreement
(the "License Agreement") by and between Purchaser and the Company, dated
October 26, 1993, a copy of which is attached hereto as Exhibit 1.

            1.    PURCHASE OF SHARES

            1.1   Purchase. In consideration of Purchaser's grant of certain
patent and technology rights to the Company pursuant to the License Agreement,
the Company shall issue to Purchaser [***] shares of its Common Stock. Upon the
Company's achievement of certain milestones, the Company shall issue additional
shares to Purchaser as follows:

                  (a)   The Company shall issue [***] shares of its Common Stock
(as adjusted for stock splits, subdivisions, combinations, recapitalizations and
distributions of stock) to Purchaser:

                        (i)   [***]

                        (ii)  [***]

provided, however, that only one such issuance shall be made pursuant to each of
(i) and (ii), above, regardless of how many lead compounds utilizing the [***]
or [***] Technologies may subsequently be entered into toxicology studies.

                  (b)   Upon the [***], the Company shall issue [***] shares of
its Common Stock (as adjusted for stock splits, subdivisions, combinations,
recapitalizations and distributions of stock) to Purchaser;

                      ***Confidential Treatment Requested
<PAGE>   2
[***]

                  (c)   [***], the Company shall issue to
Purchaser [***] shares of its Common Stock (as adjusted for stock splits,
subdivisions, combinations, recapitalizations and distributions of stock);
[***]

            All shares of the Company's Common Stock issued to Purchaser
pursuant to this paragraph 1.1, shall hereinafter be referred to as the
"Shares". Notwithstanding the foregoing, the Company shall have no obligation to
issue any shares following any termination of the License Agreement.

            2.    INVESTMENT REPRESENTATIONS

            2.1   Exemption from Registration. The Shares have not been
registered under the 1933 Act, and are accordingly being issued to Purchaser in
reliance upon the exemption from such registration provided by Section 4(2) of
the 1933 Act.

            2.2   Investment Intent. Purchaser hereby warrants and represents
that it is acquiring the Shares for its own account and not with a view to their
resale or distribution and that it is prepared to hold the Shares for an
indefinite period and has no present intention to sell, distribute or grant any
participating interests in, the Shares. Purchaser hereby acknowledges the fact
that the Shares have not been registered under the Securities Act of 1933, as
amended (the "1933 Act"), and that the Company is issuing the Shares to it in
reliance on the representations made by it herein.

            2.3   Restricted Securities.

                  (a)   Purchaser hereby confirms that it has been informed that
the Shares may not be resold or transferred unless such Shares are first
registered under the 1933 Act or unless an exemption from such registration is
available. Accordingly, Purchaser hereby acknowledges that it is prepared to
hold the Shares for an indefinite period and that it is aware that Rule 144 of
the Securities and Exchange Commission issued under the 1933 Act is not
presently available to exempt the sale of the Shares from the registration
requirements of the Act. Should Rule 144 subsequently become available,
Purchaser is aware that any sale of Shares effected pursuant to the Rule may,
depending upon the status of Purchaser as an "affiliate" or "non-affiliate"

                                       2

                      ***Confidential Treatment Requested

<PAGE>   3
under the Rule, be made only in limited amounts in accordance with the
provisions of the Rule, and that in no event may any Shares be sold pursuant to
the Rule until Purchaser has held such Shares for at least two (2) years
following their issuance.

                  (b)   Should the Company not become subject to the reporting
requirements of the Exchange Act, then Purchaser may, provided he/she is not at
the time an affiliate of the Company (nor was such an affiliate during the
preceding three (3) months), sell the Shares (without registration) pursuant to
paragraph (k) of Rule 144 after the Shares have been held for a period of three
(3) years following the payment in cash of the Purchase Price for such shares.

            2.4   Disposition of Shares. Except as specifically set forth below,
Purchaser hereby agrees that it shall make no disposition of the Shares, unless
and until:

                  (a)   it shall have complied with all requirements of this
Agreement applicable to the disposition of such Shares;

                  (b)   it shall have notified the Company of the proposed
disposition and furnished it with a written summary of the terms and conditions
of the proposed disposition; and

                  (c)   unless otherwise waived by the Company, it shall have
delivered to the Company a written opinion of counsel at the Company's expense,
in form and substance satisfactory to the Company, that (i) the proposed
disposition does not require registration of the Shares under the 1933 Act or
(ii) all appropriate action necessary for compliance with the registration
requirements of the 1933 Act or of any exemption from registration available
under the 1933 Act has been taken.

            The Company shall not be required (i) to transfer on its books any
Shares which have been sold or transferred in violation of the provisions of
this Section 2, nor (ii) to treat as the owner of the Shares, or otherwise to
accord voting or dividend rights to, any transferee to whom the Shares have been
so transferred.

            2.5   Restrictive Legends. In order to reflect the restrictions on
disposition of the Shares, the stock certificates for the Shares shall be
endorsed with restrictive legends, including the following legend:

            "The securities represented by this certificate have not been
      registered under the Securities Act of 1933. The shares have been acquired
      for investment and may not be sold or offered for sale in the absence of
      an effective registration statement for the shares

                                       3
<PAGE>   4
      under that Act, a 'no action' letter of the Securities and Exchange
      Commission as to such sale or offer, or an opinion of counsel to the
      Company that registration under such Act is not required for such sale or
      offer."

            3.    TRANSFER RESTRICTIONS

            3.1   Transferee Obligations. Each person (other than the Company)
to whom the Shares are transferred must, as a condition precedent to the
validity of such transfer, acknowledge in writing to the Company that such
person is bound by the provisions of this Agreement and that the transferred
shares are subject to the market stand-off provisions of paragraph 3.2 to the
same extent such shares would be so subject if retained by Purchaser.

            3.2   Market Stand-Off.

                  (a)   In connection with any underwritten public offering by
the Company of its equity securities pursuant to an effective registration
statement filed under the 1933 Act, including the Company's initial public
offering (the "IPO"), Purchaser and all subsequent holders of the Shares who
derive their chain of ownership through a transfer from Purchaser ("Owner")
shall not sell, make any short sale of, loan, hypothecate, pledge, grant any
option for the purchase of, or otherwise dispose or transfer for value or
otherwise agree to engage in any of the foregoing transactions with respect to,
any Shares without the prior written consent of the Company or its underwriters.
Such limitations shall be in effect for such period of time from and after the
effective date of the final prospectus for such offering as may be requested by
the Company or such underwriters.

                  (b)   Owner shall be subject to the market standoff provisions
of this paragraph 3.2 provided and only if all of the executive officers and
directors of the Company are also subject to similar arrangements.

                  (c)   In the event of any stock dividend, stock split,
recapitalization or other change affecting the Company's outstanding Common
Stock effected as a class without the Company's receipt of consideration, then
any new, substituted or additional securities distributed with respect to the
Shares shall be immediately subject to the provisions of this paragraph 3.2, to
the same extent the Shares are at such time covered by such provisions.

                                       4
<PAGE>   5
                  (d)   In order to enforce the limitations of this paragraph
3.2, the Company may impose stop-transfer instructions with respect to the
Shares until the end of the applicable standoff period.

            4.    REGISTRATION RIGHTS.

            4.1   For purposes of this Section 4:

                  (a)   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;

                  (b)   The term "Registrable Securities" means the Common Stock
issued pursuant to this Agreement, excluding, however, any Registrable
Securities sold by a person in a transaction in which his rights under this
Section 4 are not assigned;

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

                  (d)   The term "Holder" means any person owning or having the
right to acquire Registrable Securities or any assignee thereof in accordance
with Section 4.7 hereof.

            4.2   Company Registration. If (but without any obligation to do so)
the Company proposes to register (including for this purpose a registration
effected by the Company for shareholders other than the Holders) any of its
stock or other securities under the Act in connection with the public offering
of such securities solely for cash (other than a registration relating solely to
the sale of securities to participants in a Company stock plan, or a
registration on any form which 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), 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 5.2, the Company shall,
subject to the provisions of Section 4.4, cause to be registered under the Act
all of the Registrable Securities that each such Holder has requested to be
registered.

                                       5
<PAGE>   6
            4.3   Expenses of Company Registration. The Company shall bear and
pay all expenses incurred in connection with any registration, filing or
qualification of Registrable Securities with respect to the registrations
pursuant to Section 4.2 for each Holder (which right may be assigned as provided
in section 4.7), including (without limitation) all registration, filing, and
qualification fees, printers' and accounting fees relating or apportionable
thereto, but excluding underwriting discounts and commissions relating to
Registrable Securities.

            4.4   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 Section 4.2 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 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 shareholders 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, which the underwriters determine in their sole
discretion will not jeopardize the success of the offering (subject to the
rights of other security holders of the Company, including, without limitation,
preferred shareholders, the securities so included to be apportioned pro rata
among the selling shareholders according to the total amount of securities
entitled to be included therein owned by each selling shareholder or in such
other proportions as shall mutually be agreed to by such selling shareholders).
For purposes of the preceding parenthetical concerning apportionment, for any
selling shareholder which is a holder of Registrable Securities and which is a
partnership or corporation, the partners, retired partners and shareholders 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 shareholder", and any pro-rata reduction with
respect to such "selling shareholder" shall be-based upon the aggregate amount
of shares carrying registration rights owned by all entities and individuals
included in such "selling shareholder", as defined in this sentence.

                                       6
<PAGE>   7
            4.5   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 4.

            4.6   Indemnification. In the event any Registrable Securities are
included in a registration statement under this Section 4:

                  (a)   To the extent permitted by law, the Company will
indemnify and hold harmless 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 Securities Exchange Act of 1934, as amended
(the "1934 Act"), against any losses, claims, damages, or liabilities (joint or
several) to which they may become subject under the Act, or the 1934 Act or
other federal or state law, 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 law or any rule or
regulation promulgated under the Act, or the 1934 Act or any state securities
law; and the Company will pay to each such Holder, underwriter or controlling
person, as incurred, 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 4.6 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 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 which 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.

                  (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, any underwriter, any other
Holder selling

                                       7
<PAGE>   8
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, or the 1934 Act or other federal or state law, 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 pay, as incurred, any legal or
other expenses reasonably incurred by any person intended to be indemnified
pursuant to this subsection 4.6(b), in connection with investigating or
defending any such loss, claim, damage, liability, or action; provided, however,
that the indemnity agreement contained in this subsection 4.6(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 4.6(b) exceed the gross proceeds from the
offering received by such Holder.

                  (c)   The foregoing indemnity agreements of the Company and
Holders are subject to the condition that, insofar as they relate to any
Violation made in a preliminary prospectus but eliminated or remedied in the
amended prospectus on file with the SEC at the time the registration statement
in question becomes effective or the amended prospectus filed with the SEC
pursuant to SEC Rule 424(b) (the "Final Prospectus"), such indemnity agreement
shall not inure to the benefit of any indemnified party if a copy of the Final
Prospectus was furnished to such indemnified party and was not furnished to the
person asserting the loss, liability, claim or damage at or prior to the time
such Final Prospectus is required to be delivered under the Securities Act if
such loss, claim or damage would have been avoided had the indemnified party
furnished the Final Prospectus to such person.

                  (d)   Promptly after receipt by an indemnified party under
this Section 4.6 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 4.6, 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 which may be represented without
conflict by one counsel) shall have the right to retain

                                       8
<PAGE>   9
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 4.6, 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 4.6.

                  (e)   If the indemnification provided for in this Section 4.6
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
therein, 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 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.

                  (f)   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.

                  (g)   The obligations of the Company and Holders under this
Section 4.6 shall survive the completion of any offering of Registrable
Securities in a registration statement under this Section 4, and otherwise.

            4.7   Assignment of Registration Rights. The rights to cause the
Company to register Registrable Securities pursuant to this Section 4 may be
assigned (but only with all related obligations) by a Holder to a transferee or
assignee of such

                                       9
<PAGE>   10
securities who, after such assignment or transfer, holds at least twenty percent
(20%) of the shares of Registrable Securities originally purchased by such
Holder (subject to appropriate adjustment for stock splits, stock dividends,
combinations and other recapitalizations), provided 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; and provided, further, that
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. For the purposes of determining the number of shares
of Registrable Securities held by a transferee or assignee, the holdings of
transferees and assignees of a partnership who are partners or retired partners
of such partnership (including spouses and ancestors, lineal descendants and
siblings of such partners or spouses who acquire Registrable Securities by gift,
will or intestate succession) shall be aggregated together and with the
partnership; provided that all assignees and transferees who would not qualify
individually for assignment of registration rights shall have a single
attorney-in-fact for the purpose of exercising any rights, receiving notices or
taking any action under this Section 4.

            5.    INTERPRETATION

            5.1   Purchaser Undertaking. Purchaser hereby agrees to take
whatever additional action and execute whatever additional documents the Company
may in its judgment deem necessary or advisable in order to carry our or effect
one or more of the obligations or restrictions imposed on either Purchaser or
the Shares pursuant to the express provisions of this Agreement.

            5.2   Notices. Any notice required or permitted in connection any
matter pertaining to this Agreement shall be given in writing and shall be
deemed effective upon personal delivery or upon deposit in the United States
Mail, registered or certified, postage prepaid and addressed to the party to be
notified at the address indicated below such party's signature line on this
Agreement or at such other address as such party may designate by 10 days'
advance written notice under this Section 5.2 to the other party to this
Agreement.

            5.3   Governing Law. This agreement shall be construed and enforced
in accordance with, and shall be governed by, the laws of the State of
California.

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

                                       10
<PAGE>   11
            5.5   Successors and Assigns. The provisions of this Agreement shall
inure to the benefit of, and be binding upon, the Company and its successors and
assigns and Purchaser and Purchaser's legal representatives, heirs, legatees,
distributees, assigns and transferees by operation of law, whether or not any
such person shall have become a party to this Agreement and have agreed in
writing to join herein and be bound by the terms and conditions hereof.

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

                                        SIGNAL PHARMACEUTICALS, INC.

                                        By: /s/ LUKE B. EVNIN
                                            ------------------------------------
                                            Luke B. Evnin, President

                                   Address: 11545 Sorrento Valley Road
                                            San Diego, CA 92121

                                        THE REGENTS OF THE UNIVERSITY OF
                                        CALIFORNIA

                                        By: /s/ WILLIAM T. DAVIS
                                            ------------------------------------
                                       Its: Assoc. Dir OTT
                                            ------------------------------------
                                   Address: 1320 Harbor Bay Parkway
                                            ------------------------------------
                                            Alameda CA 94502
                                            ------------------------------------

Approval as to legal form: P. MARTIN SIMPSON JR.                       10/1/93
                           ---------------------------------------------------
                           P. MARTIN SIMPSON, JR., Resident Counsel      Date
                           Office of Technology Transfer
                           University of California

                                       11

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