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                                                                     EXHIBIT 4.2

      THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED WITH THE
      SECURITIES AND EXCHANGE COMMISSION IN RELIANCE UPON AN EXEMPTION FROM
      REGISTRATION UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"),
      AND, ACCORDINGLY, MAY NOT BE OFFERED OR SOLD EXCEPT PURSUANT TO AN
      EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT OR PURSUANT TO AN AVAILABLE
      EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE REGISTRATION
      REQUIREMENTS OF THE ACT.

Originally Issued on                                                $ 15,000,000
June 29, 2000 and
Amended and Restated on
November 9, 2001

                                GERON CORPORATION
              SERIES D AMENDED AND RESTATED CONVERTIBLE DEBENTURES

            Geron Corporation, a Delaware corporation (the "Issuer"), for value
received hereby promises to pay to RGC International Investors, LDC or its
registered assigns the principal sum of Fifteen Million Dollars ($15,000,000),
together with all accrued but unpaid interest thereon, on June 30, 2005 in such
coin or currency of the United States of America as at the time of payment shall
be legal tender for the payment of public and private debts at the last address
of the Holder (as defined herein) last appearing on the Register (as defined
herein).

            This Security is one of a duly authorized issue of Series D
convertible debentures of the Issuer (the "Security") referred to in the
Securities Purchase Agreement (the "Purchase Agreement"), dated as of June 29,
2000, by and between the Issuer and the Purchaser (as defined in the Purchase
Agreement) and amended pursuant to that certain Restructuring Agreement, dated
November 9, 2001, by and between the Issuer and the Purchaser (the
"Restructuring Agreement"). In connection with the Restructuring Agreement, the
Purchaser converted Ten Million Dollars ($10,000,000) of outstanding principal
amount of this Security into shares of Common Stock and amended the terms of the
Security for the remaining outstanding principal balance of Fifteen Million
Dollars ($15,000,000) pursuant to this instrument. The Securities are subject to
the terms and conditions of the Purchase Agreement, a copy of which is on file
at and may be obtained from the Issuer at its principal office at the address
set forth in Section 4.5 hereof. The Issuer agrees to issue from time to time
replacement Securities in the form hereof to facilitate any transfers and
assignments. In addition, after delivery of an indemnity in form and substance
satisfactory to the
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Issuer, the Issuer also agrees to issue replacement Securities for securities
which have been lost, stolen, mutilated or destroyed.

            The Issuer shall keep at its principal office a register (the
"Register") in which shall be entered the names and addresses of the registered
holders of the Securities and particulars of the respective Securities held by
them and of all transfers of such Securities. References to the "Holder" or
"Holders" shall mean the Person listed in the Register as the payee of any
Security unless the payee shall have presented such Security to the Issuer for
transfer and the transferee shall have been entered in the Register as a
subsequent holder, in which case the term shall mean such subsequent holder. The
ownership of the Securities shall be proven by the Register. For the purpose of
paying interest and principal on the Securities, the Issuer shall been entitled
to rely on the names and addresses in the Register and notwithstanding anything
to the contrary contained in this Security, no Event of Default shall occur
under Section 3.1 if payment of principal is made in accordance with the names
and addresses and particulars contained in the Register.

            No provision of this Security shall alter or impair the obligations
of the Issuer, which are absolute and unconditional, to pay the principal of and
accrued interest on this Security at the place, times, rate, and in the
currency, herein prescribed.

            The principal of this Security shall bear interest at the rate (the
"Interest Rate") of two and one-half percent (2 1/2%) per annum beginning on
November 9, 2001, except in the case of an Event of Default (as defined in
Article III hereof), in which case the Interest Rate of this Security shall
thereafter be seven percent (7%) per annum (to the extent that the payment of
such interest shall be legally enforceable), which shall accrue from the date of
such Event of Default to the date such default has been cured or waived and
overdue principal, if any, has been paid or duly provided for. Such interest
will be computed on the basis of a 365-day year (or 366 days in the case of a
leap year). Interest on any overdue principal shall be payable on demand.
Payment of the principal of and any such interest on this Security will be at
the offices of the Holder of this Security.

            Interest will be computed on the basis of a fraction, the
denominator of which is 365 (or 366 for any leap year) and the numerator of
which is the actual number of days elapsed from the date such interest becomes
due and payable.

            The applicable Interest Rate shall be effective both before and
after any judgment may be rendered in a court of competent jurisdiction,
provided, however, that if the applicable Interest Rate is deemed to be in
excess of the amount permitted to be charged by the Issuer under applicable
laws, the Holder shall be entitled to collect an Interest Rate only at the
highest rate permitted by law, and any interest collected by the Holder in
excess of such lawful amount shall be deemed a payment in reduction of the
Principal Amount then outstanding under this Security and shall be so applied.

            The payment obligations evidenced by this Security shall rank senior
to all other Debt of the Issuer in existence as of the date of issuance of this
Security. The Issuer shall not

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hereafter and while this Security is outstanding issue directly or indirectly
any Debt for Money Borrowed which is senior to the indebtedness evidenced by
this Security.

                                    ARTICLE I

                                   DEFINITIONS

      1.1 CERTAIN TERMS DEFINED. The following terms (except as otherwise
expressly provided or unless the context otherwise clearly requires) for all
purposes of this Security shall have the respective meanings specified below.
All accounting terms used herein and not expressly defined shall have the
meanings given to them in accordance with generally accepted accounting
principles, and the term "generally accepted accounting principles" shall mean
such accounting principles which are generally accepted as of the date hereof.
The terms defined in this Section 1.1 include the plural as well as the
singular.

            "Acceleration Notice" shall have the meaning set forth in Section
3.1.

            "Affiliate" of any Person means any other Person directly or
indirectly controlling or controlled by or under direct or indirect common
control with such Person. For the purposes of this definition, "control" when
used with respect to any Person means the possession, directly or indirectly, of
the power to direct or cause the direction of the management and policies of
such Person, whether through the ownership of voting securities, by contract or
otherwise; and the terms "controlling" and "controlled" have meanings
correlative to the foregoing.

            "Amended Series D Warrants" has the meaning set forth in the
Restructuring Agreement.

            "Average Price" on any date means (x) the sum of the Per Share
Market Value for the ten (10) Trading Days immediately preceding such date minus
(y) the highest and lowest Per Share Market Values during the ten (10) Trading
Days immediately preceding such date, divided by (z) eight.

            "Board of Directors" means either the Board of Directors of the
Issuer or any committee of such Board duly authorized to act hereunder.

            "Business Day" means any day except a Saturday, Sunday or other day
on which commercial banks in the City of New York are authorized by law to
close.

            "Capital Stock" means, with respect to any Person, any and all
shares, interests, participations or other equivalents (however designated) of
such Person's capital stock whether now outstanding or issued after the original
Issue Date, including, without limitation, all Common Stock and all Preferred
Stock.

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            "Change of Control" means the occurrence of one or more of the
following events: the occurrence of any of (i) an acquisition after the date
hereof by an individual or legal entity or "group" (as described in Rule
13d-5(b)(1) promulgated under the Exchange Act), other than the Buyers or any of
their Affiliates, of in excess of 50% of the voting securities of the Issuer,
(ii) a replacement of more than one-half of the members of the Issuer's Board of
Directors which is not approved by those individuals who are members of the
Board of Directors on the date hereof in one or a series of related
transactions, (iii) the merger of the Issuer with or into another entity,
consolidation or sale of all or substantially all of the assets of the Issuer in
one or a series of related transactions or (iv) the execution by the Issuer of
an agreement to which the Issuer is a party or by which it is bound, providing
for any of the events set forth in (i), (ii) or (iii).

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

            "Closing Date" shall have the meaning set forth in the Purchase
Agreement.

            "Conversion Price" shall have the meaning set forth in Section 4.2.

            "Debt" of any Person means, at any date, without duplication, (i)
all obligations of such Person for borrowed money, (ii) all obligations of such
Person evidenced by bonds, debentures, notes or other similar instruments, (iii)
all obligations of such Person in respect of letters of credit or bankers'
acceptance or other similar instruments (or reimbursement obligations with
respect thereto), (iv) all obligations of such Person to pay the deferred
purchase price of property or services, (v) all obligations of such Person as
lessee undercapitalized leases, (vi) all Debt of others secured by a Lien on any
asset of such Person, whether or not such Debt is assumed by such Person,
provided that for purposes of determining the amount of any Debt of the type
described in this clause, if recourse with respect to such Debt is limited to
such asset, the amount of such Debt shall be limited to the fair market value of
such asset, (vii) all Debt of others guaranteed by such Person, and (viii) all
redeemable stock valued at the greater of its voluntary or involuntary
liquidation preference plus accrued and unpaid dividends.

            "Debt for Money Borrowed" of any Person means at any date, without
duplication, Debt of the type referred to in clauses (i) and (ii) of the
definition of "Debt" set forth herein.

            "Exchange Act" means the Securities Exchange Act of 1934, as
amended.

            "GAAP" or "generally accepted accounting principles" means generally
accepted accounting principles in the United States, including, without
limitation, those set forth in the opinions and pronouncements of the Accounting
Principles Board of the American Institute of Certified Public Accountants and
statements and pronouncements of the Financial Accounting Standards Board or in
such other statements by such other entity as approved by a significant segment
of the accounting profession.

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            "Holder", "Holder of Securities", "Securityholders" or other similar
terms means the registered holder of any Security.

            "Incurrence" means the incurrence, creation, assumption or in any
other manner becoming liable with respect to, or the extension of the maturity
of or becoming responsible for the payment of, any Debt. "Incur" shall have a
comparable meaning.

            "Issuer" shall have the meaning set forth in the first paragraph
hereof.

            "Lien" means, with respect to any asset, any mortgage, lien, pledge,
charge, security interest or encumbrance of any kind in respect of such asset.
For the purposes of this Security, the Issuer shall be deemed to own subject to
a Lien any asset which it has acquired or holds subject to the interest of a
vendor or lessor under any conditional sale agreement, capital lease or other
title retention agreement relating to such asset.

            "Mandatory Prepayment Amount" for any Security means the greater of
(i) the sum of (x) 115% of the principal amount of the Security to be prepaid
plus all accrued and unpaid interest thereon and (y) all other amounts, costs,
interest, expenses and liquidated damages due in respect of such principal
amount and (ii) the sum of (x) at the option of the Holder, either (I) the
principal amount of the Security to be repaid, plus all accrued and unpaid
interest thereon, divided by the Conversion Price on the date the Mandatory
Prepayment Amount is demanded or otherwise due, multiplied by the Average Price
on the date the Mandatory Prepayment Amount is demanded or otherwise due or (II)
the principal amount of the Security to be prepaid, plus all accrued and unpaid
interest thereon, divided by the Conversion Price on the Trading Day immediately
prior to the date the Mandatory Prepayment Amount is paid in full, multiplied by
the Average Price on the Trading Day immediately prior to the date the Mandatory
Prepayment Amount is paid in full, and (y) all other amounts, costs, interest,
expenses and liquidated damages due in respect of such principal amount.

            "Nasdaq" means the Nasdaq National Market.

            "Original Issue Date" of any Security (or portion thereof) means the
earlier of (i) the date of such Security and (ii) the date of any Security (or
portion thereof) for which such security was issued (directly or indirectly) on
registration of transfer, exchange or substitution.

            "Per Share Market Value" means (i) on any particular date the
closing bid price per share of the Common Stock on such date on Nasdaq or any
Subsequent Market on which the Common Stock is then listed or if there is no
such price on such date, then the closing bid price on such exchange or
quotation system on the date nearest preceding such date or (ii) if the Common
Stock is not listed then on Nasdaq or any Subsequent Market, the closing bid
price for a share of Common Stock in the over-the-counter market, as reported by
the National Quotation Bureau Incorporated (or similar organization or agency
succeeding to its functions of reporting prices) at the close of business on
such date, or (iii) if the Common Stock is not then publicly traded the fair

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market value of a share of Common Stock as determined by an appraiser selected
in good faith by the holder of this Security; provided, however, that the
Issuer, after receipt of the determination by such appraiser, shall have the
right to select an additional appraiser, in which case, the fair market value
shall be equal to the average of the determinations by each such appraiser; and
provided, further that all determinations of the Per Share Market Value shall be
appropriately adjusted for any stock dividends, stock splits or other similar
transactions during such period.

            "Person" means an individual, a corporation, a partnership, an
association, a trust or any other entity or organization, including a government
or political subdivision or an agency or instrumentality thereof.

            "Preferred Stock" means, with respect to any Person, any and all
shares, interests, participations or other equivalents (however designated) of
such Person's preferred or preference stock whether now outstanding or issued
after the date of this Security, and includes, without limitation, all classes
and series of preferred or preference stock.

            "Principal", wherever used with reference to the Securities or any
Security or any portion thereof, shall be deemed to include "and interest, if
any."

            "Property" of any Person means all types of real, personal,
tangible, intangible or mixed property owned by such Person whether or not
included in the most recent consolidated balance sheet of such Person under
generally accepted accounting principles.

            "Purchase Price" means, with respect to any Security, the purchase
price paid to the Issuer upon issuance of such Security.

            "Purchaser" shall have the meaning ascribed thereto in the
Restructuring Agreement.

            "Registration Rights Agreement" means that Registration Rights
Agreement dated as of June 29, 2000 by and between the Issuer and the Initial
Investor (as defined in the Registration Rights Agreement).

            "Security" or "Securities" shall have the meaning set forth in the
second paragraph hereof.

            "Stated Maturity Date" means June 30, 2005.

            "Stock Plan" means any stock or compensation plan pursuant to which
Common Stock may be issued to any employee, officer, director or consultant of
the Issuer which is either (a) approved by the stockholders of the Issuer or (b)
approved by the compensation committee of the Issuer's Board of Directors for
legitimate compensation purposes which provides for the purchase of the Common
Stock at a purchase price of no less than 85% of the market price of the Common
Stock on the date of issuance of such option, warrant or security.

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            "Subsidiary" means any corporation or other organization, whether
incorporated or unincorporated, in which the Issuer owns, directly or
indirectly, any equity or other ownership interest and which would be deemed to
be a "significant subsidiary" (as such term is defined in Rule 1-02(w) of
Regulation S-X promulgated under the Securities Act of 1933, as amended (the
"Securities Act")).

            "Subsequent Market" means the New York Stock Exchange, American
Stock Exchange or Nasdaq SmallCap Market.

            "Trading Day" means (a) a day on which the Common Stock is traded on
Nasdaq or on such Subsequent Market on which the Common Stock is then listed or
quoted or (b) if the Common Stock is not listed on Nasdaq or a Subsequent
Market, a day on which the Common Stock is traded in the over-the-counter
Market, as reported by the OTC Bulletin Board, or (c) if the Stock is not quoted
on the OTC Bulletin Board, a day on which the Common Stock is quoted in the
over-the-countermarket as reported by the National Quotation Bureau Incorporated
(or any similar organization or agency succeeding its functions or reporting
prices) provided, however that in any event that the Common Stock is not listed
or quoted as set forth in (a), (b), or (c) hereof, then a Trading Day shall mean
any Business Day.

            "Wholly-Owned Subsidiary" means with respect to any Person a
Subsidiary the voting stock of which is more than 90% owned by such Person.

                                   ARTICLE II

                             PAYMENT; THE SECURITIES

      2.1 PAYMENT OF PRINCIPAL AND INTEREST. The Issuer covenants and agrees
that it will duly and punctually pay or cause to be paid the principal and
interest (to the extent enforceable under applicable law), with respect to each
of the Securities at the place or places, at the respective times and in the
manner provided in the Securities.

      2.2 MUTILATED, DEFACED, DESTROYED, LOST AND STOLEN SECURITIES. In case any
temporary or definitive Security shall become mutilated, defaced or be
apparently destroyed, lost or stolen, the Issuer shall execute and deliver a new
Security, bearing a number not contemporaneously outstanding, in exchange and
substitution for the mutilated or defaced Security. In every case the applicant
for a substitute Security shall furnish to the Issuer such security or indemnity
as it may require to indemnify and defend and to save it harmless and, in every
case of destruction, loss or theft evidence to the Issuer's satisfaction of the
apparent destruction, loss or theft of such Security and of the ownership
thereof.

            Upon the issuance of any substitute Security, the Issuer may require
the payment of a sum sufficient to cover any tax or other governmental charge
that may be imposed in relation

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thereto and any other expenses connected therewith. In case any Security which
has matured or is about to mature, or has been called for redemption in full, or
is being surrendered for conversion in full shall become mutilated or defaced or
be apparently destroyed, lost or stolen, the Issuer may, instead of issuing a
substitute Security, with the holder's consent, pay or authorize the payment or
conversion of the same (without surrender thereof except in the case of a
mutilated or defaced Security), if the applicant for such payment shall furnish
to the Issuer such security or indemnity as it may require to save it harmless
from all risks, however remote, and, in every case of apparent destruction, loss
or theft, the applicant shall also furnish to the Issuer evidence to the
Issuer's satisfaction of the apparent destruction, loss or theft of such
Security and of the ownership thereof.

            Every substitute Security issued pursuant to the provisions of this
Section by virtue of the fact that any Security is apparently destroyed, lost or
stolen shall constitute an additional contractual obligation of the Issuer,
whether or not the apparently destroyed, lost or stolen Security shall be at any
time enforceable by anyone and shall be entitled to all the benefits of (but
shall be subject to all the limitations of rights set forth in) this Security
equally and proportionately with any and all other Securities duly authenticated
and delivered hereunder. All Securities shall be held and owned upon the express
condition that, to the extent permitted by law, the foregoing provisions are
exclusive with respect to the replacement or payment or conversion of mutilated,
defaced, or apparently destroyed, lost or stolen Securities and shall preclude
any and all other rights or remedies notwithstanding any law or statute existing
or hereafter enacted to the contrary with respect to the replacement or payment
of negotiable instruments or other securities without their surrender.

      2.3 CANCELLATION OF SECURITIES; DESTRUCTION THEREOF. All Securities
surrendered for payment, redemption, registration of transfer or exchange shall
be delivered to the Issuer for cancellation and no Securities shall be issued in
lieu thereof except as expressly permitted by any of the provisions of this
Security. If the Issuer shall acquire any of the Securities, such acquisition
alone shall not operate as a redemption or satisfaction of the indebtedness
represented by such Securities unless and until such indebtedness is satisfied.

                                   ARTICLE III

                                    DEFAULTS

      3.1 EVENT OF DEFAULT DEFINED; ACCELERATION OF MATURITY; WAIVER OF DEFAULT.
In case one or more of the following events ("Events of Default") (whatever the
reason for such Event of Default and whether it shall be voluntary or
involuntary or be effected by operation of law or pursuant to any judgment,
decree or order of any court or any order, rule or regulation of any
administrative or governmental body) shall have occurred and be continuing:

            (a) default in the payment of all or any part of the principal of or
interest on any of the Securities as and when the same shall become due and
payable either at maturity, upon any redemption, by declaration or otherwise; or

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            (b) failure on the part of the Issuer duly to observe or perform any
other of the covenants or agreements on the part of the Issuer contained in this
Security, the Restructuring Agreement, the Purchase Agreement or the
Registration Rights Agreement for a period of five (5) Business Days after the
date on which any officer of the Issuer shall have obtained actual knowledge of
such failure or after written notice thereof has been given to the Issuer by the
holders of at least a majority in aggregate principal amount of the Securities
then outstanding; or

            (c) there shall have occurred with respect to any issue or issues of
Debt of the Issuer and/or one or more Subsidiaries having an outstanding
principal amount of $1,000,000 or more in the aggregate for all such issues of
all such Persons, whether such Debt now exists or shall hereafter be created, an
event of default which has caused the holder thereof to declare such debt to be
due and payable prior to its stated maturity and such Debt has not been
discharged in full or such acceleration has not been rescinded or annulled
within 30 days of such acceleration; or

            (d) a judgment or order (not covered by insurance) for the payment
of money shall be rendered against the Issuer or any Subsidiary of the Issuer in
excess of $500,000 in the aggregate for all such judgments or orders against all
such Persons (treating any deductibles, self insurance or retention as not so
covered) that shall not be discharged, and all such judgments and orders remain
outstanding and there shall be any period of 30 consecutive days following entry
of the judgment or order in excess of $500,000 or the judgment or order which
causes the aggregate amount described above to exceed $500,000 during which a
stay of enforcement of such judgment or order, by reason of a pending appeal or
otherwise, shall not be in effect; or

            (e) a court having jurisdiction in the premises shall enter a decree
or order for relief in respect of the Issuer or any of its subsidiaries in an
involuntary case under any applicable bankruptcy, insolvency or other similar
law now or hereafter in effect, or appointing a receiver, liquidator, assignee,
custodian, trustee, sequestrator (or similar official) of the Issuer or any of
its Subsidiaries or for any substantial part of the property of the Issuer or
any of its Subsidiaries or ordering the winding up or liquidation of the affairs
of the Issuer or any of its Subsidiaries, and such decree or order shall remain
unstayed and in effect for a period of 30 consecutive days; or

            (f) the Issuer or any of its Subsidiaries shall commence a voluntary
case under any applicable bankruptcy, insolvency or other similar law now or
hereafter in effect, or consent to the entry of an order for relief in an
involuntary case under any such law, or consent to the appointment or taking
possession by a receiver, liquidator, assignee, custodian, trustee, sequestrator
(or similar official) of the Issuer or any of its Subsidiaries or for any
substantial part of the property of the Issuer or any of its Subsidiaries, or
the Issuer or any of its Subsidiaries shall make any general assignment for the
benefit of creditors; or

            (g) any representation, warranty, certification or statement made by
the Issuer in the Purchase Agreement, the Restructuring Agreement or in any
certificate, financial statement or other document delivered pursuant to the
Purchase Agreement or the Restructuring Agreement shall prove to have been
incorrect in any material respect when made; or

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            (h) the Common Stock shall be delisted from Nasdaq or shall be
suspended from trading on Nasdaq without resuming trading and/or being relisted
thereon or on a Subsequent Market or having such suspension lifted, as the case
may be, within three Business Days; or

            (i) (Y) the Issuer fails to obtain the effectiveness of any
additional Registration Statement (required to be filed pursuant to Section 3(b)
of the Registration Rights Agreement) within ninety (90) days after the
Registration Trigger Date (as defined in the Registration Rights Agreement), or
(Z) any such Registration Statement, after its initial effectiveness and during
the Registration Period (as defined in the Registration Rights Agreement),
lapses in effect or sales of all of the Registrable Securities (as defined in
the Registration Rights Agreement, the "Registrable Securities") otherwise
cannot be made thereunder (whether by reason of the Issuer's failure to amend or
supplement the prospectus included therein in accordance with the Registration
Rights Agreement, the Issuer's failure to file and obtain effectiveness with the
SEC of an additional Registration Statement required to be filed pursuant to
Section 3(b) of the Registration Rights Agreement or otherwise) for more than
thirty (30) consecutive days or more than sixty (60) days in any twelve (12)
month period after such Registration Statement becomes effective, excluding for
purposes of clause (Z) above only, days during the Allowed Delay period (as
defined in the Registration Rights Agreement) and excluding for purposes of
clause (Y) above only, a delay caused by Purchaser's failure to comply with
Sections 3(h) and 4(a) of the Registration Rights Agreement; or

            (j)     a Change of Control shall occur;

then, in each and every such case (other than an Event of Default specified in
Section 3.1(e) or 3.1(f)hereof), unless the principal shall have already become
due and payable, by notice in writing to the Issuer (the "Acceleration Notice"),
a Holder may declare the entire principal amount of the Securities and any
interest accrued thereon to be due and payable immediately, and upon any such
declaration the same shall become immediately due and payable. If an Event of
Default specified in Section 3.1(e) or 3.1(f) occurs, the principal of and any
accrued interest on the Securities shall become and be immediately due and
payable without any declaration or other act on the part of any Holder. In the
event that the Issuer shall not have promptly, but in any event within five (5)
Business Days of receipt of an Acceleration Notice, paid the Holder the amount
specified herein as due and payable in respect of such Event of Default, the
Conversion Price shall automatically be adjusted to equal the average Per Share
Market Value of the Common Stock during the preceding thirty (30) consecutive
Trading Days; provided, that the Per Share Market Value is lower than the
Conversion Price.

            The aggregate amount payable upon an Event of Default described in
Section 3.1(a), (e), (f) and (i) shall be equal to the sum of (I) the Mandatory
Prepayment Amount plus (II) the Mandatory Prepayment Amount for the principal
amount of the Securities (the "Converted Debentures") that would then be held by
such Holder had the principal amount of Securities converted into Debenture
Shares (as defined in the Purchase Agreement) that are then held by the

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Holder not been so converted; provided, that the Holder shall not be entitled to
a Mandatory Prepayment Amount with respect to Converted Debentures if both the
following have occurred: (i) prior to the occurrence of an Event of Default, the
Debenture Shares into which the Converted Debentures were converted had been
held by the Holder for more than thirty (30) days (which period shall be
extended at the time of occurrence of the Event of Default for the number of
Trading Days during such 30-day period that an Allowed Delay (as defined in the
Registration Rights Agreement) is in effect) and (ii) prior to the occurrence of
the Event of Default and after receipt by the Holder of the Debenture Shares
that are held by the holder at the time of the occurrence of the Event of
Default, the Registration Statement with respect to such Conversion Shares had
been continuously effective for thirty (30) Trading Days.

            The aggregate principal amount payable on each Event of Default
other than as described in Section 3.1(a), (e),(f) and (i) shall be equal to the
sum of (I) the Mandatory Prepayment Amount plus (II) the Mandatory Prepayment
Amount for the Converted Debentures that would then be held by such Holder had
the principal amount of Securities converted into Debenture Shares that are then
held by the Holder not been so converted; provided, that the holder shall not be
entitled to a Mandatory Prepayment Amount with respect to Converted Debentures
if prior to the occurrence of an Event of Default, the Debenture Shares into
which the Converted Debentures were converted had been held by the Holder for
more than three Trading Days (which period shall be extended at the time of
occurrence of the Event of Default for the number of Trading Days during such
3-day period that an Allowed Delay is in effect).

            For purposes of this Section 3.1, principal amount of the Securities
are outstanding until such date as the holder shall have received Debenture
Shares upon a conversion (or attempted conversion) thereof. Interest shall
accrue on the prepayment amount hereunder from the day after such amount is due
(being the date of an Event of Default) through the date of payment in full
thereof at the rate of seven percent (7%) per annum, accruing daily from the
date of conversion until such amount, plus any interest thereon, if any, is paid
in full. Payment of the Mandatory Prepayment Amount pursuant to this Section 3.1
shall be in addition to any other amounts that may be due to the Holder pursuant
to this Security. Within five (5) Business Days of receipt by the Holder of
payments of amounts due to the Holder, (i) the Holder shall return the
Securities to the Issuer and (ii) in the event the Mandatory Prepayment Amount
relates to the Converted Debentures, the Holder shall return the Debenture
Shares into which such Converted Debentures were converted. In the event of the
occurrence of an Event of Default, the Holder need not provide and the Issuer
hereby waives any presentment, demand, protest or other notice of any kind, and
the Holder may immediately and without expiration of any grace period enforce
any and all of its rights and remedies hereunder and all other remedies
available to it under applicable law. Any demand for payment may be rescinded
and annulled by the Holder at any time prior to payment hereunder. No such
rescission or annulment shall affect any subsequent Event of Default or impair
any right consequent thereon.

            Upon delivery of any Acceleration Notice to the Issuer, the Issuer
shall provide a copy of such notice to the other Holders, if any. Failure to
deliver such notice shall not affect the validity of the notice delivered by the
Holders in accordance with the provisions referred to above.

                                       11
<PAGE>
      3.2 POWERS AND REMEDIES CUMULATIVE; DELAY OR OMISSION NOT WAIVER OF
DEFAULT. No right or remedy herein conferred upon or reserved to the Holders is
intended to be exclusive of any other right or remedy, and every right and
remedy shall, to the extent permitted by law, be cumulative and in addition to
every other right and remedy given hereunder or now or hereafter existing at law
or in equity or otherwise. The assertion or employment of any right or remedy
hereunder, or otherwise, shall not prevent the concurrent assertion or
employment of any other appropriate right or remedy.

            No delay or omission of the Holders to exercise any right or power
accruing upon any Event of Default occurring and continuing as aforesaid shall
impair any such right or power or shall be construed to be a waiver of any such
Event of Default or an acquiescence therein; and every power and remedy given by
the Securities or by law may be exercised from time to time, and as often as
shall be deemed expedient, by the Holders.

                                   ARTICLE IV

                              EXCHANGE; CONVERSION

      4.1 RIGHT OF SECURITYHOLDERS TO EXCHANGE SECURITIES. Subject to and upon
compliance with the provisions of this Section, this Security is exchangeable
for an equal principal amount of debentures of different authorized
denominations, as requested by the holder surrendering the same. No service
charge will be made for such registration of transfer or exchange.

      4.2 RIGHT OF SECURITYHOLDERS TO CONVERT SECURITIES INTO COMMON STOCK.
Subject to and upon compliance with the provisions of this Section, the
principal amount of this Security, or any portion thereof, together with accrued
but unpaid interest thereon, may, at any time and at or before the close of
business on June 30, 2005, be converted into duly authorized, validly issued,
fully-paid and non-assessable shares of Common Stock at $20.00 per share (the
"Conversion Price"), or, in case an adjustment in the Conversion Price and the
securities or other property issuable upon conversion has taken place pursuant
to Article III or IV hereof, then at the applicable Conversion Price, as
adjusted, and in such securities or other property as so adjusted, upon
surrender to the Issuer at any time prior to 8:00 p.m., New York City time, on
any Business Day at the Issuer's offices, of a written notice of election to
convert as provided in the form attached hereto as Exhibit A (a "Conversion
Notice").

      4.3 SURRENDER OF SECURITY UPON CONVERSION. Notwithstanding anything to the
contrary set forth herein, upon conversion of this Security in accordance with
the terms thereof, the Holder of this Security shall not be required to
physically surrender the Security to the Issuer unless the entire unpaid
principal amount of the Security is so converted. The Holder and the Issuer
shall maintain records showing the principal amount so converted and the dates
of such conversions or shall use such other method, reasonably satisfactory to
the Holder and the Issuer, so as not to require physical surrender of the
Security upon each such conversion. In the event of any dispute or

                                       12
<PAGE>
discrepancy, such records of the Issuer shall be controlling and determinative
in the absence of manifest error. Notwithstanding the foregoing, if any portion
of the Security is converted as aforesaid, the Holder may not transfer the
Security unless the Holder first physically surrenders the Security to the
Issuer, whereupon the Issuer will forthwith issue and deliver upon the order of
the Holder a new Security of like tenor, registered as the Holder (upon payment
by the Holder of any applicable transfer taxes) may request, representing in the
aggregate the remaining unpaid principal amount of the Security. The Holder and
any assignee, by acceptance of the Security, acknowledge and agree that, by
reason of the provisions of this paragraph, following conversion of a portion of
a Security, the unpaid and unconverted principal amount of such Security
represented by such Security may be less than the amount stated on the face
thereof.

      4.4 ADJUSTMENT FOR DIVIDENDS. No payment or adjustment will be made for
dividends on any Common Stock except as provided herein. On conversion of a
Security, that portion of interest accrued and unpaid interest attributable to
the period from the Original Issuance Date to the Conversion Date with respect
to the converted Security shall not be canceled, extinguished or forfeited, but
rather shall be deemed to be paid in full to the Holder thereof through delivery
of the Common Stock, in exchange for the Security being converted pursuant to
the provisions hereof. If the Holder converts more than one Security at the same
time, the number of shares of Common Stock issuable upon the conversion shall be
based on the total principal amount of the Securities converted.

      4.5   ISSUANCE OF SHARES UPON CONVERSION.

            (a) As promptly as practicable after the delivery of a Conversion
Notice, as herein provided, but in any event within three Trading Days of the
date of such Conversion Notice (so long as the Conversion Notice was sent and
confirmed via facsimile to the Issuer prior to 8:00 p.m., New York City time, on
the date specified therein as the conversion date), the Issuer shall deliver or
cause to be delivered at its said office or agency to or upon the written order
of the holder of the Security or Securities so converted a certificate or
certificates representing the number of duly authorized, validly issued,
fully-paid and non-assessable shares of Common Stock, into which such Security
or Securities may be converted in accordance with the provisions of this Article
IV. The Conversion Notice shall state that the holder irrevocably elects to
convert such Security or Securities, or, if less than the entire principal
amount thereof is to be converted, the portion thereof to be converted. Such
notice shall also state the name or names (with address and social security or
other taxpayer identification number) in which said certificate or certificates
are to be issued. Such conversion shall be deemed to have been made on the date
specified in the Conversion Notice as the date of conversion, the rights of the
holder of such Security or Securities as a Holder shall cease at such time, the
person or persons entitled to receive the shares of Common Stock upon conversion
of such Security or Securities shall be treated for all purposes as having
become the record holder or holders of such shares of Common Stock at such time
and such conversion shall be at the Conversion Price in effect at such time. In
the case of any Security which is converted in part only, upon such conversion,
the Issuer shall execute and deliver to the holder thereof, as requested by such

                                       13
<PAGE>
holder, a new Security or securities of authorized denominations in aggregate
principal amount equal to the unconverted portion of such Security.

            (b) In lieu of delivering physical certificates representing the
Common Stock issuable upon conversion of this Security, provided the Issuer's
transfer agent is participating in the Depository Trust Company ("DTC") Fast
Automated Securities Transfer ("FAST") program, upon request of the Holder and
its compliance with the provisions contained in this Section 4.5, the Issuer
shall use its best efforts to cause its transfer agent to electronically
transmit the Common Stock issuable upon conversion to the Holder by crediting
the account of Holder's Prime Broker with DTC through its Deposit Withdrawal
Agent Commission ("DWAC") system (to the extent not inconsistent with any
provisions of the Purchase Agreement).

      4.6 ADJUSTMENT OF CONVERSION PRICE. The Conversion Price in effect at any
time shall be subject to adjustment from time to time upon the occurrence of an
Event of Default as set forth in Article III and upon the happening of certain
events, as follows:

            (a) Common Stock Dividends; Common Stock Splits; Reverse Common
Stock Splits. If the Issuer, at any time while this Security is outstanding, (a)
shall pay a stock dividend on its Common Stock, (b) subdivide outstanding shares
of Common Stock into a larger number of shares, (c) combine outstanding shares
of Common Stock into a smaller number of shares, or (d) issue by
reclassification of shares of Common Stock any shares of capital stock of the
Issuer, the Conversion Price shall be multiplied by a fraction the numerator of
which shall be the number of shares of Common Stock (excluding treasury shares,
if any) outstanding before such event and the denominator of which shall be the
number of shares of Common Stock outstanding after such event. Any adjustment
made pursuant to this paragraph 4.6(a) shall become effective immediately after
the record date for the determination of stockholders entitled to receive such
dividend or distribution and shall become effective immediately after the
effective date in the case of a subdivision, combination or re-classification.

            (b)     [INTENTIONALLY OMITTED.]

            (c)     [INTENTIONALLY OMITTED.]

            (d)     Rounding.  All calculations under this section 4.6 shall
be made to the nearest cent or the nearest l/l00th of a share, as the case
maybe.

            (e) Notice of Adjustment. Whenever the Conversion Price is adjusted
pursuant to paragraph 4.6(a), the Issuer shall promptly mail to the holder of
this Security, a notice setting forth the Conversion Price after such adjustment
and setting forth a brief statement of the facts requiring such adjustment.

            (f)     Reclassification, Etc. If:

                                       14
<PAGE>
                    A.    the Issuer shall declare a dividend (or any other
                          distribution) on its Common Stock; or

                    B.    the Issuer shall declare a special nonrecurring
                          cash dividend on or a redemption of its Common
                          Stock; or

                    C.    the Issuer shall authorize the granting to all the
                          holders of the Common Stock rights or warrants to
                          subscribe for or purchase any shares of capital
                          stock of any class or of any rights; or

                    D.    the approval of any shareholders of the Issuer
                          shall be required in connection with any
                          reclassification of the Common Stock of the Issuer,
                          any consolidation or merger to which the Issuer is
                          a party, any sale or transfer of all or
                          substantially all of the assets of the Issuer, of
                          any compulsory share exchange whereby the Common
                          Stock is converted into other securities, cash or
                          property; or

                    E.    the Issuer shall authorize the voluntary or
                          involuntary dissolution, liquidation or winding up
                          of the affairs of the Issuer;

then the Issuer shall cause to be filed at each office or agency maintained for
the purpose of conversion of this Security, and shall cause to be mailed to the
holder of this Security, at least 30 calendar days prior to the applicable
record or effective date hereinafter specified, a notice (provided such notice
shall not include any material non-public information) stating (x) the date on
which a record is to be taken for the purpose of such dividend, distribution,
redemption, rights or warrants, or if a record is not to be taken, the date as
of which the holders of Common Stock of record to be entitled to such dividend,
distributions, redemption, rights or warrants are to be determined or (y) the
date on which such reclassification, consolidation, merger, sale, transfer or
share exchange is expected to become effective or close, and the date as of
which it is expected that holders of Common Stock of record shall be entitled to
exchange their shares of Common Stock for securities, cash or other property
deliverable upon such reclassification, consolidation, merger, sale, transfer or
share exchange; provided, however, that the failure to mail such notice or any
defect therein or in the mailing thereof shall not affect the validity of the
corporate action required to be specified in such notice.

      (g)   [INTENTIONALLY OMITTED.]

      4.7 NASDAQ LIMITATION. If on any date (the "Determination Date") (a) the
Common Stock is listed for trading on Nasdaq or the Nasdaq Small Cap Market, (b)
the Conversion Price then in effect is such that the sum of (x) the aggregate
number of shares of Common Stock that would then be issuable upon conversion in
full of the then outstanding principal amount of the Securities and as payment
of interest thereon, plus (y) the aggregate number of shares of Common Stock
that have previously been issued upon conversion of the Securities, plus (z) any
shares of Common Stock

                                       15
<PAGE>
issued or issuable upon conversion of any other securities of the Issuer which
would be aggregated with the Common Stock issuable upon conversion of the
Securities as required by the applicable rules and regulations of Nasdaq (or any
successor entity), would equal or exceed 20% of the number of shares of the
Common Stock outstanding immediately prior to the Closing Date (as defined in
the Purchase Agreement) (such number of shares as would not equal or exceed such
20% limit, the "Issuable Maximum"), and (c) the Issuer shall not have previously
obtained the vote of the stockholders of the Issuer (the "Stockholder
Approval"), if any, as may be required by the applicable rules and regulations
of Nasdaq (or any successor entity) to approve the issuance of shares of Common
Stock in excess of the Issuable Maximum in a private placement whereby shares of
Common Stock are deemed to have been issued at a price that is less than the
greater of book value or fair market value of the Common Stock, then with
respect to the aggregate principal amount of the Securities then held by the
Holders for which a conversion in accordance with the Conversion Price would
result in an issuance of shares of Common Stock in excess of the Issuable
Maximum (the "Excess Principal") the Issuer may elect to prepay cash to the
holders in an amount equal to the Mandatory Prepayment Amount. Any such election
by the Issuer must be made in writing to the Holders within two (2) Trading Days
after the Determination Date and the payment of such Mandatory Prepayment Amount
applicable to such prepayment must be made in full to the Holders with ten (10)
Business Days after the date such notice is delivered. If the Issuer does not
deliver timely a notice of its election to prepay under this Section or shall,
if it shall have delivered such a notice, fail to pay the prepayment amount
hereunder within ten (10) Business Days thereafter, then the Holders of a
majority of the aggregate principal amount of the Securities then outstanding
shall have the option by written notice to the Issuer, to declare any such
notice given by the Issuer, if given, to be null and void and require the Issuer
to pay cash to each Holder in an amount equal to the Mandatory Prepayment Amount
for such Holder's portion of the Excess Principal. The payment of the Mandatory
Prepayment Amount to each Holder pursuant to this Section shall be determined on
a pro rata basis upon the principal amount of the Securities held by such Holder
on the determination Date. If the Issuer fails to pay the Mandatory Prepayment
Amount in full pursuant to this Section within five Business Days after the date
payable, the Issuer will pay interest thereon at a rate of seven percent (7%)
per annum to the converting Holder, accruing interest daily from the date of
conversion until such amount, plus all such interest thereon, if any, is paid in
full.

            In no event shall the Issuer be required to issue shares of Common
Stock upon conversion of the Securities if such issuance would violate the rules
of the Nasdaq Stock Market.

      4.8   [INTENTIONALLY OMITTED.]

      4.9 RESTRICTION ON CONVERSION BY THE HOLDER. Notwithstanding anything in
this Security to the contrary, in no event shall the Holder of this Security be
entitled to convert an amount of Securities in excess of that amount of
Securities upon conversion of which the sum of (i) the number of shares of
Common Stock beneficially owned by the Holder and its affiliates (other than
shares of Common Stock which may be deemed beneficially owned through the
ownership of the unexercised Securities and the unexercised or unconverted
portion of any other securities of the Issuer (including the Amended Series D
Warrants) subject to a limitation on conversion or exercise

                                       16
<PAGE>
analogous to the limitation contained herein) and (ii) the number of shares of
Common Stock issuable upon exercise of the Securities (or portions thereof) with
respect to which the determination described herein is being made, would result
in beneficial ownership by the Holder and its affiliates of more than 9.9% of
the outstanding shares of Common Stock following such conversion. For purposes
of the immediately preceding sentence, beneficial ownership shall be determined
in accordance with Section 13(d) of the Exchange Act and Regulation 13D-G
thereunder, except as otherwise provided in clause (i) hereof. Notwithstanding
anything in this Debenture to the contrary, the restriction on the Holders set
forth in this paragraph shall not be amended without (i) the written consent of
the Holders and the Issuer and (ii) the approval of the holders of a majority of
Issuer's Common Stock present, or represented by proxy, and voting at any
meeting called to vote on the amendment of such restriction.

      4.10 OFFICER'S CERTIFICATE. Whenever the number of shares purchasable upon
conversion shall be adjusted as required by the provisions of Section 4.6, the
Issuer shall forthwith file in the custody of its Secretary or an Assistant
Secretary at its principal office and with its stock transfer agent, if any, an
officer's certificate showing the adjusted number of shares determined as herein
provided, setting forth in reasonable detail the facts requiring such adjustment
and the manner of computing such adjustment. Each such officer's certificate
shall be signed by the chairman, president or chief financial officer of the
Issuer and by the secretary or any assistant secretary of the Issuer. Each such
officer's certificate shall be made available at all reasonable times for
inspection by any holder of the Securities and the Issuer shall, forthwith after
each such adjustment, mail a copy, by first class mail, of such certificate to
the each of the Holders.

      4.11 RESERVATION OF SHARES. The Issuer covenants that it will reserve and
keep available out of its authorized shares of Common Stock, free from
preemptive rights, solely for the purpose of issue upon conversion of this
Security and the exercise of the Amended Series D Warrants, such number of
shares of Common Stock equal to 125% of the shares then issuable upon the
conversion or exercise of all the Securities and Amended Series D Warrants into
Common Stock. The Issuer covenants that all shares of the Common Stock issued
upon conversion of the Security and exercise of the Amended Series D Warrants
which shall be so issuable shall, when issued, be duly and validly issued and
fully paid and non-assessable.

      4.12 COMPLIANCE WITH GOVERNMENTAL REQUIREMENTS. The Issuer covenants that
if any shares of Common Stock required to be reserved for purposes of conversion
of Securities hereunder require registration with or approval of any
governmental authority under any Federal or state law, or any national
securities exchange, before such shares may be issued upon conversion, the
Issuer will use its best efforts to cause such shares to be duly registered or
approved, as the case may be.

      4.13 FRACTIONAL SHARES. Upon a conversion hereunder, the Issuer shall not
be required to issue stock certificates representing fractions of shares of the
Common Stock, but may if otherwise permitted, make a cash payment in respect of
any final fraction of a share based on the Per Share Market value at such time.
If the Issuer elects not, or is unable, to make such a cash payment,

                                       17
<PAGE>
the holder shall be entitled to receive, in lieu of the final fraction of a
share, one whole share of Common Stock.

      4.14 PAYMENT OF TAX UPON ISSUE OF TRANSFER. The issuance of certificates
for shares of the Common Stock on conversion of the Securities shall be made
without charge to the Holders thereof for any documentary stamp or similar taxes
that maybe payable in respect of the issue or delivery of such certificate,
provided that the Issuer shall not be required to pay any tax that may be
payable in respect of any transfer involved in the issuance and delivery of any
such certificate upon conversion in a name other than that of the holder of such
Securities so converted and the Issuer shall not be required to issue or deliver
such certificates unless or until the person or persons requesting the issuance
thereof shall have paid to the Issuer the amount of such tax or shall have
established to the satisfaction of the Issuer that such tax has been paid.

      4.15 NOTICES. Any notice or other communication required or permitted to
be given hereunder shall be in writing and shall be deemed to have been received
(a) upon hand delivery (receipt acknowledged) or delivery by telex (with correct
answer back received), telecopy or facsimile (with transmission confirmation
report) at the address or number designated below (if received by 8:00 p.m. EST
where such notice is to be received), or the first business day following such
delivery (if delivered on a business day after during normal business hours
where such notice is to be received) or (b) on the second business day following
the date of mailing by express courier service, fully prepaid, addressed to such
address, or upon actual receipt of such mailing, whichever shall first occur.
The addresses for such communications are (i) if to the Issuer to Geron
Corporation, 230 Constitution Drive, Menlo Park, California 94025 attn: David
Greenwood, fax no. (650) 473-7701 with copies to Latham & Watkins, 135
Commonwealth Drive, Menlo Park, CA 94025, Attn: Alan C. Mendelson, Esq., fax no.
(650) 463-2600 and (ii) if to any Holder to the address set forth immediately
below such Holder's name on the signature pages to the Restructuring Agreement
or such other address as may be designated in writing hereafter, in the same
manner, by such Person.

                                    ARTICLE V

                                  MISCELLANEOUS

      5.1 MODIFICATION OF SECURITIES. The Securities may be modified without
prior notice to any Holder upon the written consent of the Holders of a majority
in principal amount of the Securities then outstanding. The Holders of a
majority in principal amount of the Securities then outstanding may waive
compliance by the Issuer with any provision of the Securities without prior
notice to any Holder. However, without the consent of each Holder affected, an
amendment, supplement or waiver may not (1) reduce the amount of Securities
whose Holders must consent to an amendment, supplement or waiver, (2) reduce the
principal amount of or extend the fixed maturity of any Security or (3) make any
Security payable in money or property other than as stated in the Securities.

                                       18
<PAGE>
      5.2 MISCELLANEOUS. This Security shall be deemed to be a contract under
the laws of the State of Delaware, and for all purposes shall be construed in
accordance with the laws of said State, except as may otherwise be required by
mandatory provisions of law. The parties hereto, including all guarantors or
endorsers, hereby waive presentment, demand, notice, protest and all other
demands and notices in connection with the delivery, acceptance, performance and
enforcement of this Security, except as specifically provided herein, and assent
to extensions of the time of payment, or forbearance or other indulgence without
notice. The Holder of this Security by acceptance of this Security agrees to be
bound by the provisions of this Security which are expressly binding on such
Holder.

      5.3 SECURITIES OWNED BY ISSUER DEEMED NOT OUTSTANDING. In determining
whether the holders of the requisite aggregate principal amount of Securities
have concurred in any direction, consent or waiver under this Security,
Securities which are owned by the Issuer or any other obligor on the Securities
shall be disregarded and deemed not to be outstanding for the purpose of any
such determination; provided that any Securities owned by any Purchaser shall be
deemed outstanding for purposes of making such a determination. Securities so
owned which have been pledged in good faith may be regarded as outstanding if
the pledgee establishes to the satisfaction of the Issuer the pledgers right so
to act with respect to such Securities and that the pledgee is not the Issuer or
any other obligor upon the securities or any person directly or indirectly
controlling or controlled by or under direct or indirect common control with the
Issuer or any other obligor on the Securities.

      5.4   EFFECT OF HEADINGS.  The Section headings herein are for
convenience only and shall not affect the construction hereof.

      5.5 NO RIGHTS AS STOCKHOLDER. This Security shall not entitle the Holder
to any rights as a stockholder of the Issuer, including without limitation, the
right to vote, to receive dividends and other distributions, or to receive
notice of, or to attend, meetings of stockholders or any other proceedings of
the Issuer, unless and to the extent converted into shares of Common Stock in
accordance with the terms hereof.

      5.6 REMEDIES. The Issuer acknowledges that a breach by it of its
obligations hereunder will cause irreparable harm to the Holder, or assigns, by
vitiating the intent and purpose of the transactions contemplated hereby.
Accordingly, the Issuer acknowledges that the remedy at law for breach of its
obligations hereunder will be inadequate and agrees, in the event of a breach or
threatened breach by the Issuer of any of the provisions hereunder, that the
Holder, or assigns, shall be entitled, in addition to all other available
remedies in law or in equity, to an injunction or injunctions to prevent or cure
breaches of the provisions of this Security and to enforce specifically the
terms and provisions hereof, without the necessity of showing economic loss and
without any bond or other security being required.

                                       19
<PAGE>
            IN WITNESS WHEREOF, the Issuer has caused this instrument to be duly
executed under its corporate seal.

                                        GERON CORPORATION

[Seal]
                                        By: /s/ David L. Greenwood
                                            --------------------------------
                                            David L. Greenwood
                                            Senior Vice President, Corporate
                                            Development and Chief Financial
                                            Officer

Dated:  November 9, 2001

Attest:

/s/ Olivia K. Bloom
-----------------------------
Olivia K. Bloom
Controller

                                       20
<PAGE>
                                    EXHIBIT A
                              NOTICE OF CONVERSION

                   (To be Executed by the Registered Holder
in order to Convert the Series D Amended and Restated Convertible Debentures)

      Pursuant to the terms of the Series D Amended and Restated Convertible
Debenture (the "Security"), the undersigned hereby irrevocably elects to convert
$ __________ principal amount of the Security into shares of Common Stock of
Geron Corporation, a Delaware corporation (the "Issuer"). Capitalized terms used
herein and not otherwise defined herein have the respective meanings provided in
the Security. If securities are to be issued in the name of a person other than
the undersigned, the undersigned will pay all transfer taxes payable with
respect thereto and is delivering herewith such certificates. No fee will be
charged to the Holder for any conversion, except for transfer taxes, if any. A
copy of the Security is attached hereto (or evidence of loss, theft or
destruction thereof).

      The undersigned hereby irrevocably elects to convert $___________ in
accrued but unpaid interest on the Security and/or $___________ (in payments
pursuant to Section 2(c) of the Registration Rights Agreement at the Conversion
Price, as adjusted, set forth below.

[ ]   The undersigned hereby requests that the Issuer electronically transmit
      the Common Stock issuable pursuant to this Notice of Conversion to the
      account of the undersigned or its nominee with DTC through its Deposit
      Withdrawal Agent Commission system ("DWAC Transfer").

      Name of DTC Prime Broker:_________________________________________________
      Account Number:___________________________________________________________

[ ]   In lieu of receiving shares of Common Stock issuable pursuant to this
      Notice of Conversion by way of a DWAC Transfer, the undersigned hereby
      requests that the Issuer issue a certificate or certificates for the
      number of shares of Common Stock set forth above (which numbers are based
      on the Holder's calculation attached hereto) in the name(s) specified
      immediately below or, if additional space is necessary, on an attachment
      hereto:

      Name:       _________________________________
      Address:    _________________________________

      The undersigned represents and warrants that all offers and sales by the
undersigned of the securities issuable to the undersigned upon conversion of the
Security shall be made pursuant to registration of the securities under the
Securities Act of 1933, as amended (the "Act"), or pursuant to an exemption from
registration under the Act.
<PAGE>
                  Date of Conversion:_____________________________________
                  Applicable Conversion Price:____________________________
                  Number of Shares of
                  Common Stock to be Issued pursuant to:

                  (i)   Conversion of principal amount of
                        the Security:

                  (ii)  Conversion of accrued but unpaid interest on the
                        Security and/or payments pursuant to Section 2(c) of the
                        Registration Rights Agreement:

                  Signature:______________________________________________
                  Name:___________________________________________________
                  Address:        ________________________________________
                                  ________________________________________
                                  ________________________________________

*If the entire unpaid principal amount of the Security is to be converted, the
Issuer is not required to issue shares of Common Stock until the original
Security (or evidence of loss, theft or destruction thereof) to be converted is
received by the Issuer or its Transfer Agent. The Issuer shall issue and deliver
shares of Common Stock to an overnight courier not later than three (3) Trading
Days following receipt of the original Security to be converted, and interest
shall accrue at the applicable Interest Rate pursuant to the Security for the
number of Trading Days such issuance and delivery is late.

                                       2<PAGE>
                                                                     EXHIBIT 4.3

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED WITH THE SECURITIES
AND EXCHANGE COMMISSION IN RELIANCE UPON AN EXEMPTION FROM REGISTRATION UNDER
THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), AND, ACCORDINGLY, MAY NOT BE
OFFERED OR SOLD EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE
ACT OR PURSUANT TO AN AVAILABLE EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT
TO, THE REGISTRATION REQUIREMENTS OF THE ACT.

                                                               Right to Purchase
                                                                  333,935 Shares
                                                                of Common Stock,
                                                                par value $0.001
                                                                       per share

                                GERON CORPORATION
                         AMENDED AND RESTATED SERIES D-1
                             STOCK PURCHASE WARRANT

Registered Owner:

      This warrant is a duly authorized warrant (the "Warrant") of Geron
Corporation, a Delaware corporation (the "Company") referred to in the
Securities Purchase Agreement (the "Purchase Agreement"), dated as of June 29,
2000, by and between the Company and the Purchaser (as defined in the Purchase
Agreement) and as amended by that certain Restructuring Agreement (the
"Restructuring Agreement"), dated as of November 9, 2001, by and between the
Company and the Purchaser. This Warrant is subject to the terms and conditions
of the Purchase Agreement and the Restructuring Agreement, copies of which are
on file at and may be obtained from the Company at its principal office at the
address set forth in Section 11 hereof. This certifies that, for value received
the Company grants the following rights to the Registered Owner, or assigns, of
this Warrant:
<PAGE>
      1. ISSUE. Upon tender (as defined in section 5 hereof) to the Company, the
Company shall issue to the Registered Owner, RGC International Investors, LDC,
or assigns, up to the number of shares specified in paragraph 2 hereof of fully
paid and non-assessable shares of Common Stock, par value $.001 per share
("Common Stock"), that the Registered Owner, or assigns, is otherwise entitled
to purchase.

      2. NUMBER OF SHARES. The total number of shares of Common Stock that the
Registered Owner, or assigns, of this Warrant is entitled to receive upon
exercise of this Warrant is 333,935 shares of Common Stock, subject to
adjustment from time to time as set forth in paragraph 6 below. The Company
shall at all times have authorized, reserved and held available sufficient
shares of Common Stock to satisfy all conversion and purchase rights represented
by outstanding convertible securities, options and warrants, including this
Warrant, for issuance upon exercise of this Warrant, such number of shares equal
to 125% of the shares for which it may then be exercised. The Company covenants
and agrees that all shares of Common Stock that may be issued upon the exercise
of this Warrant shall, upon issuance, be duly and validly issued, fully paid and
non-assessable, and free from all taxes, liens and charges with respect to the
purchase and the issuance of the shares.

                                       2
<PAGE>
      3. EXERCISE PRICE. The initial exercise price of this Warrant, the price
at which the shares of stock issuable upon exercise of this Warrant may be
purchased, is $15.625 and subject to adjustment from time to time pursuant to
the provisions of paragraph 6 below (the "Exercise Price").

      3A. PAYMENT OF EXERCISE PRICE. The Registered Owner may pay the Exercise
Price in one of the following manners:

            (i) Cash Exercise. The Registered Owner shall deliver immediately
available funds or a check payable to the Company; or

            (ii) Cashless Exercise. At such time as, but only at such time as,
after the 120th day after the date of issuance of this Warrant, all of the
Registerable Securities (as defined in the Registration Rights Agreement) are
not registered pursuant to an effective registration statement, the Registered
Owner shall have the right to surrender this Warrant to the Company together
with a notice of cashless exercise, in which event the Company shall issue to
the Registered Owner the number of Warrant Shares determined as follows:

      where:            X = Y (A-B)/A

                        X = the number of Warrant Shares (as defined in the
                        Restructuring Agreement) to be issued to the Registered
                        Owner

                        Y = the number of Warrant Shares with respect to which
                        this Warrant is being exercised

                        A = the average of the Per Share Market Value of the
                        Common Stock for the five (5) Trading Days immediately
                        prior to (but not including) the date of exercise

                        B = the Exercise Price

For purposes of Rule 144 promulgated under the Securities Act, it is intended,
understood and acknowledged that the Warrant Shares issued in a cashless
exercise transaction shall be deemed to have been acquired by the Registered
Owner, and the holding period for the Warrant Shares shall be deemed to have
been commenced, on the issue date to the extent permitted by Rule 144.

      4. EXERCISE PERIOD. This Warrant may be exercised at any time and from
time to time on or after the date on which this Warrant is issued and delivered
(the "Issue Date") and before 5:00, New York City time on June 30, 2003 (the
"Exercise Period"). If not exercised during this period, this Warrant and all
rights granted under this Warrant shall expire and lapse.

      5. TENDER. This Warrant may be exercised, in whole or in part, by actual
delivery of (i) the Exercise Price in cash, (ii) a duly executed Warrant
Exercise Form, a copy of which is attached to this Warrant as Exhibit A,
properly executed by the Registered Owner, or assigns, of this

                                       3
<PAGE>
Warrant, and (iii) by surrender of this Warrant. The payment and Warrant
Exercise Form must be delivered, personally or by mail, to the registered office
of the Company. Documents sent by mail shall be deemed to be delivered when they
are received by the Company. If this Warrant shall have been exercised only in
part, then, unless this Warrant has expired, the Company shall, at its expense,
at the time of delivery of such certificates, deliver to the holder a new
Warrant representing the number of shares with respect to which this Warrant
shall not then have been exercised.

      6. Adjustment of Exercise Price.

            (a) Common Stock Dividends; Common Stock Splits; Reverse Common
Stock Splits. If the Company, at any time while this Warrant is outstanding, (a)
shall pay a stock dividend on its Common Stock, (b) subdivide outstanding shares
of Common Stock into a larger number of shares, (c) combine outstanding shares
of Common Stock into a smaller number of shares, or (d) issue by
reclassification of shares of Common Stock any shares of capital stock of the
Company, the Exercise Price shall be multiplied by a fraction the numerator of
which shall be the number of shares of Common Stock (excluding treasury shares,
if any) outstanding before such event and the denominator of which shall be the
number of shares of Common Stock outstanding after such event. Any adjustment
made pursuant to this paragraph (6)(a) shall become effective immediately after
the record date for the determination of shareholders entitled to receive such
dividend or distribution and shall become effective immediately after the
effective date in the case of a subdivision, combination or re-classification.

            (b) [Intentionally Omitted.]

            (c) [Intentionally Omitted.]

            (d) Rounding. All calculations under this section 6 shall be made to
the nearest cent or the nearest l/l00th of a share, as the case may be.

            (e) Notice of Adjustment. Whenever the Exercise Price is adjusted
pursuant to paragraph 6(a), the Company shall promptly mail to the holder of the
Warrant, a notice setting forth the Exercise Price after such adjustment and
setting forth a brief statement of the facts requiring such adjustment. Upon
each adjustment of the Exercise Price pursuant paragraph 6(a), the number of
shares of Common Stock issuable upon exercise of this Warrant shall be adjusted
by multiplying a number equal to the Exercise Price in effect immediately prior
to such adjustment by the number of shares of Common Stock issuable upon
exercise of this Warrant immediately prior to such adjustment and dividing the
product so obtained by the adjusted Exercise Price.

            (f) Redemption Events. In case of (A) any reclassification of the
Common Stock, (B) any consolidation or merger of the Company with or into
another person pursuant to which (i) a majority of the Company's Board of
Directors will not constitute a majority of the board of directors of the
surviving entity or (ii) less than 51% of the outstanding shares of the capital
stock of the surviving entity will be held by the same shareholders of the
Company prior to such reclassification, consolidation or merger, (C) the sale or
transfer of all or substantially all of the assets of the Company, (D) any
compulsory share exchange pursuant to which the Common Stock is converted into
other securities, cash or property, (E) suspension from listing or delisting of
the

                                       4
<PAGE>
Common Stock from the National Market System of the Nasdaq Stock Market
("Nasdaq") or any other exchange on which the Common Stock is listed for a
period of five (5) consecutive Trading Days, (F) the Company's notice to any
Registered Owner, including by way of public announcement, at any time, of its
intention, for any reason, not to comply with proper requests for the exercise
of any such warrants, or (G) a breach by the Company of any representation,
warranty, covenant or other term or condition of the Purchase Agreement, the
Registration Rights Agreement or any other agreement, document, certificate or
other instrument delivered in connection with the transactions contemplated
thereby or hereby, except to the extent that such breach would not have a
Material Adverse Effect (as defined in Section 3(a) of the Purchase Agreement)
and except, in the case of a breach of a covenant which is curable, only if such
breach continues for a period of at least ten (10) days after the Company knows
or reasonably should have known of the existence of such breach (clauses (A)
through (G) above referred to as a "Redemption Event"), the holder of the
Warrant shall have the right thereafter to exercise the Warrant within ten (10)
Business Days of the Redemption Event for the shares of stock and other
securities, cash and property receivable upon or deemed to be held by holders of
Common Stock following such Redemption Event, and the holder of the Warrant
shall be entitled upon such event to receive such amount of securities, cash or
property as the shares of the Common Stock of the Company into which the Warrant
could have been converted immediately prior to such Redemption Event would have
been entitled.

            (g) Reclassification, Etc. If:

                  A. the Company shall declare a dividend (or any other
      distribution) on its Common Stock; or

                  B. the Company shall declare a special nonrecurring cash
      dividend on or a redemption of its Common Stock; or

                  C. the Company shall authorize the granting to all of the
      holders of the Common Stock rights or warrants to subscribe for or
      purchase any shares of capital stock of any class or of any rights; or

                  D. the approval of any shareholders of the Company shall be
      required in connection with any reclassification of the Common Stock of
      the Company, any consolidation or merger to which the Company is a party,
      any sale or transfer of all or substantially all of the assets of the
      Company, of any compulsory share exchange whereby the Common Stock is
      converted into other securities, cash or property; or

                  E. the Company shall authorize the voluntary or involuntary
      dissolution, liquidation or winding up of the affairs of the Company;

then the Company shall cause to be filed at each office or agency maintained for
the purpose of exercise of this Warrant, and shall cause to be mailed to the
holder of this Warrant at its address as it shall appear below, at least thirty
(30) calendar days prior to the applicable record or effective date hereinafter
specified, a notice (provided such notice shall not include any material
non-public information) stating (x) the date on which a record is to be taken
for the purpose of such dividend, distribution, redemption, rights or warrants,
or if a record is not to betaken, the date as of which the

                                       5
<PAGE>
      holders of Common Stock of record to be entitled to such dividend,
      distributions, redemption, rights or warrants are to be determined or (y)
      the date on which such reclassification, consolidation, merger, sale,
      transfer or share exchange is expected to become effective or close, and
      the date as of which it is expected that holders of Common Stock of record
      shall be entitled to exchange their shares of Common Stock for securities,
      cash or other property deliverable upon such reclassification,
      consolidation, merger, sale, transfer or share exchange; provided,
      however, that the failure to mail such notice or any defect therein or in
      the mailing thereof shall not affect the validity of the corporate action
      required to be specified in such notice.

      (h)   [Intentionally Omitted.]

      In no event shall any provision in this Section 6 cause the Exercise Price
to be greater than the Exercise Price on the date of issuance of this Warrant.

      7. RESTRICTION ON EXERCISE BY THE REGISTERED OWNER. Notwithstanding
anything in this Warrant to the contrary, in no event shall the Holder of this
Warrant be entitled to exercise a number of Warrants (or portions thereof) in
excess of the number of Warrants (or portions thereof) upon exercise of which
the sum of (i) the number of shares of Common Stock beneficially owned by the
Holder and its affiliates (other than shares of Common Stock which may be deemed
beneficially owned through the ownership of the unexercised Warrants and the
unexercised or unconverted portion of any other securities of the Company
(including the Amended Series D Debentures (as defined in the Restructuring
Agreement)) and the Company's Series D-2 Stock Purchase Warrant issued to RGC)
subject to a limitation on conversion or exercise analogous to the limitation
contained herein) and (ii) the number of shares of Common Stock issuable upon
exercise of the Warrants (or portions thereof) with respect to which the
determination described herein is being made, would result in beneficial
ownership by the Holder and its affiliates of more than 9.9% of the outstanding
shares of Common Stock. For purposes of the immediately preceding sentence,
beneficial ownership shall be determined in accordance with Section 13(d) of the
Securities Exchange Act of 1934, as amended, and Regulation 13D-G thereunder,
except as otherwise provided in clause (i) hereof. Notwithstanding anything in
this Warrant to the contrary, the restriction on Holder set forth in this
paragraph shall not be amended without (i) the written consent of the Holder and
the Company and (ii) the approval of the holders of a majority of the Company's
Common Stock present, or represented by proxy, and voting at any meeting called
to vote on the amendment of such restriction.

      8. DEFINITIONS. Capitalized terms used herein and not otherwise defined
herein shall have the meanings given to such terms in the Purchase Agreement. As
used in this Warrant, the following terms have the following meanings:

      "Affiliate" means, with respect to any Person, any other Person that
directly or indirectly controls or is controlled by or under common control with
such Person. For the purposes of this definition, "control," when used with
respect to any Person, means the possession, direct or indirect, of the power to
direct or cause the direction of the management and policies of such Person,
whether through the ownership of voting securities, by contract or otherwise;
and the terms "affiliated," controlling" and "controlled" have meanings
correlative to the foregoing.

                                       6
<PAGE>
      "Appraiser" has the meaning assigned to it in Section 6(c)hereof.

      "Business Day" means any day except Saturday, Sunday and any day which
shall be a legal holiday or a day on which banking institutions in the state of
New York generally are authorized or required by law or other government actions
to close.

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

      "Company" means Geron Corporation, a Delaware corporation.

      "Convertible Securities" has the meaning assigned to it in Section
6(h)(i)(A) hereof.

      "Exercise Period" has the meaning assigned to it the Section 4 hereof.
      "Exercise Price" has the meaning assigned to it in Section 3 hereof.

      "Market Price" has the meaning assigned to it in Section 6(h)(i) hereof.

      "Options" has the meaning assigned to it in Section 6(h)(i)(A) hereof.

      "Per Share Market Value" means on any particular date (i) the closing bid
price per share of the Common Stock on such date on Nasdaq or another registered
national stock exchange on which the Common Stock is then listed or if there is
no such price on such date, then the closing bid price on such exchange or
quotation system on the date nearest preceding such date, or (ii) if the Common
Stock is not listed then on the National Market System of the Nasdaq Stock
Market or any registered national stock exchange, the closing bid price for a
share of Common Stock in the over-the-counter market, as reported by the
National Quotation Bureau Incorporated (or similar organization or agency
succeeding to its functions of reporting prices) at the close of business on
such date, or (iii) if the Common Stock is not then publicly traded the fair
market value of a share of Common Stock as determined by an Appraiser selected
in good faith by the holder of this Warrant; provided, however, that the
Company, after receipt of the determination by such Appraiser, shall have the
right to select an additional Appraiser, in which case, the fair market value
shall be equal to the average of the determinations by each such Appraiser; and
provided, further that all determinations of the Per Share Market Value shall be
appropriately adjusted for any stock dividends, stock splits or other similar
transactions during such period.

      "Purchase Agreement" means that certain Securities Purchase Agreement,
dated June 29, 2000, by and between the Company and the Purchaser (as defined in
the Purchase Agreement).

      "Redemption Event" has the meaning assigned to it in Section 6(f) hereof.

      "Registered Owner" means RGC International Investors, LDC, or such other
Person as shown on the records of the Company as being the registered owner of
this Warrant.

                                       7
<PAGE>
      "Registration Rights Agreement" means that certain Registration Rights
Agreement, dated as of June 29, 2000, by and between the Company and the Initial
Investor (as defined in the Registration Rights Agreement).

      "Restructuring Agreement" means that certain Restructuring Agreement dated
as of November 9, 2001, between the Company and RGC.

      "RGC" means RGC International Investors, LDC.

      "Trading Day(s)" means any day on which the primary market on which shares
of Common Stock are listed is open for trading.

      9. REGISTRATION RIGHTS. The Company will undertake the registration of the
Common Stock into which this Warrant is exercisable at such times and upon such
terms pursuant to the provisions of the Registration Rights Agreement.

      10. [Intentionally Omitted].

      11. Notices. Any notice or other communication required or permitted to be
given hereunder shall be in writing and shall be deemed to have been received
(a) upon hand delivery (receipt acknowledged) or delivery by telex (with correct
answer back received), telecopy or facsimile (with transmission confirmation
report) at the address or number designated below (if received by 8:00 p.m. EST
where such notice is to be received), or the first business day following such
delivery (if delivered on a business day after during normal business hours
where such notice is to be received) or (b) on the second business day following
the date of mailing by express courier service, fully prepaid, addressed to such
address, or upon actual receipt of such mailing, whichever shall first occur.
The addresses for such communications are (i) if to the Company to Geron
Corporation, 230 Constitution Drive, Menlo Park, California 94025 attn: David
Greenwood, fax no. (650) 473-7701 with copies to Latham & Watkins, 135
Commonwealth Drive, Menlo Park, California 94025, Attn: Alan C. Mendelson, Esq.,
fax no. (650) 463-2600 and (ii) if to any Registered Owner to the address set
forth immediately below such Registered Owner's name on the signature pages to
the Restructuring Agreement or such other address as may be designated in
writing hereafter, in the same manner, by such Person.

      12. REMEDIES. The Company acknowledges that a breach by it of its
obligations hereunder will cause irreparable harm to the Registered Owner, or
assigns, by vitiating the intent and purpose of the transactions contemplated
hereby. Accordingly, the Company acknowledges that the remedy at law for breach
of its obligations hereunder will be inadequate and agrees, in the event of a
breach or threatened breach by the Company of any of the provisions hereunder,
that the Registered Owner, or assigns, shall be entitled, in addition to all
other available remedies in law or in equity, to an injunction or injunctions to
prevent or cure breaches of the provisions of this Security and to enforce
specifically the terms and provisions hereof, without the necessity of showing
economic loss and without any bond or other security being required.

              [REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                       8
<PAGE>
      IN WITNESS WHEREOF, the Company has caused this Warrant to be executed by
its duly authorized officer as of the date first set forth above.

                                          GERON CORPORATION

                                          By: /s/ David L. Greenwood
                                              ----------------------------------
                                              David L. Greenwood
                                              Senior Vice President, Corporate
                                              Development and Chief Financial
                                              Officer

                                       9
<PAGE>
                                    EXHIBIT A

                              Warrant Exercise Form

TO:   GERON CORPORATION

      The undersigned hereby: (1) irrevocably subscribes for and offers to
purchase _______ shares of Common Stock of Geron Corporation, pursuant to
Warrant No. ___ heretofore originally issued to ___________________ on June 29,
2000; (2) encloses a payment of $__________ for these shares at a price of
$________ per share (as adjusted pursuant to the provisions of the Warrant); and
(3) requests that a certificate for the shares be issued in the name of the
undersigned and delivered to the undersigned at the address specified below. The
undersigned represents that it is an accredited investor within the meaning of
Regulation D under the Securities Act.

                       Date:

                       Investor Name:

                       Taxpayer Identification
                       Number:

                       By:

                       Printed Name:

                       Title:

                       Address:

                       Note: The above signature should correspond exactly with
                             the name on the face of this Warrant Certificate or
                             with the name of assignee appearing in assignment
                             form below.

AND, if said number of shares shall not be all the shares purchasable under the
within Warrant, a new Warrant Certificate is to be issued in the name of said
undersigned for the balance remaining of the shares purchasable thereunder less
any fraction of a share paid in cash and delivered to the address stated above.

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