CHIRON 1991 STOCK OPTION PLAN  [AS AMENDED AUGUST 14, 1993, APRIL 11, 1994, FEBRUARY 24, 1995,  MARCH 8, 1996, FEBRUARY 28, 1997, AUGUST 7, 1998, AUGUST 20, 1999,  FEBRUARY 25, 2000, SEPTEMBER 21, 2000, AND
FEBRUARY 16, 2001]

I.  PURPOSES

This Chiron 1991 Stock Option Plan ("Plan") is intended to enable Chiron Corporation ("Corporation") to attract and retain the following individuals by
offering them incentives and rewards, in the form of options, restricted shares, share rights, share units and performance units ("awards") which will encourage them to acquire a proprietary interest
in the Corporation, to continue in the service of the Corporation or its subsidiaries, and to provide incentive to build value for stockholders: (a) employees (including officers and directors)
of the Corporation and its subsidiaries, (b) non-employee members of the Board of Directors of the Corporation ("Board"), and (c) consultants and independent contractors who perform valuable
services for the Corporation and its subsidiaries.

In
addition, the Plan is intended to permit the Corporation to satisfy its obligations in connection with options it assumed pursuant to the terms of the Agreement and Plan of Merger
dated as of July 21, 1991 by and among the Corporation, Chiron Acquisition Subsidiary, Inc., and Cetus Corporation ("Agreement"). Upon consummation of the transactions described in the
Agreement ("Merger"), the Plan superseded Cetus Corporation's Amended and Restated Common Stock Option Plan and Cetus Corporation's Non-Employee Directors' Stock Option Plan ("Cetus Prior Plans").
Upon stockholder approval in December 1991, this Plan superseded the following Chiron prior plans: the Protos Corporation 1988 Stock Option Plan (upon the merger of Protos into Chiron), the
Chiron Ophthalmics, Inc. 1986 Stock Option Plan (upon the merger of Chiron Ophthalmics into a wholly owned subsidiary of Chiron), the Corporation's 1982 Stock Option Plan and the Corporation's
1984 Non-Qualified Stock Option Plan (collectively, "Chiron Prior Plans").

II.  ADMINISTRATION

The Plan will be administered by a committee or committees appointed by the Board and consisting of one or more members of the Board or a subcommittee or
subcommittees thereof. The Board may delegate the responsibility for administration of the Plan with respect to designated classes of award
holders to different committees, subject to such limitations as the Board deems appropriate, and each committee may similarly delegate its responsibilities to one or more subcommittees. Members of a
committee or subcommittee will serve for such term as the Board or committee may determine, and will be subject to removal by the Board at any time. With respect to any matter, the term "Committee,"
when used in this Plan, will refer to the committee or subcommittee that has been delegated authority with respect to such matter.

In
determining the composition of any committee or subcommittee, the Board or committee, as the case may be, shall consider the desirability of compliance with the compositional
requirements of (i) Rule 16b-3 of the Securities and Exchange Commission with respect to award holders who are subject to the trading restrictions of Section 16(b) of the
Securities Exchange Act of 1934 ("1934 Act") with respect to securities of the Corporation and (ii) Section 162(m) of the Internal Revenue Code ("Code") with respect to performance units, but
shall not be bound by such compliance.

(a) AUTHORITY.
Each Committee will have full authority to administer the Plan within the scope of its delegated responsibilities, including authority to interpret and
construe any relevant provision of the Plan, to adopt such rules and regulations as it may deem necessary, and to determine the terms and conditions of awards made under the Plan (which need
not be identical). Decisions of a Committee made within the discretion delegated to it by the Board will be final and binding on all persons who have an interest in the Plan.

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III. ELIGIBILITY FOR AWARDS

(a) DISCRETIONARY
AWARDS. From time to time the Committee may, in its discretion, select individuals from among the following categories to receive awards under the
Plan:



(b) PERFORMANCE
UNITS. Corporate vice-presidents and other executive officers of the Corporation or a parent or subsidiary ("162(m) executives") will be eligible to
receive performance units in addition to, or in lieu of, other discretionary awards granted under the Plan.

(c) AUTOMATIC
GRANTS. Members of the Board who are not employees of the Corporation or a subsidiary will receive awards in accordance with the Plan's automatic grant
provisions.

(d) SUBSTITUTE
OPTIONS. Upon consummation of the Merger, outstanding options under the Cetus Prior Plans (including related Limited Stock Appreciation Rights) were
converted, in the manner and at the exchange ratio specified in the Agreement, into substitute options under this Plan to acquire Common Stock (as defined below). Upon stockholder approval and, with
regard to the Protos prior plan options and the Chiron Ophthalmics prior plan options, consummation of the relevant mergers, outstanding options under the Chiron Prior Plans were converted into
options under this Plan. These options preserved the exercise price of the outstanding options as adjusted, in the case of options under the Protos Corporation 1988 Stock Option Plan and the Chiron
Ophthalmics, Inc. 1986 Stock Option Plan, to reflect the substitution of Common Stock. These options also preserved the other terms and conditions of the outstanding options; provided, however,
that on the Effective Date of this Plan, outstanding automatic option grants under the Corporation's 1982 Stock Option Plan were conformed, other than to extend the term, to the Automatic Option
Grants under this Plan. Collectively, these options are referred to as "Substitute Options."

IV. STOCK SUBJECT TO THE PLAN

(a) CLASS.
The stock subject to awards under the Plan is (i) the Corporation's authorized but unissued or reacquired Common Stock ("Common Stock"), or
(ii) shares of one or more series of the Corporation's authorized but unissued or reacquired Restricted Common Stock, in the aggregate, "Company Stock." In connection with the grant of awards
under the Plan, the Corporation may repurchase shares in the open market or otherwise.

(b) AGGREGATE
AMOUNT



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(c) ADJUSTMENTS.
In the event any change is made to the Company Stock subject to the Plan (whether by reason of merger, consolidation, reorganization, recapitalization,
stock dividend, stock split, combination of shares, exchange of shares, or other change in corporate or capital structure of the

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Corporation affecting the outstanding Common Stock as a class without the Corporation's receipt of consideration) then, unless such change results in the termination of all awards, the Committee will
make appropriate adjustments to (i) the number and/or class of securities available under the Plan, (ii) the number and/or class of securities for which any one person may be granted
awards under the Plan, (iii) the number and/or class of securities to be made under automatic grants to non-employee directors, and (iv) the number and/or class of securities and, where
applicable, price per share of securities subject to outstanding awards. Such adjustments are to be effected in a manner which shall preclude the enlargement or dilution of rights and benefits of
awards and shall be final, binding and conclusive.

V.  TERMS AND CONDITIONS OF DISCRETIONARY OPTIONS

Discretionary stock options granted under the Plan may, in the Committee's discretion, be either incentive stock options ("Incentive Options") qualifying under
Section 422 of the Internal Revenue Code of 1986, as amended ("Internal Revenue Code"), or nonstatutory options. Individuals who are not employees of the Corporation or its subsidiaries may
only be granted nonstatutory options. Options will be evidenced by instruments in such form as the Committee may from time to time approve. These instruments will conform to the following terms and
conditions and, in the discretion of the Committee, may contain such other terms, conditions and restrictions as are not inconsistent with the following:

(a) OPTION
PRICE.  The option price per share will be fixed by the Committee, but in no event will the option price per share be less than eighty-five
percent (85%) of the Fair Market Value of the option shares on the date of the option grant; provided, however, that in no event will the option price per share of an Incentive Option be less than one
hundred percent (100%) of the Fair Market Value of the option shares on the date of the option grant. However, the Committee may grant options with an option price per share less than eighty-five
percent (85%) of the Fair Market Value of the option shares on the date of the option grant in substitution for the outstanding options of the acquired company in a merger, if such options are granted
with an option price per share which preserves the option price of the outstanding options, as adjusted for the merger. Notwithstanding the foregoing, Substitute Options will have an option price per
share determined pursuant to Section III(d) of this Plan.

(b) NUMBER
OF SHARES, TERM AND EXERCISE



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(c) TERMINATION
OF SERVICES. The Committee will determine and set forth in each option whether the option will continue to be exercisable, and the terms and conditions
of such exercise, on and after the date that an optionee ceases to be employed by, or to provide services to, the
Corporation or its subsidiaries. The date of termination of an optionee's employment or services will be determined by the Committee, which determination will be final.

(d) INCENTIVE
OPTIONS.  Options granted under the Plan that are intended to be Incentive Options will be subject to the following additional terms and
conditions:



(e) WITHHOLDING



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VI. DISCRETIONARY RESTRICTED SHARES, SHARE RIGHTS, SHARE UNITS AND PERFORMANCE UNITS

(a) NATURE
OF AWARDS



(b) WITHHOLDING.
The Committee may require, or permit an award holder to elect, that a portion of the total value of the shares of Common Stock subject to restricted
shares or share rights held by one or more award holders be paid in the form of cash in lieu of the issuance of Company Stock and that such cash payment be applied to the satisfaction of the federal,
state and local income and employment tax withholding obligations that arise at the time the restricted shares and share rights become free of all restrictions under the Plan.

(c) CASH
PAYMENTS. The Committee may provide award holders with an election to receive a percentage of the total value of the Company Stock subject to restricted shares
or share rights in the form of a cash payment, subject to such terms, conditions and restrictions as the Committee shall specify.

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(d) ELECTIVE AND TANDEM AWARDS. The Committee may award restricted shares, share rights, and share units independently of other compensation or in lieu of compensation
that would otherwise be paid in cash or stock options, whether at the election of the potential award holder or otherwise. The number of restricted shares, share rights or share units to be awarded in
lieu of any cash compensation amount or number of stock options shall be determined by the Committee in its sole discretion and need not be equal to such foregone compensation in Fair Market Value. In
addition, restricted shares, share rights, and share units may be awarded in tandem with stock options, so that a portion of such award becomes payable or becomes free of restrictions only if and to
the extent that the tandem options are not exercised or are forfeited, subject to such terms and conditions as the Committee may specify.

(e) MODIFICATION
OF AWARDS. Except to the extent an award is granted as a performance unit, the Committee may, in its sole discretion, modify or waive any or all of the
terms, conditions or restrictions applicable to any outstanding restricted share, share right or share unit; provided, however, that no such modification or waiver shall, without the consent of the
holder of an outstanding award, adversely affect the holder's rights thereunder.

(f)  PERFORMANCE
UNITS. Effective March 8, 1996, the Committee may grant restricted shares, share rights and share units to 162(m) executives that comply with
the requirements of Code Section 162(m). Performance units will become payable or vest upon attainment of specified performance goals over a specified performance period.



For
purposes of this Plan, net income and gross revenues shall be net income and gross revenues of the Corporation and its consolidated subsidiaries as reported by the Corporation and certified by its
independent public accountants, but the Committee in fixing any goal may exclude any or all of the

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following if they have a material effect on annual net income or gross revenues: events or transactions that are either unusual in nature or infrequent in occurrence (such restructuring/reorganization
charges, the purchase or sale of in process technology, the sale or discontinuance of a business segment, the sale of investment securities, losses from litigation, the cumulative effect of changes in
accounting principles and natural disasters), depreciation, interest or taxes.



VII. AUTOMATIC AWARDS TO DIRECTORS



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VIII.  LOANS AND INSTALLMENT PAYMENTS

In order to assist an award holder (including an employee who is an officer or director of the Corporation) in the acquisition of shares of Company Stock
pursuant to an award granted under the Plan (other than pursuant to the Automatic Option Grant provisions of this Plan), the Committee may authorize, at either the time of the grant of an award or the
time of the acquisition of Company Stock pursuant to the award (i) the extension of a loan to the award holder by the Corporation, (ii) the payment by the award holder of the purchase
price, if any, of the Company Stock in installments, or (iii) the guarantee by the Corporation of a loan obtained by the award holder from a third party. The terms of any loans, guarantees or
installment payments, including the interest rate and terms of repayment, will be subject to the discretion of the Committee. Loans, installment payments and guarantees may be granted without
security, the maximum credit available being the purchase price, if any, of the Company Stock acquired plus the maximum federal and state income and employment tax liability that may be incurred in
connection with the acquisition.

IX. ASSIGNABILITY

No award granted under the Plan is assignable or transferable by the award holder other than by Will or by the laws of descent and distribution, and during the
lifetime of the award holder, only the award holder may exercise options or exercise the rights provided under awards granted under the Plan. However, if and to the extent that the Committee so
authorizes at the time an award is granted or amended, an option (other than an option designated as an Incentive Option) or other award may be assigned in whole or in part during the grantee's
lifetime to one or more of the grantee's family members or an entity substantially owned, benefiting or controlled by the grantee or one or more of grantee's family members if and to the extent that
the Securities and Exchange Commission Form S-8 Registration Statement would continue to be available for the exercise of the award and resale of the underlying securities following such
assignment. The terms applicable to the assigned portion shall be the same as those in effect for the award immediately prior to such assignment and shall be set forth in such documents issued to the
assignee as the Committee may deem appropriate.

X.  CANCELLATION AND NEW GRANT OF OPTIONS

The Committee will have the authority to effect, at any time and from time to time, with the consent of the affected option holders, the cancellation of any or
all outstanding options under the Plan, a Cetus Prior Plan or a Chiron Prior Plan (other than options granted under automatic option

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grant provisions of these plans) and to grant in substitution therefor new options under the Plan covering the same or different numbers of shares, but having an option price per share not less than
eighty-five percent (85%) of the Fair Market Value on the new grant date or, in the case of an Incentive Option, one hundred percent (100%) of the Fair Market Value on the new grant date (or, in the
case of an Incentive Option granted to a 10% Stockholder, one hundred ten percent (110%) of such Fair Market Value).

XI. ACCELERATION AND TERMINATION OF AWARDS

(a) ACCELERATION.
If the Corporation or its stockholders enter into an agreement to dispose of all or substantially all of the assets of the Corporation, enter into an
agreement to merge or consolidate with another entity or enter into a plan of reorganization or liquidation, each award will be automatically accelerated so that (1) options become fully
exercisable with respect to the total number of shares purchasable under the options; (2) restrictions on restricted shares will be eliminated, and the shares will immediately vest; and
(3) share rights and share units will immediately vest and become payable. The Committee may also provide for the automatic termination of repurchase rights upon the occurrence of such an
event.

(b) NO
ACCELERATION. No acceleration of awards will occur if the terms of the agreement require that each such award either be assumed by the successor corporation or
parent thereof or be replaced with a comparable award subject to shares of the successor corporation or parent thereof. The determination of such comparability will be made by the Committee, and its
determination will be final, binding and conclusive. Upon consummation of the transaction contemplated by the agreement, all awards, whether or not accelerated, will terminate unless assumed pursuant
to a written agreement by the successor corporation or parent thereof. However, nothing in this Section will prohibit the Committee from granting an award that provides for the acceleration of
exercisability and/or vesting of the award either (i) upon the occurrence of any specified event, including a corporate transaction or change in control of the Corporation (as defined by the
Committee) whether or not the award is assumed or replaced with a comparable award or (ii) upon termination of employment or other occurrence following such an event.

(c) CORPORATE
STRUCTURE. The grant of awards under this Plan will in no way affect the right of the Corporation to adjust, reclassify, reorganize, or otherwise change
its capital or business structure or to merge, consolidate, dissolve, liquidate or sell or transfer all or any part of its business or assets.

XII. VALUATION

With regard to all Substitute Options, Fair Market Value will be determined in accordance with the relevant option plan documents on the date that the
outstanding options were granted. With regard to awards granted under this Plan, for all valuation purposes under the Plan, the Fair Market Value of a share of Common Stock or Restricted Common Stock
(as the case may be) on any relevant date will be determined in accordance with the following provisions:

(a) If
the Common Stock or Restricted Common Stock is not at the time listed or admitted to trading on any stock exchange, but is traded in the over-the-counter market,
the Fair Market Value will be the average between the reported high price and the reported low price of one share of Common Stock or Restricted Common Stock (as the case may be) on the date in
question in the over-the-counter market, as such prices are reported by the National Association of Securities Dealers through its NASDAQ system or any successor system.

(b) If
the Common Stock or Restricted Common Stock is at the time listed or admitted to trading on any stock exchange, then the Fair Market Value will be the average
between the reported high price and the reported low price of one share of Common Stock or Restricted Common Stock (as the case

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may be) on the date in question on the stock exchange that is the primary market for the stock, as such prices are officially quoted on such exchange.

(c) If
the Common Stock or Restricted Common Stock (as the case may be) is at the time neither listed nor admitted to trading on any stock exchange nor traded in the
over-the-counter market, or if the Committee determines that neither subparagraph (a) nor subparagraph (b) above reflects Fair Market Value of the stock and the award was not granted
pursuant to the Plan's Automatic Award provisions, then the Fair Market Value will be determined by the Committee after taking into account such factors as the Committee deems appropriate, or in the
case of Automatic Awards, by an independent third party valuation.

XIII.  SURRENDER OF OPTIONS FOR CASH OR STOCK

(a) STOCK
APPRECIATION RIGHTS. If, and only if the Committee, in its discretion, elects to implement an option surrender program under the Plan, one or more option
holders may, upon such terms and conditions as the Committee may establish at the time of the option grant or at any time thereafter, be granted the right to surrender all or part of an unexercised
option in exchange for a distribution equal in amount to the difference between (i) the Fair Market Value (at date of surrender) of the shares for which the surrendered option or portion
thereof is at the time exercisable and (ii) the aggregate option price payable for such shares. The distribution to which an option holder becomes entitled under this Section may be made
in shares of Common Stock or Restricted Common Stock, valued at Fair Market Value at the date of surrender, in cash, or partly in shares and partly in cash, as the Committee, in its sole discretion,
deems appropriate. The option surrender provisions of this Section will not apply to options granted pursuant to the Automatic Option Grant provisions of this Plan.

(b) LIMITED
STOCK APPRECIATION RIGHTS. If outstanding options of Cetus for which Substitute Options are issued pursuant to Section III(d) have Limited
Stock Appreciation Rights ("LSARs") attached thereto, then each such LSAR shall be honored by the Corporation in accordance with its terms and remain exercisable for a period of 60 days following the
date that stockholders of Cetus approve the Merger; provided, however, that if the LSAR was originally granted within 6 months of the date that Cetus stockholders approve the Merger, then the LSAR
will be exercisable for a period of 60 days following expiration of such six-month period. Upon expiration of the applicable 60-day period, each such LSAR not previously exercised shall expire. Upon
exercise of an LSAR, the related option will be cancelled, and Chiron will pay to the LSAR holder an amount in cash for each share with respect to which the LSAR is exercised determined in accordance
with the terms of the Cetus Prior Plans.

XIV.  REPURCHASE RIGHTS

The Committee may, in its discretion, establish as a term of one or more awards granted under the Plan that the Corporation (or its assigns) will have the
right, exercisable upon the award holder's
termination of employment with, or cessation of services for, the Corporation and its subsidiaries, to repurchase at the original price paid, if any, for such shares of (1) Company Stock
acquired by the award holder pursuant to the granted award, or (2) Common Stock into which acquired Restricted Common Stock may have been converted or for which Restricted Common Stock may have
been exchanged. Any such repurchase right will be exercisable by the Corporation (or its assigns) upon such terms and conditions (including provisions for the expiration of such right in one or more
installments) as the Committee may specify in the instrument evidencing such right. The Committee will also have full power and authority to provide for the automatic termination of the Corporation's
repurchase rights, in whole or in part, thereby accelerating the vesting of any or all of the purchased shares (other than purchased shares obtained pursuant to the Automatic Award provisions of this
Plan) upon the occurrence of any change in control specified in Article XI.

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XV. RIGHT OF FIRST REFUSAL

The Committee may, in its discretion, establish as a term of one or more awards granted under the Plan that the Corporation has a right of first refusal with
respect to the proposed disposition by the award holder (or any successor in interest by reason of purchase, gift or other mode of transfer) of any shares of (1) Company Stock acquired by the
award holder pursuant to the granted award, or (2) Common Stock into which purchased Restricted Common Stock may have been converted or for which acquired Restricted Common Stock may have been
exchanged. Any such right of first refusal will be exercisable by the Corporation or its assigns in accordance with the terms and conditions specified in the instrument evidencing such right.

XVI.  EFFECTIVE DATE AND TERM OF PLAN

(a) EFFECTIVE
DATE. The Plan became effective on December 10, 1991, the date that it was approved by the Corporation's stockholders. The Plan was subsequently
amended on several occasions and, as amended through March 8, 1996, was approved by the Corporation's stockholders on May 16, 1996. The Plan was further amended on February 28, 1997, and
as amended through February 28, 1997 was approved by the Corporation's stockholders on May 15, 1997. The Plan was subsequently amended on a number of occasions and was further amended on
February 16, 2001 to (i) eliminate the automatic share rights awards to Eligible Directors contained in then Section VII (b) of the Plan, (ii) change the annual
automatic option grant to non-employee directors from an option to purchase a number of shares of Common Stock determined by dividing $100,000 by the average stock price of a share of Common Stock
over the preceding 12 months to an option to purchase 15,000 shares of Common Stock, and reduce the vesting schedule for such option from 5 years to 6 months, and (iii) implement an initial
option grant feature, whereby each newly elected or appointed non-employee director automatically receives an option grant to purchase 30,000 shares of Common Stock which vests over a 3-year period.

(b) TERM.
Incentive Options may be granted under the Plan until May 14, 2007, and may not be issued under the Plan after that date. Subject to this limitation, the
Committee may grant awards under the Plan at any time after the Effective Date of the Plan and before the Plan is terminated by the Board.

XVII.  AMENDMENT OR DISCONTINUANCE

(a) BOARD.
The Board may amend, suspend or discontinue the Plan in whole or in part at any time; provided, however, that (a) except to the extent necessary to
qualify as Incentive Options any or all options granted under the Plan that are intended to so qualify, such action may not, without the consent of the award holder, adversely affect rights and
obligations with respect to awards outstanding under the Plan; (b) certain amendments may, as determined by the Board in its sole discretion, require stockholder approval pursuant to applicable
laws or regulations.

(b) COMMITTEE.
The Committee will have full power and authority to modify or waive any or all of the terms, conditions or restrictions applicable to any outstanding
award (other than Automatic Option Grants), to the extent not inconsistent with the Plan.

(c) SUBSTITUTE
OPTIONS.  Substitute Options will be subject to amendment in accordance with the terms of this Plan.

XVIII.  NO OBLIGATION

Nothing contained in the Plan (or in any award granted under this Plan, a Chiron Prior Plan or a Cetus Prior Plan) shall confer upon any employee, consultant,
or independent contractor any right to continue in the employ of, or to provide services to, the Corporation or any affiliate or constitute a

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contract or agreement of employment or for the provision of services, or interfere in any way with the right of the Corporation or an affiliate to reduce such employee's, consultant's or independent
contractor's compensation from the rate in existence at the time of the granting of an award or to terminate such employee's, consultant's or independent contractor's employment or services at any
time, with or without cause; but nothing contained in the Plan or in any award granted under this Plan shall affect any contractual rights of an employee pursuant to a written employment agreement.

XIX.  USE OF PROCEEDS

The cash proceeds received by the Corporation pursuant to awards granted under the Plan will be used for general corporate purposes.

XX. COMPLIANCE

(a) FEDERAL
AND STATE LAWS. No option may be exercised, and the Corporation will not be obligated to issue stock under any award unless, in the opinion of counsel for
the Corporation, such exercise and issuance is in compliance with all applicable federal and state securities laws. As a condition to the grant of any award, or to the issuance of stock under any
award, the Committee may require that the award holder agree to comply with such provisions of federal and state securities laws as may be applicable to such grant, or to the sale of stock acquired
pursuant to the Plan, and that the award holder deliver to the Corporation a written agreement, in form and substance satisfactory to the Corporation and its counsel, implementing such agreement.

(b) INFORMATION.
The Corporation will furnish to each award holder participating in the Plan (other than a key employee or a director) a copy of the Corporation's
Annual Report to Stockholders for the most recent fiscal year, and additional copies will be furnished, without charge, to such award holders upon request to the Secretary of the Corporation.

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APPENDIX A  SPECIAL PROVISIONS RELATED TO 1995 CIBA-GEIGY TRANSACTION

Those persons holding options to acquire shares of Common Stock under the Corporation's 1991 Stock Option Plan on November 20, 1994 are granted the
following rights ("Rights") with respect to each such option:



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