CELL GENESYS INC
424B3, 1998-01-16
PHARMACEUTICAL PREPARATIONS
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<PAGE>   1


PROSPECTUS FILED PURSUANT TO RULE 424(B)(3) OF THE SECURITIES ACT OF 1933, AS
AMENDED.
REGISTRATION NO. 333-43277


                                8,028,569 SHARES

                               CELL GENESYS, INC.
                                  COMMON STOCK

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

         This Prospectus relates to the public offering, which is not being
underwritten, from time to time by certain stockholders of the Company or by
pledgees, donees, transferees or other successors in interest that receive such
shares as a gift, partnership distribution or other non-sale related transfer
(the "Selling Stockholders") of (i) up to 8,028,569 shares of Common Stock, par
value $0.001 per share (the "Shares"), of Cell Genesys, Inc. ("Cell Genesys" or
the "Company"), and (ii) in accordance with Rule 416 ("Rule 416") under the
Securities Act of 1933, as amended (the "Securities Act"), such presently
indeterminate number of additional Shares as may be issuable upon conversion of
the shares of the Company's Series B Convertible Preferred Stock (the "Series B
Preferred Stock") held by or issuable to certain of the Selling Stockholders,
based upon fluctuations in the conversion price of the Series B Preferred Stock
and any future antidilution adjustments in accordance with the terms of the
Series B Preferred Stock. The Company will receive no part of the proceeds of
such sales. All of the Shares were originally issued by the Company or will be
issued by the Company consisting of: (1) up to 737,902 Shares issued or issuable
upon the exercise by certain of the Selling Stockholders of warrants (the
"Warrants") of Cell Genesys, which warrants were assumed by Cell Genesys as a
result of the merger of a wholly-owned subsidiary of the Company into Somatix
Therapy Corporation ("Somatix"), (2) up to 1,666,667 Shares issued or issuable
upon the conversion of a Convertible Subordinated Promissory Note dated March
26, 1997 (the "Convertible Note"), (3) up to 5,624,000 shares of Common Stock
potentially to be issued from time to time to Selling Stockholders upon
conversion of the Company's outstanding Series B Preferred Stock at the election
of the registered holders of such Series B Preferred Stock, or (4) in accordance
with Rule 416 such presently indeterminate number of additional Shares as may be
issuable upon conversion of outstanding Series B Preferred Stock based on
fluctuations in the conversion price of the Common Stock and any antidilution
adjustments in accordance with the terms of the Series B Preferred Stock. The
Shares were issued or potentially will be issued upon conversion of the Series B
Preferred Stock and Convertible Note and exercise of the Warrants in private
placements pursuant to an exemption from the registration requirements of the
Securities Act, provided by Section 4(2) thereof. The Shares are being
registered by the Company pursuant to registration rights granted to the Selling
Stockholders.

         The Selling Stockholders have not advised the Company of any specific
plans for the distribution of the Shares covered by this Prospectus. It is
anticipated, however, that the Shares may be offered by the Selling Stockholders
from time to time in one or more transactions on the Nasdaq National Market, in
privately negotiated transactions at such prices as may be agreed upon, or in a
combination of such methods of sale. The Selling Stockholders may effect such
transactions by selling the Shares to or through broker-dealers and such
broker-dealers may receive compensation in the form of discounts, concessions or
commissions from the Selling Stockholders or the purchasers of the Shares for
whom such broker-dealers may act as agent or to whom they sell as principal or
both (which compensation to a particular broker-dealer might be in excess of
customary commissions). See "Plan of Distribution." The price at which any of
the Shares may be sold, and the commissions, if any, paid in connection with any
such sale, are unknown and may vary from transaction to transaction. The Company
will pay all expenses incident to the offering and sale of the Shares to the
public other than any commissions and discounts of underwriters, dealers or
agents and any transfer taxes. See "Selling Stockholders" and "Plan of
Distribution."

         The Company's Common Stock is listed on the Nasdaq National Market
under the symbol "CEGE." On January 15, 1998 the last sale price of the
Company's Common Stock was $7.625 per share.


         THIS OFFERING INVOLVES A HIGH DEGREE OF RISK. SEE "RISK FACTORS" ON
PAGE 4 FOR INFORMATION THAT SHOULD BE CONSIDERED BY PROSPECTIVE INVESTORS.

         The Securities and Exchange Commission (the "Commission") may take the
view that, under certain circumstances, the Selling Stockholders and any
broker-dealers or agents that participate with the Selling Stockholders in the
distribution of the Shares may be deemed to be "underwriters" within the meaning
of the Securities Act. Commissions, discounts or concessions received by any
such broker-dealer or agent may be deemed to be underwriting commissions under
the Securities Act. The Company and the Selling Stockholders have agreed to
certain indemnification arrangements. See "Plan of Distribution."


    THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
       AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS
         THE SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES
               COMMISSION PASSED UPON THE ACCURACY OR ADEQUACY OF
                   THIS PROSPECTUS. ANY REPRESENTATION TO THE
                         CONTRARY IS A CRIMINAL OFFENSE.

                THE DATE OF THIS PROSPECTUS IS JANUARY 16, 1998

<PAGE>   2

                              AVAILABLE INFORMATION

         The Company is subject to the informational requirements of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), and in
accordance therewith files reports, proxy and information statements and other
information with the Commission. Such reports, proxy and information statements
and other information may be inspected and copied at the public reference
facilities maintained by the Commission at Room 1024, Judiciary Plaza, 450 Fifth
Street, NW, Washington, D.C. 20549, and at the following Regional Offices of the
Commission: New York Regional Office, Seven World Trade Center, Suite 1300, New
York, New York 10048 and Chicago Regional Office, Northwest Atrium Center, 500
West Madison Street, Suite 1400, Chicago, Illinois 60661. Copies of such
material may be obtained by mail at prescribed rates from the Public Reference
Section of the Commission at Judiciary Plaza, 450 Fifth Street, NW, Washington,
D.C. 20549. The Commission maintains a Web site that contains reports, proxy and
information statements and other information regarding registrants that file
electronically with the Commission. The address of the site is
http://www.sec.gov. The Common Stock of the Company is listed on the Nasdaq
National Market, and such reports, proxy and information statements and other
information concerning the Company may be inspected at the offices of Nasdaq
Operations, 1735 K Street, NW, Washington, D.C. 20006.

         This Prospectus constitutes a part of a Registration Statement on Form
S-3 (herein, together with all amendments and exhibits, referred to as the
"Registration Statement") filed by the Company with the Commission under the
Securities Act. This Prospectus does not contain all of the information set
forth in the Registration Statement, certain parts of which are omitted in
accordance with the rules and regulations of the Commission. For further
information with respect to the Company and the shares of Common Stock offered
hereby, reference is hereby made to the Registration Statement. The Registration
Statement may be inspected at the public reference facilities maintained by the
Commission at the addresses set forth in the preceding paragraph. Statements
contained herein concerning any document filed as an exhibit are not necessarily
complete and, in each instance, reference is made to the copy of such document
filed as an exhibit to the Registration Statement. Each such statement is
qualified in its entirety by such reference.

                 INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

         The following documents filed with the Commission by the Company (File
No. 0-19986) pursuant to the Exchange Act are hereby incorporated by reference
in this Prospectus:

         (1)      The Company's Annual Report on Form 10-K for the fiscal year
                  ended December 31, 1996;

         (2)      The amendment on Form 10-K/A to the Company's Annual Report on
                  Form 10-K for the fiscal year ended December 31, 1996, filed
                  with the Commission on April 30, 1997;

         (3)      The Company's Quarterly Report on Form 10-Q for the quarters
                  ended March 31, 1997, June 30, 1997, and September 30, 1997;

         (4)      The description of the Company's Common Stock and associated
                  preferred share purchase rights, contained in its Registration
                  Statements on Form S-4 filed with the Commission on April 30,
                  1997, as supplemented by a Supplement to the Joint Proxy
                  Statement dated April 30, 1997 as filed on May 13, 1997 and
                  Form 8-A filed with the Commission on March 24, 1992 and
                  August 8, 1995; and

         (5)      All other reports filed by the Company pursuant to Sections
                  13(a) or 15(d) of the Exchange Act since December 31, 1996.

         All reports and other documents subsequently filed by the Company
pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date
of this Prospectus and prior to the termination of this offering shall be deemed
to be incorporated by reference into this Prospectus, to the extent required,
and to be a part of this Prospectus from the date of filing of such reports and
documents.

         Any statement contained in a document incorporated by reference into
this Prospectus shall be deemed to be modified or superseded for purposes of
this Prospectus to the extent that a statement contained herein or in any other
subsequently filed document that also is or is deemed to be incorporated by
reference herein modifies or supersedes such statement. Any statement so
modified or superseded shall not be deemed, except as so modified or superseded,
to constitute a part of this Prospectus.

         The Company hereby undertakes to provide without charge to each person,
including any beneficial owner, to whom a copy of this Prospectus has been
delivered, upon written or oral request of such person, a copy of any or all of
the foregoing documents incorporated by reference into this Prospectus (other
than exhibits to such documents, unless such exhibits are specifically
incorporated by reference into such documents). Requests for such documents
should be submitted in writing to Investor Relations, Cell Genesys, Inc., 342
Lakeside Drive, Foster City, CA 94404, or by telephone at (650) 425-4400.







                                      -2-

<PAGE>   3

                                   THE COMPANY

         Cell Genesys is focused on the development and commercialization of
gene therapies to treat major life-threatening diseases, including AIDS and
cancer. The Company's objective is to commercialize both ex vivo and in vivo
gene therapies. The Company's AIDS gene therapy is currently in Phase II
clinical testing. Current studies will evaluate whether the combination of AIDS
gene therapy and antiviral drugs can delay the recurrence of HIV infection,
which has been generally observed in patients who stop their antiviral drug
therapy. These studies will help determine whether the gene therapy can reduce
the requirement for long-term treatment with combinations of three or more
antiviral drugs. The Company anticipates preliminary efficacy data from certain
of these trials in the first half of 1998. In September 1997, the Company
initiated human clinical trials for its T cell gene therapy for colon cancer and
its second generation GVAX (TM) cancer vaccine for melanoma, lung and prostate
cancers. The Company has additional ongoing research programs in other gene
therapy applications, including hemophilia, cardiovascular disease, central
nervous system disorders and other cancer indications, as well as gene delivery,
or vector, technologies. The Company believes that such programs may provide
opportunities for collaborative arrangements with third parties that could also
provide additional funding to the Company.

         In the Company's human monoclonal antibody program (being pursued
through its subsidiary, Abgenix, Inc.), the Company has developed transgenic
technology to create strains of mice capable of producing human monoclonal
antibodies. These transgenic mice contain the majority of human antibody genes
and could produce multiple product candidates. The Company believes that fully
human antibodies should avoid the allergic reactions seen with antibodies
containing mouse proteins, which should make them better suited to long-term
therapy and could provide a marketing advantage. The Company is focused on the
development and commercialization of monoclonal antibodies to treat a wide range
of serious diseases, including transplantation-associated conditions,
inflammation, autoimmune disorders and cancer. The Company currently has three
antibody products in development. The Company's most advanced product, ABX-CBL,
is in Phase II trials for treating steroid-resistant graft versus host disease
(GVHD), a frequent, fatal and currently untreatable complication of allogeneic
bone marrow transplants. To date, ABX-CBL has been used in clinical studies with
over 50 patients, with an approximated response rate of 90% in acute
transplantation-associated disorders. A second antibody product, ABX-IL8, is
expected to enter clinical trials in early 1998 for the treatment of moderate to
severe psoriasis.

         On May 30, 1997, the Company acquired Somatix Therapy Corporation.
Under the terms of the merger agreement, Somatix became a wholly-owned
subsidiary of Cell Genesys in a tax-free reorganization and stock-for-stock
merger. Somatix stockholders received 0.385 shares of Cell Genesys stock for
each share of Somatix stock. A total of 11,089,706 shares of the Company's
Common Stock were issued to Somatix stockholders. The Company believes the
acquisition will strengthen the Company's position in the field of gene therapy.

         The principal executive offices of the Company are located at 342
Lakeside Drive, Foster City, California, 94404, (650) 425-4400.














                                      -3-

<PAGE>   4

                                  RISK FACTORS

         THIS PROSPECTUS AND THE DOCUMENTS INCORPORATED HEREIN BY REFERENCE
CONTAIN FORWARD-LOOKING STATEMENTS THAT INVOLVE RISKS AND UNCERTAINTIES. THE
STATEMENTS CONTAINED IN THIS PROSPECTUS THAT ARE NOT PURELY HISTORICAL ARE
FORWARD-LOOKING STATEMENTS WITHIN THE MEANING OF SECTION 27A OF THE SECURITIES
ACT AND SECTION 21E OF THE EXCHANGE ACT, INCLUDING WITHOUT LIMITATION,
STATEMENTS REGARDING THE COMPANY'S EXPECTATIONS, BELIEFS, ESTIMATES, INTENTIONS
AND STRATEGIES ABOUT THE FUTURE. WORDS SUCH AS "ANTICIPATES," "EXPECTS,"
"INTENDS," "PLANS," "BELIEVES," "SEEKS," "ESTIMATES," VARIATIONS OF SUCH WORDS
AND SIMILAR EXPRESSIONS ARE INTENDED TO IDENTIFY SUCH FORWARD-LOOKING
STATEMENTS. THESE STATEMENTS ARE NOT GUARANTEES OF FUTURE PERFORMANCE AND ARE
SUBJECT TO CERTAIN RISKS, UNCERTAINTIES AND ASSUMPTIONS THAT ARE DIFFICULT TO
PREDICT; THEREFORE, ACTUAL RESULTS MAY DIFFER MATERIALLY FROM THOSE EXPRESSED OR
FORECASTED IN ANY SUCH FORWARD-LOOKING STATEMENTS AS A RESULT OF CERTAIN
FACTORS, INCLUDING THOSE SET FORTH IN THE FOLLOWING RISK FACTORS, ELSEWHERE IN
THIS PROSPECTUS AND IN THE DOCUMENTS INCORPORATED HEREIN BY REFERENCE. THE
COMPANY UNDERTAKES NO OBLIGATION TO UPDATE PUBLICLY ANY FORWARD-LOOKING
STATEMENTS, WHETHER AS A RESULT OF NEW INFORMATION, FUTURE EVENTS OR OTHERWISE.
POTENTIAL INVESTORS SHOULD CONSIDER CAREFULLY THE FOLLOWING FACTORS, AS WELL AS
THE MORE DETAILED INFORMATION CONTAINED ELSEWHERE IN THIS PROSPECTUS AND IN THE
DOCUMENTS INCORPORATED BY REFERENCE, BEFORE MAKING A DECISION TO INVEST IN THE
SHARES OFFERED HEREBY.

         NEED FOR SUBSTANTIAL ADDITIONAL FUNDS. The Company and its subsidiary,
Abgenix, Inc. ("Abgenix"), will require substantial additional funds to continue
existing and planned preclinical and clinical trials and its research and
development activities, and to establish manufacturing and marketing
capabilities for any products it may develop. The Company expects that its
existing capital resources, together with payments to be received under existing
collaborative agreements and amounts available under existing equipment
financing facilities, will enable the Company to maintain the Company's
operations at least into 1999. Beyond such time, the Company will need to raise
substantial additional capital to fund its operations.

         Abgenix will also require substantial additional funds. In particular,
as a result of the issuance of certain patents relating to antibody technology,
a $7.5 million milestone payment under the Cross-License and Settlement
Agreement with GenPharm International, Inc. ("GenPharm") became due and payable
in December 1997 and a second $7.5 million milestone payment is due and payable
in November 1998. Abgenix is responsible for fifty percent (i.e., $3.75
million) of each such payment. Abgenix made the December 1997 milestone payment
to GenPharm by payment from existing capital resources. 

         The Company's future capital requirements will depend on, and could
increase as a result of, many factors, including, but not limited to, the
continuation of the collaboration with Hoechst Marion Roussel ("HMR"), continued
scientific progress in its research and development programs, the magnitude of
such programs, the progress of preclinical and clinical testing, the time and
costs involved in obtaining regulatory approvals, the costs involved in
preparing, filing, prosecuting, maintaining, enforcing and defending patent
claims, competing technological and market developments, changes in
collaborative relationships, the terms of any additional collaborative
arrangements into which the Company may enter, the Company's ability to
establish research, development and commercialization arrangements pertaining to
products other than those covered by existing collaborative arrangements, the
cost of establishing manufacturing facilities, the cost of commercialization
activities, and the demand for the Company's products if and when approved.
There is no assurance that opportunities for in-licensing technologies or for
third party collaborations will continue to be available to the Company on
acceptable terms.

         A major portion of the Company's operating revenues are derived from a
collaborative agreement with HMR signed in October 1995. Under the terms of the
agreement, HMR has the ability to terminate its commitment at any time two years









                                      -4-

<PAGE>   5

after its anniversary date. Also under the agreement, in the fourth quarter, the
parties establish the budget for the next year. The Company has begun
discussions with HMR for the 1998 budget. There is no assurance that HMR will
continue the agreement or that the level of funding will not vary year to year.
The Company's operating results would be adversely affected should HMR decide
not to continue funding under this arrangement.

         Cell Genesys expects to raise additional funds through additional
equity or debt financings, collaborative relationships, or otherwise. Because of
these long-term capital requirements, the Company and its subsidiary, Abgenix,
may seek to access the public or private equity markets whenever conditions are
favorable, even if it does not have an immediate need for additional capital at
that time. There can be no assurance that any such additional funding will be
available to the Company, or, if available, that it will be on acceptable terms.
If additional funds are raised by issuing equity securities, further dilution to
stockholders may result. If adequate funds are not available, the Company may be
required to delay, reduce the scope of, or eliminate one or more of its
research, development and clinical activities or to seek to obtain funds through
arrangements with collaborative partners or others that may require the Company
to relinquish rights to certain of its technologies, product candidates or
products that the Company would otherwise seek to develop or commercialize
itself, any of which could have a material adverse effect on the Company's
business, results of operations, financial condition or cash flow.

         EARLY STAGE OF DEVELOPMENT; NO DEVELOPED OR APPROVED PRODUCTS. Cell
Genesys' potential gene therapy products are in research and development. No
revenues have been generated from the sale of any of such products, nor are any
such revenues expected for at least the next several years. The products
currently under development by Cell Genesys will require significant additional
research and development efforts, including extensive preclinical and clinical
testing and regulatory approval, prior to commercial use. There can be no
assurance that Cell Genesys' research and development efforts will be successful
or that any commercially successful products will ultimately be developed by
Cell Genesys. Even if developed, these products may not receive regulatory
approval or be successfully introduced and marketed at prices that would permit
Cell Genesys to operate profitably.

         TECHNOLOGICAL UNCERTAINTY. Gene therapy is a new technology, and
existing preclinical and clinical data on the safety and efficacy of gene
therapy are limited. Data relating to Cell Genesys' specific gene therapy
approaches are even more limited. The Company's T cell gene therapy for cancer,
GVAX (TM) cancer vaccine and AIDS gene therapy are currently being tested in
Phase I/II and Phase II human clinical trials to determine their safety and
efficacy. None of the other products or therapies under development are in human
clinical trials. The results of preclinical studies do not predict safety or
efficacy in humans. Possible side effects of gene therapy may be serious and
life-threatening. There can be no assurance that unacceptable side effects will
not be discovered during preclinical and clinical testing of Cell Genesys'
potential products or thereafter. There are many reasons that potential products
that appear promising at an early stage of research or development do not result
in commercialization. Although Cell Genesys is testing proposed products or
therapies in human clinical trials, there can be no assurance that Cell Genesys
will be permitted to undertake human clinical trials for any of its other
products or that the results of such testing will demonstrate safety or
efficacy. Even if clinical trials are successful, there is no assurance that
Cell Genesys will obtain regulatory approval for any indication, or that an
approved product can be produced in commercial quantities at reasonable cost, or
be successfully marketed.

         PATENTS AND TRADE SECRETS. The patent positions of pharmaceutical and
biotechnology firms, including Cell Genesys, are generally uncertain and involve
complex legal and factual questions. While Cell Genesys is prosecuting patent
applications, it cannot be certain whether any given application will result in
the issuance of a patent or, if any patent is issued, whether it will provide
significant proprietary protection or will be invalidated. Because patent
applications in the United States are confidential until patents are issued and
publication of discoveries in the scientific or patent literature tends to lag
behind actual discoveries by several months, Cell Genesys cannot be certain that
it was the first creator of inventions covered by pending patent applications or
that it was the first to file patent applications for such inventions.

         The commercial success of the Company will also depend in part on not
infringing the patents or proprietary rights of others and not breaching
licenses granted to Cell Genesys. The Company may be required to obtain licenses
to third party technology necessary to conduct its business. Any failure by the
Company to license at reasonable cost any technology required to commercialize
its technologies or products may have a material adverse effect on the Company's
business, results of operations, financial condition or cash flow.







                                      -5-

<PAGE>   6

         Litigation, which could result in substantial cost to the Company, may
also be necessary to enforce any patents issued to Cell Genesys, or to determine
the scope and validity of other parties' proprietary rights. To determine the
priority of inventions, interference proceedings are frequently declared by the
United States Patent Office that could result in substantial costs to the
Company and may result in an adverse decision as to the priority of Cell
Genesys' inventions. Cell Genesys is currently involved in three separate
interference proceedings with regard to: (i) gene activation technology, (ii) ex
vivo gene therapy, and (iii) chimeric receptor technology. While the Company
believes its position in each interference proceeding is strong, the outcome of
each proceeding cannot be predicted, and an adverse result could have a material
adverse effect on the Company's intellectual property position and its business.
The Company may be involved in other interference proceedings in the future.
Cell Genesys believes that there will continue to be significant litigation in
the industry regarding patent and other intellectual property rights.

         Cell Genesys also relies on unpatented trade secrets and improvements,
unpatented know-how and continuing technological innovation to develop and
maintain its competitive position. No assurance can be given that others will
not independently develop substantially equivalent proprietary information and
techniques, or otherwise gain access to the Company's trade secrets or disclose
such technology, or that the Company can meaningfully protect its rights to its
unpatented trade secrets.

         Cell Genesys requires its employees and consultants to execute a
confidentiality agreement upon the commencement of an employment or consulting
relationship with it. These agreements provide that all confidential information
developed by or made known to an individual during the course of the employment
or consulting relationship generally must be kept confidential. In the case of
employees, the agreements provide that all inventions conceived by the
individual while employed by Cell Genesys, relating to its business are the
exclusive property of Cell Genesys. These agreements may not provide meaningful
protection for the Company's trade secrets in the event of unauthorized use or
disclosure of such information.

         COMPETITION. Competition in the field of gene therapy from other
biotechnology and pharmaceutical companies and from research and academic
institutions is intense and expected to increase. There are numerous competitors
working on products to treat each of the diseases for which Cell Genesys is
seeking to develop therapeutic products. Some competitors are pursuing a product
development strategy competitive with Cell Genesys, particularly with respect to
the Company's human monoclonal antibody program. Certain of these competitive
products are in substantially more advanced stages of product development and
clinical trials. The Company's competitors may develop technologies and products
that are more effective than those being developed by Cell Genesys, or that
would render the Company's technology and products less competitive or obsolete.
Many of these competitors have substantially greater financial resources and
larger research and development staffs than Cell Genesys. In addition, many of
these competitors may have significantly greater experience than Cell Genesys in
developing products, in undertaking preclinical testing and human clinical
trials of new pharmaceutical products, in obtaining United States Food and Drug
Administration (the "FDA") and other regulatory approvals of products, and in
manufacturing and marketing such products. Accordingly, Cell Genesys'
competitors may succeed in obtaining patent protection, receiving FDA approval
or commercializing products more rapidly than Cell Genesys. There can be no
assurance that the Company will be able to obtain certain biological materials
necessary to support its research, development or manufacturing of any of its
planned therapies. If the Company is permitted to commence commercial sales of
products, it will also be competing with respect to marketing capabilities and
manufacturing efficiency, areas in which it has limited or no experience. It is
anticipated that the Company will build additional clinical scale and commercial
scale manufacturing facilities to the extent that contract facilities are not
available in order to commercialize its products. It is also anticipated that
the Company will secure funding for these and other product development
activities through its partners and future potential partners. Cell Genesys also
competes with universities and other research institutions in the development of
products, technologies and processes. In many instances, Cell Genesys competes
with other commercial entities in acquiring products or technology from
universities.

         Cell Genesys expects that competition among products approved for sale
will be based, among other things, on product efficacy, safety, reliability,
availability, price, patent, position, and sales, marketing and distribution
capabilities. Cell Genesys' competitive positions also depend upon its ability
to attract and retain qualified personnel, obtain patent protection or otherwise
develop proprietary products or processes and secure sufficient capital
resources for the often substantial period between product conception and
commercial sales.









                                      -6-


<PAGE>   7

         The levels of revenues and profitability of biotechnology and
pharmaceutical companies such as the Company may be affected by the continuing
efforts of governmental and third-party payers to contain or reduce the costs of
health care through various means. In the United States there have been, and
Cell Genesys expects that there will continue to be, a number of federal and
state proposals to control health care costs. Cell Genesys cannot predict the
effect health care reforms may have on its business, and no assurance can be
given that any such reforms will not have a material adverse effect on the
Company's business, results of operations, financial condition or cash flow. In
the United States and elsewhere, sales of therapeutic products are dependent in
part on the availability of reimbursements to the consumer from third party
payers, such as government and private insurance plans. If the Company succeeds
in bringing one or more products to the market, there can be no assurance that
these products will be considered cost effective and that reimbursement to the
consumer will be available or will be sufficient to allow the Company to sell
its products on a competitive basis.

         OPERATING LOSS AND ACCUMULATED DEFICIT. Cell Genesys has incurred net
losses since its inception. At September 30, 1997, Cell Genesys' accumulated
deficit was approximately $170.9 million. Such losses have resulted principally
from expenses incurred in its research and development programs, the acquisition
of Somatix and other new technologies, and to a lesser extent, from general and
administrative expenses. For the nine months ended September 30, 1997, Cell
Genesys incurred losses of $114.8 million, including $78.9 million related to
the acquisition of Somatix and $18.8 million related to the Cross-License and
Settlement Agreement with GenPharm. The Company expects to incur substantial and
increasing losses for at least the next several years due primarily to the
expansion of research and development programs, including preclinical studies,
clinical trials and manufacturing. Cell Genesys expects that losses will
fluctuate from quarter to quarter and that such fluctuations may be substantial.
There can be no assurance that the Company will successfully develop,
commercialize, manufacture or market its products or ever achieve or sustain
product revenues or profitability.

         VOLATILITY OF STOCK PRICE. The market prices for securities of
biopharmaceutical and biotechnology companies (including Cell Genesys) have
historically been highly volatile, and the market has from time to time
experienced significant price and volume fluctuations that are unrelated to the
operating performance of particular companies. Factors such as fluctuations in
Cell Genesys' operating results, announcements of technological innovations or
new therapeutic products by Cell Genesys or its competitors, governmental
regulation, developments in patent or other proprietary rights, public concern
as to the safety of products developed by the Company or other biotechnology and
pharmaceutical companies, and general market conditions may have a significant
effect on the market price of the Cell Genesys' Common Stock.

         GOVERNMENT REGULATION. Regulation by governmental authorities in the
United States and foreign countries is a significant factor in the manufacture
and marketing of Cell Genesys' proposed products and its research and
development activities. All of Cell Genesys' products will require regulatory
approval by governmental agencies prior to commercialization. In particular,
human therapeutic products must undergo rigorous preclinical and clinical
testing and other premarket approval procedures by the FDA and similar
authorities in foreign countries. Since certain of Cell Genesys' potential
products involve the application of new technologies, regulatory approvals may
take longer than for products produced using more conventional methods. Various
federal and, in some cases, state statutes and regulations also govern or
influence the manufacturing, safety, labeling, storage, record keeping and
marketing of such products. The lengthy process of seeking these approvals, and
the subsequent compliance with applicable federal statutes and regulations,
requires the expenditure of substantial resources. Any failure by Cell Genesys
or its collaborators or licensees to obtain, or any delay in obtaining,
regulatory approvals could adversely affect the marketing of any products
developed by Cell Genesys, and its ability to receive product or royalty
revenue.

         In responding to a new drug application, or a product license
application, the FDA may grant marketing approvals, request additional
information or further research, or deny the application if it determines that
the application does not satisfy its regulatory approval criteria. Approvals may
not be granted on a timely basis, if at all, or if granted may not cover all the
clinical indications for which Cell Genesys is seeking approval or may contain
significant limitations in the form of warnings, precautions or
contraindications with respect to conditions of use.

         In addition to laws and regulations enforced by the FDA, Cell Genesys
is also subject to regulation under the Occupational Safety and Health Act, the
Environmental Protection Act, the Toxic Substances Control Act, the Resource
Conservation and Recovery Act and other present and potential future federal,
state or local laws and regulations. Cell Genesys' research and development
involves the controlled use of hazardous materials, chemicals, viruses and
various radioactive compounds. Although Cell Genesys believes its safety
procedures for handling and disposing of such materials comply with the
standards prescribed by state and federal regulations, the risk of accidental
contamination or injury from








                                      -7-
<PAGE>   8

these materials cannot be completely eliminated. In the event of such an
accident, Cell Genesys could be held liable for any damages that result and any
such liability could exceed the resources of the Company.

         The manufacturing facilities of Cell Genesys are subject to licensing
requirements of the California Department of Health Services. While not subject
to license by the FDA, such facilities are subject to inspection by the FDA as
well as by the California Department of Health Services. A separate license from
the FDA is required for commercial manufacture of any product. Failure to
maintain such licenses or to meet the inspection criteria of the FDA and the
California Department of Health Services would result in disruption to the
manufacturing processes of the Company and would have a material adverse effect
on the Company's business, results of operations, financial condition and cash
flow.

         For marketing outside the United States, Cell Genesys is subject to
foreign regulatory requirements governing human clinical trials and marketing
approval for drugs and devices. The requirements governing the conduct of
clinical trials, product licensing, pricing and reimbursement vary greatly from
country to country. Failure to comply with such regulatory requirements or
obtain such approvals could impair the Company's ability to develop these
markets and have a material adverse effect on the Company's business, results of
operations, financial condition or cash flow.

         COMMERCIALIZATION; LACK OF MARKETING EXPERIENCE. It is anticipated that
the Company will rely on sales and marketing expertise of potential corporate
partners for its initial products. Cell Genesys does not have any experience in
sales, marketing or distribution of biopharmaceutical products. The decision to
market future products directly or through corporate partners will be based on a
number of factors including market size and concentration, the size and
expertise of the partner's sales force in a particular market and the Company's
overall strategic objectives. Cell Genesys is currently engaged in various
stages of discussions with potential partners. There can be no assurance that
Cell Genesys will be able to establish such relationships, if at all, on
acceptable terms and conditions.

         PRODUCT LIABILITIES AND INSURANCE. Clinical trials or marketing of any
of Cell Genesys' potential products may expose the Company to liability claims
resulting from the use of such products. These claims might be made directly by
consumers, health care providers or by others selling such products. Cell
Genesys currently maintains its product liability insurance with respect to its
clinical trials. There can be no assurance that the Company will be able to
maintain such insurance or, if maintained, that sufficient coverage can be
acquired at a reasonable cost. An inability to maintain insurance at acceptable
cost or at all could prevent or inhibit the clinical testing or
commercialization of products developed by the Company. A product liability
claim or recall could have a material adverse effect on the Company's business,
results of operations, financial condition and cash flow.

         HAZARDOUS MATERIALS; ENVIRONMENTAL MATTERS. Cell Genesys' research and
development activities involve the controlled use of hazardous materials,
chemicals, viruses and various radioactive compounds. Cell Genesys is subject to
federal, state and local laws and regulations governing the use, manufacture,
storage, handling and disposal of such materials and certain waste products.
Although Cell Genesys believes that its safety procedures for handling and
disposing of such materials comply with the standards prescribed by such laws
and regulations, the risk of accidental contamination or injury from these
materials cannot be completely eliminated. In the event of such an accident, the
Company could be held liable for any damages that result and any such liability
could exceed the resources of the Company. The Company may be required to incur
significant costs to comply with environmental laws and regulations in the
future. The Company's operations, business or assets may be materially adversely
affected by current or future environmental laws or regulations.

         REIMBURSEMENT. In both domestic and foreign markets, sales of the
Company's potential products will depend in part upon coverage and reimbursement
from third-party payers, including health care organizations, government
agencies, private health care insurers and other health care payers such as
health maintenance organizations, self-insured employee plans and the Blue
Cross/Blue Shield plans. There is considerable pressure to reduce the cost of
drug products. In particular, reimbursement from government agencies and
insurers and large health organizations may become more restricted in the
future. Cell Genesys' potential products represent a new mode of therapy and,
while the cost-benefit ratio of the products may be favorable, Cell Genesys
expects that the costs associated with the Company's products will be
substantial. There can be no assurance that the Company's proposed products, if
successfully developed, will be considered cost-effective by third-party payers,
that insurance coverage will be available or, if available, that such
third-party payers' reimbursement policies will not adversely affect the
Company's ability to sell its products on a profitable basis. In addition, there
can be no assurance that insurance coverage will be provided by such third-party
payers at all or without substantial









                                      -8-

<PAGE>   9

delay or, if such coverage is provided, that the approved reimbursement will
provide sufficient funds to enable the Company to become profitable.

         UNCERTAINTY OF PHARMACEUTICAL PRICING AND RELATED MATTERS. The future
revenues and profitability of and availability of capital for biotechnology
companies may be materially adversely affected by the continuing efforts of
governmental and third-party payers to contain or reduce the costs of health
care through various means. For example, in certain foreign markets, pricing or
profitability of prescription pharmaceuticals is subject to government control.
In the United States, there have been, and Cell Genesys expects that there will
continue to be, a number of federal and state proposals to implement similar
government control. While Cell Genesys cannot predict whether any such
legislative or regulatory proposals will be adopted, the announcement or
adoption of such proposals could have a material adverse effect on the Company's
business, results of operations, financial condition and cash flow.

         DEPENDENCE UPON KEY PERSONNEL AND COLLABORATIVE RELATIONSHIPS. Cell
Genesys' success is highly dependent on the retention of principal members of
management and scientific staff and the recruitment of additional qualified
personnel. The loss of key personnel or the failure to recruit necessary
additional qualified personnel could have a material adverse effect on the
Company's business, results of operations, financial condition or cash flow.
There is intense competition from other companies, research and academic
institutions and other organizations for qualified personnel in the areas of
Cell Genesys' activities. There is no assurance that the Company will be able to
continue to attract and retain the qualified personnel necessary for the
development of its business. These activities are expected to require the
addition of new personnel with expertise in the areas of clinical testing,
manufacturing, marketing and distribution and the development of additional
expertise by existing personnel. The failure to acquire such personnel or
develop such expertise could have a material adverse effect on the Company's
business, results of operations, financial condition and cash flow.

         Cell Genesys has clinical trial arrangements with the National
Institutes of Allergy and Infectious Diseases covering the initial
proof-of-principle study for AIDS gene therapy being conducted in identical twin
pairs now in Phase II testing. Cell Genesys also has clinical trial arrangements
with the University of California, San Francisco, San Francisco General
Hospital, the University of Colorado Health Sciences Center, Massachusetts
General Hospital, the University of California, Los Angeles, ViRx, Inc. and the
Aids Research Community Consortium covering Cell Genesys' patient-specific
configuration of its AIDS gene therapy also now in Phase II testing. If these
relationships were terminated, progress of clinical testing would be adversely
affected.

         In addition, the Company has several clinical trial arrangements under
its GVAX (TM) program, including with The Johns Hopkins University covering a
Phase I clinical trial to treat prostate cancer patients and with the Dana
Farber Cancer Institute covering two Phase I clinical trials to treat lung
cancer and melanoma patients. In the event that any of these relationships are
terminated, the completion and evaluation of clinical trials could be adversely
affected.

         The Company will depend, in part, on the continued availability of
outside scientific collaborators performing research. These relationships
generally may be terminated at any time by the collaborator, typically by giving
30 days' notice. The Company's scientific collaborators are not employees of
Cell Genesys. As a result, the Company has limited control over their activities
and can expect that only limited amounts of their time will be dedicated to the
Company's activities. The Company's agreements with these collaborators, as well
as those with the Company's scientific consultants provide that any rights the
Company obtains as a result of the research efforts of these individuals will be
subject to the rights of the research institutions in such work. In addition,
some of these collaborators have consulting or other advisory arrangements with
other entities that may conflict with their obligations to the Company. For
these reasons, there can be no assurance that inventions or processes discovered
by the Company's scientific collaborators or consultants will become the
property of the Company.

         SHARES ELIGIBLE FOR FUTURE SALE; DILUTION. Substantially all of the
Company's shares are eligible for sale in the public market. The issuance of
Common Stock upon conversion of the Series B Preferred Stock and Convertible
Note and upon exercise of the Warrants, as well as future sales of such Common
Stock or of shares of Common Stock by existing stockholders, or the perception
that such sales could occur, could adversely affect the market price of the
Common Stock. The Common Stock issuable upon conversion of the Series B
Preferred Stock and the Convertible Note and upon exercise of the Warrants are
being registered hereunder. Conversion of the Series B Preferred Stock and the
Convertible Note and exercise of the Warrants for shares of Common Stock could
adversely affect the market price of the Common Stock. In addition, investors
could experience substantial dilution upon conversion of the Series B Preferred
Stock into Common Stock








                                      -9-

<PAGE>   10

as a result of either (i) a decline in the market price of the Company's Common
Stock immediately prior to conversion, or (ii) an event triggering the
antidilution rights of any outstanding shares of Series B Preferred Stock. See
"Description of Capital Stock."


                                 USE OF PROCEEDS

         The Company will not receive any of the proceeds from the sale of the
Shares. All proceeds from the sale of the Shares will be for the account of the
Selling Stockholders, as described below. See "Selling Stockholders" and "Plan
of Distribution" described below.

































                                      -10-

<PAGE>   11

                              SELLING STOCKHOLDERS

         The following table sets forth as of the date of this Prospectus, the
name of each of the Selling Stockholders, the number of shares of Common Stock
that each such Selling Stockholder beneficially owns as of December 16, 1997,
the number of shares of Common Stock beneficially owned by each Selling
Stockholder that may be offered for sale from time to time by this Prospectus,
and the number of shares of Common Stock to be held by each such Selling
Stockholder assuming the sale of all the Common Stock offered hereby.

         The Company has agreed to initially register 5,624,000 Shares, which
are potentially issuable upon conversion of the Series B Preferred Stock, for
resale by the Selling Stockholders holding Series B Preferred Stock. The number
of Shares shown in the following table as being offered by the Selling
Stockholders holding Series B Preferred Stock does not include such presently
indeterminate number of shares of Common Stock as may be issuable upon
conversion of the Series B Preferred Stock pursuant to the provisions thereof
regarding determination of the applicable conversion price which may fluctuate
and certain antidilution provisions which shares of Common Stock are, in
accordance with Rule 416 under the Securities Act, included in the Registration
Statement on Form S-3 of which this Prospectus forms a part.

         The Shares being offered by the Selling Stockholders were acquired from
the Company (i) following conversion of the Series B Preferred Stock acquired
from the Company in a private placement transaction pursuant to the Securities
Purchase Agreement dated November 13, 1997 (the "Securities Purchase
Agreement"), (ii) upon exercise of the Warrants which were assumed by Cell
Genesys in connection with the acquisition of Somatix, or (iii) following the
conversion of the Convertible Note. For a description of such securities, see
"Description of Capital Stock."

         Each Selling Stockholder that purchased the Series B Preferred Stock
pursuant to the Securities Purchase Agreement represented to the Company that it
would acquire the Shares for investment and with no present intention of
distributing any such Shares except pursuant to this Prospectus. Pursuant to its
obligation under the Securities Purchase Agreement, the Company filed with the
Commission, under the Securities Act, a Registration Statement on Form S-3, of
which this Prospectus forms a part, with respect to the resale of the Shares
from time to time on the Nasdaq National Market or in privately-negotiated
transactions and has agreed to use its best efforts to keep such Registration
Statement effective until the earlier of (i) the date all holders may sell all
of the Shares freely pursuant to Rule 144 without compliance with the
registration requirement of the Securities Act, or (ii) the date all of the
Shares have been sold and no shares of Series B Preferred Stock are outstanding.

         Except as indicated, none of the Selling Stockholders has held any
position or office or had a material relationship with the Company or any of its
affiliates within the past three years other than as a result of the ownership
of the Company's Common Stock. The Company may amend or supplement this
Prospectus from time to time to update the disclosure set forth herein.


<TABLE>
<CAPTION>                                         
                                                                                                  Shares of Common
                                                                                                 Stock Beneficially
                                             Shares of                                              Owned After
                                             Preferred  Shares Beneficially       Shares         Offering(1)(2)(4)
                                              Stock        Owned Prior to          Being        ---------------------
Selling Stockholder                           Owned        Offering(1)(2)      Offered(3)(4)    Number        Percent
- ---------------------------------------      ---------  -------------------    -------------    ------        -------
<S>                                          <C>           <C>                  <C>             <C>           <C>
WARRANT HOLDERS
Aberlyn Capital Management L.P. (5) ...         -               4,812              4,812           --            --
Aeneas Venture Corporation (5) ........         -             799,075             80,850        718,225          2.5
Berman, Martin (5) ....................         -               6,620              6,620           --            --
Betje Partners, L.P. (5) ..............         -              25,919              8,624         17,295          *
Birch, Robert S. (5) ..................         -              16,197              5,390         10,807          *
Fawer, Mark (5) .......................         -              10,153             10,153           --            --
Feinberg, Larry N.(5) .................         -               5,975              5,975           --            --
</TABLE>











                                      -11-


<PAGE>   12

<TABLE>
<CAPTION>
                                                                                                 Shares of Common
                                                                                                Stock Beneficially
                                             Shares of                                             Owned After
                                             Preferred  Shares Beneficially     Shares          Offering(1)(2)(4)
                                              Stock        Owned Prior to        Being         ---------------------
Selling Stockholder                           Owned        Offering(1)(2)     Offered(3)(4)    Number        Percent
- ---------------------------------------      ---------  -------------------   -------------    ------        -------
<S>                                          <C>           <C>                  <C>             <C>           <C>
Financing For Science International(5)          -               7,434             7,434             --            --
Kingsley & Co. (5) ....................         -              13,475            13,475             --            --
Legg Mason Special Investment Trust, ..         -              58,587            58,587             --            --
Inc. (5)                                                                                                   
Merrill Lynch, Pierce, Fenner & Smith .         -              20,355            20,355             --            --
Inc. (5)                                                                                                   
Morgens, Waterfall, Vintidiadis .......         -              53,458            17,787           35,671          *
Investments (5)                                                                                            
Oracle Institutional Partners, L.P. (5)         -               3,018             3,018             --            --
Oracle Partners, L.P. (5) .............         -              75,460            75,460             --            --
Phoenix Partners, L.P. (5) ............         -              82,616            27,489           55,127          *
Quasar International Partners, C.V. (5)         -              11,642            11,642             --            --
Rosenfeld, Steven (5) .................         -              14,515            14,515             --            --
Sam Oracle Fund, Inc. (5) .............         -              19,766            19,766             --            --
SBSF Biotechnology Fund, L.P. (5) .....         -             121,494            40,425           81,069          *
Shenk, Susan (5) ......................         -              16,550            16,550             --            --
TBD Ltd. (5) ..........................         -              33,101            33,101             --            --
Warburg Pincus Emerging Growth ........         -              80,850            80,850             --            --
Fund (5)                                                                                                   
Weisbrod, Stuart T. (5) ...............         -               1,466             1,466             --            --
Westcoat & Co. (5) ....................         -             146,608           146,608             --            --
WHI Somatix Partners (5) ..............         -              26,950            26,950             --            --
                                             ----           ---------        ----------         ---------      -----
    Subtotal ........................          --           1,656,096           737,902           918,194

SERIES B PREFERRED STOCKHOLDERS
AG Super Fund International Partners,            25            31,023            70,300              --           --
L.P. (6)
Colonial Penn Life Insurance ........            50            62,047           140,600              --           --
Company (6)
GAM Arbitrage Investments, Inc. (6) .            25            31,023            70,300              --           --
Halifax Fund, L.P. (6) ..............           500           620,469         1,406,000              --           --
Heracles Fund (6) ...................           175           217,164           492,100              --           --
Leonardo, L.P. (6) ..................           365           452,942         1,026,380              --           --
Nelson Partners (6)(7) ..............           247           306,511           694,564              --           --
Olympus Securities, Ltd. (6)(7) .....           303           376,004           852,036              --           --
Ramius Fund, Ltd. (6) ...............            85           105,480           239,020              --           --
Raphael, L.P. (6) ...................            50            62,047           140,600              --           --
Themis Partners L.P. (6) ............           175           217,164           492,100              --           --
                                             ------         ---------        ----------         ---------      -----
      Subtotal                                2,000         2,481,874         5,624,000              --           --

NOTE HOLDER
Medarex, Inc. (8)....................             -         1,666,667         1,666,667              --           --
                                             ------         ---------        ----------         ---------      -----


      Total..........................        2,000          5,804,637        8,028,569            918,194
                                             ======         =========        ==========         ========= 
</TABLE>


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

    *   Less than one percent.

(1) Based upon 28,168,459 shares of Common Stock outstanding as of 
    December 16, 1997. Except as otherwise noted herein, the number and 
    percentage of shares beneficially owned is determined in accordance 
    with Rule 13d-3 of 








                                      -12-

<PAGE>   13

    the Exchange Act, and the information is not necessarily indicative of
    beneficial ownership for any other purpose. Under such rule, beneficial
    ownership includes any shares as to which the individual has sole or shared
    voting power or investment power and also any shares which the individual
    has the right to acquire within 60 days of the date of this Prospectus
    through the exercise of any stock option or other right. Unless otherwise
    indicated in the footnotes, each person has sole voting and investment power
    (or shares such powers with his or her spouse) with respect to the shares
    shown as beneficially owned. 
(2) The number of Shares shown as beneficially owned prior to the offering by
    the Selling Stockholders holding Series B Preferred Stock represents shares
    of Common Stock issuable to the Selling Stockholders assuming conversion, as
    of December 16, 1997, of all shares of Series B Preferred Stock issued
    calculated using an assumed conversion price of $8.09375 (representing the
    average of the two lowest closing bid prices for the Common Stock during the
    10 consecutive trading days ending on December 15, 1997), with respect to
    the stated value of the Series B Preferred Stock plus an accretion of 5% per
    year, based upon certain conversion price provisions of the Series B
    Preferred Stock (which conversion price could fluctuate from time to time
    based on changes in the market price of the Common Stock). This Prospectus
    also covers the resale of such presently indeterminate number of additional
    shares as may be issuable upon conversion of the Series B Preferred Stock
    based upon fluctuations in the conversion price of the Series B Preferred
    Stock and certain antidilution provisions. As described in footnote 6 below,
    the actual number of shares of Common Stock issuable upon conversion of the
    Series B Preferred Stock depends, subject to certain limitations, upon the
    lower of (i) the Fixed Conversion Price (as defined pursuant to the
    provisions of the Series B Preferred Stock) which is currently $11.02, and
    (ii) the average of the two lowest closing bid prices during the ten
    consecutive trading days immediately prior to conversion, and may be less
    than or greater than the number of shares shown as beneficially owned by the
    Selling Stockholders or otherwise covered by this Prospectus. The number of
    Shares shown as beneficially owned does not include shares of Common Stock
    issuable upon the conversion of (a) up to 1,000 shares of Series B Preferred
    Stock which, beginning on November 9, 1998 and subject to certain
    conditions, the Company may cause the holders of Series B Preferred Stock to
    purchase and (b) a number of shares of Series B Preferred Stock which,
    beginning on November 1998 and subject to certain conditions, the initial
    purchasers of Series B Preferred Stock have the right to acquire from the
    Company.
(3) The number of shares of Common Stock registered pursuant to the Registration
    Statement on behalf of the Selling Stockholders holding Series B Preferred
    Stock and the number of Shares offered hereby by such holders have been
    determined by agreement between the Company and such Selling Stockholders.
    Because the number of shares that will ultimately be issued upon conversion
    of the Series B Preferred Stock is dependent, subject to certain
    limitations, upon the average of certain closing bid prices of the Common
    Stock prior to conversion as described in footnote 6 below and certain
    antidilution adjustments, such number of shares (and therefore the number of
    Shares offered hereby) cannot be determined at this time. The number of
    Shares being offered by the Selling Stockholders owning Series B Preferred
    Stock, in accordance with Rule 416, also includes such presently
    indeterminate number of additional Shares as may be issuable upon conversion
    of the shares of the Company's Series B Preferred Stock held by or issuable
    to certain of the Selling Stockholders, based upon fluctuations in the
    conversion price of the Series B Preferred Stock and any future antidilution
    adjustments in accordance with the terms of the Series B Preferred Stock.
(4) Assumes the sale of all Shares offered hereby.
(5) Shares offered pursuant to this registration statement consist entirely of
    shares of Cell Genesys Common Stock issued or issuable upon exercise of
    outstanding Warrants.
(6) Each share of Series B Preferred Stock is convertible into that number of
    shares of Common Stock equal to (i) the share's stated value of $10,000,
    plus a premium in the amount of 5% per annum accruing from the date of
    issuance through the date of conversion, divided by (ii) the conversion
    price equal to the lesser of (A) the Fixed Conversion Price (as defined
    pursuant to the provisions of the Series B Preferred Stock), and (B) a
    floating conversion price equal to 100% (subject to downward adjustment upon
    certain events, including if the Common Stock is not quoted on certain
    markets) of the average of the two lowest closing bid prices for the Common
    Stock over 10 trading days preceding the conversion date. The initial Fixed
    Conversion Price for the 2,000 shares of Series B Preferred Stock issued on
    November 14, 1997 is $11.02.

    In accordance with the rights and restrictions of the Series B Preferred
    Stock, unless waived by the Selling Stockholder upon 61 days prior written
    notice, no Selling Stockholder may convert the Series B Preferred Stock to
    the extent that the shares to be received by such holder upon such
    conversion would cause such holder to beneficially own (other than Shares of
    Common Stock so owned through ownership of Series B Preferred Stock) more
    than 4.99% of the outstanding shares of Common Stock. In addition, pursuant
    to the regulations of the National Association of Securities Dealers, Inc.,
    in the absence of stockholder approval, the aggregate number of shares of
    Common Stock issuable to the Selling Stockholders at a discount from market
    price upon conversion of the Series B Preferred Stock may not equal or
    exceed 20% of the outstanding shares of Common Stock on November 14, 1997
    (i.e., 5,628,850 shares). If stockholder approval is not obtained to issue
    shares of Common Stock to the Selling Stockholders in excess of such amount,
    none of the Selling Stockholders will be entitled to acquire more than its
    proportionate share of such maximum amount. Any Series B Preferred Stock
    which may not be converted because of such limitation must be redeemed by
    the Company. For a complete description of the rights, preferences and
    restrictions of the Series B Preferred Stock, see "Description of Capital
    Stock".
(7) Citadel Limited Partnership is the managing general partner of Nelson
    Partners ("Nelson") and the trading manager of Olympus Securities, Ltd.
    ("Olympus") and consequently has voting control and investment discretion
    over









                                      -13-


<PAGE>   14


    securities held by both Nelson and Olympus. The ownership information for
    Nelson does not include the Shares owned by Olympus and the ownership
    information for Olympus does not include the Shares owned by Nelson. 
(8) Shares offered pursuant to this registration statement consist entirely of
    shares issuable upon conversion of the Convertible Note, based on the
    Company's estimate of the maximum number of shares of Common Stock likely
    issuable upon conversion.


                              PLAN OF DISTRIBUTION

         The Shares covered by this Prospectus may be offered and sold from time
to time by the Selling Stockholders. The Selling Stockholders will act
independently of the Company in making decisions with respect to the timing,
manner and size of each sale. The Selling Stockholders may sell the Shares being
offered hereby on the Nasdaq National Market, or otherwise, at prices and under
terms then prevailing or at prices related to the then current market price or
at negotiated prices. The Shares may be sold by one or more of the following
means of distribution: (a) a block trade in which the broker-dealer so engaged
will attempt to sell Shares as agent, but may position and resell a portion of
the block as principal to facilitate the transaction; (b) purchases by a
broker-dealer as principal and resale by such broker-dealer for its own account
pursuant to this Prospectus; (c) an over-the-counter distribution in accordance
with the rules of the Nasdaq National Market; (d) ordinary brokerage
transactions and transactions in which the broker solicits purchasers; (e) in
privately negotiated transactions; (f) to cover short sales; and (g) a
combination of any of the foregoing. To the extent required, this Prospectus may
be amended and supplemented from time to time to describe a specific plan of
distribution. In connection with distributions of the Shares or otherwise, the
Selling Stockholders may enter into hedging transactions with broker-dealers or
other financial institutions. In connection with such transactions,
broker-dealers or other financial institutions may engage in short sales of the
Company's Common Stock in the course of hedging the positions they assume with
Selling Stockholders. The Selling Stockholders may also sell the Company's
Common Stock short and redeliver the shares to close out such short positions.
The Selling Stockholders may also enter into option or other transactions with
broker-dealers or other financial institutions which require the delivery to
such broker-dealer or other financial institution of Shares offered hereby,
which Shares such broker-dealer or other financial institution may resell
pursuant to this Prospectus (as supplemented or amended to reflect such
transaction). The Selling Stockholders may also pledge Shares to a broker-dealer
or other financial institution, and, upon a default, such broker-dealer or other
financial institution, may effect sales of the pledged Shares pursuant to this
Prospectus (as supplemented or amended to reflect such transaction). In
addition, any Shares that qualify for sale pursuant to Rule 144 may be sold
under Rule 144 rather than pursuant to this Prospectus.

         In effecting sales, brokers, dealers or agents engaged by the Selling
Stockholders may arrange for other brokers or dealers to participate. Brokers,
dealers or agents may receive commissions, discounts or concessions from the
Selling Stockholders in amounts to be negotiated prior to the sale. Such brokers
or dealers and any other participating brokers or dealers may be deemed to be
"underwriters" within the meaning of the Securities Act in connection with such
sales, and any such commissions, discounts or concessions may be deemed to be
underwriting discounts or commissions under the Securities Act. The Company will
pay all expenses incident to the offering and sale of the Shares to the public
other than any commissions and discounts of underwriters, dealers or agents and
any transfer taxes.

         In order to comply with the securities laws of certain states, if
applicable, the Shares must be sold in such jurisdictions only through
registered or licensed brokers or dealers. In addition, in certain states the
Shares may not be sold unless they have been registered or qualified for sale in
the applicable state or an exemption from the registration or qualification
requirement is available and there has been compliance thereof.

         The Company has advised the Selling Stockholders that the
anti-manipulation rules of Regulation M under the Exchange Act may apply to
sales of Shares in the market and to the activities of the Selling Stockholders
and their affiliates. In addition, the Company will make copies of this
Prospectus available to the Selling Stockholders and has informed them of the
need for delivery of copies of this Prospectus to purchasers at or prior to the
time of any sale of the Shares offered hereby. The Selling Stockholders may
indemnify any broker-dealer that participates in transactions involving the sale
of the shares against certain liabilities, including liabilities arising under
the Securities Act.

         At the time a particular offer of Shares is made, if required, a
Prospectus Supplement will be distributed that will set forth the number of
Shares being offered and the terms of the offering, including the name of any
underwriter, dealer or agent, the purchase price paid by any underwriter, any
discount, commission and other item constituting








                                      -14-


<PAGE>   15

compensation, any discount, commission or concession allowed or reallowed or
paid to any dealer, and the proposed selling price to the public.

         The sale of Shares by the Selling Stockholders is subject to compliance
by the Selling Stockholders with certain contractual restrictions with the
Company. There can be no assurance that the Selling Stockholders will sell all
or any of the Shares.

         The Company has agreed to indemnify the Selling Stockholders and any
person controlling a Selling Stockholder against certain liabilities, including
liabilities under the Securities Act. The Selling Stockholders have agreed to
indemnify the Company and certain related persons against certain liabilities,
including liabilities under the Securities Act.

         The Company has agreed with certain of the Selling Stockholders to keep
the Registration Statement of which this Prospectus constitutes a part effective
until all the Shares are sold by the Selling Stockholders or all unsold Shares
are freely tradable subject to compliance with Rule 144 of the Securities Act.


                          DESCRIPTION OF CAPITAL STOCK

         The authorized capital stock of the Company consists of 75,000,000
shares of Common Stock and 5,000,000 shares of Preferred Stock. As of December
16, 1997, there were approximately 28,168,459 shares of Common Stock outstanding
held of record by approximately 1,096 holders and 2,000 shares of Preferred
Stock outstanding held of record by approximately 11 holders.

COMMON STOCK

         The holders of Common Stock are entitled to one vote per share on all
matters to be voted upon by the stockholders, except that, in the election of
directors, the holders are entitled to cumulative voting. Subject to preferences
of outstanding Preferred Stock, the holders of Common Stock are entitled to
receive ratably such dividends, if any, as may be declared from time to time by
the board of directors out of funds legally available for that purpose. In the
event of a liquidation, dissolution or winding up of the Company, the holders of
Common Stock are entitled to share ratably in all assets remaining after payment
of liabilities, subject to prior distribution rights of Preferred Stock, if any,
then outstanding. The Common Stock has no preemptive or conversion rights or
other subscription rights. There are no redemption or sinking fund provisions
applicable to the Common Stock. All outstanding shares of Common Stock are fully
paid and non-assessable, and the shares of Common Stock offered hereby will be
fully paid and non-assessable.

PREFERRED STOCK

         The Board of Directors has the authority, without further action by the
stockholders, to issue the undesignated Preferred Stock in one or more series,
to fix the rights, preferences, privileges and restrictions granted to or
imposed upon any wholly unissued shares of undesignated Preferred Stock and to
fix the number of shares constituting any series and the designation of such
series. The purpose of authorizing the board of directors of Cell Genesys to
determine such rights and preferences is to eliminate delays associated with a
stockholder vote on specific issuances. The issuance of Preferred Stock, while
providing flexibility in connection with possible acquisitions and other
corporate purposes, could, among other things, adversely affect the voting power
of holders of Cell Genesys Common Stock and, under certain circumstances, make
it more difficult for a third party to gain control of Cell Genesys.

         The Company is authorized to issue 5,000,000 shares of undesignated
Preferred Stock, of which (i) 200,000 shares have been designated as Series A
Participating Preferred Stock (the "Series A Preferred Stock"), none of which is
issued and outstanding, and (ii) 4,000 shares have been designated Series B
Convertible Preferred Stock (the "Series B Preferred Stock"), of which 2,000
shares are issued and outstanding. The Series A Preferred Stock was designated
in connection with the adoption of a preferred shares rights agreement dated
July 28, 1995 and is described in the Company's Registration Statement on Form
8-A pursuant to which such preferred shares rights agreement is filed as an
exhibit thereto. 




                                      -15-

<PAGE>   16


In the event of a liquidation, dissolution or winding up of the Company, the
holders of Series B Preferred Stock are entitled to receive in cash out of the
assets of the Company in preference to the Series A Preferred Stock, any other
series of Preferred Stock ranking junior to the Series B Preferred Stock and to
the holders of Common Stock in an amount equal to $10,000 per share plus the
aggregate amount of accumulated premium of 5% per annum. Payment must be made in
full to holders of the Series B Preferred Stock, and thereafter, any remaining
assets may be distributed ratably among the holders of Common Stock.

         The 2,000 shares of Series B Preferred Stock issued on November 14,
1997 are convertible into shares of Common Stock of the Company based on a
conversion price of $11.02 per share or, if lower, 100% of the average of
certain specified trading prices during the 10 trading days preceding such date
of conversion (the "Floating Conversion Price"); provided that, subject to
certain limitations, the Floating Conversion Price for the shares of Series B
Preferred Stock issued on November 14, 1997 shall not be less than (i) $6.612
during the period between 90 days and 180 days after the issuance date, and (ii)
$4.41 during the period between 181 days and 270 days from the issuance date.
Each holder may not convert the Series B Preferred Stock issued on November 14,
1997 until 90 days after issuance, and thereafter (i) up to 25% may be converted
during the period between 90 days and 135 days from issuance, (ii) up to 50% may
be converted during the period between 135 days and 180 days from issuance,
(iii) up to 75% may be converted during the period between 180 and 225 days from
issuance, and (iv) the entire balance outstanding may be converted after 225
days from issuance; provided that, a holder will not be permitted at anytime to
convert an amount of Series B Preferred Stock which would result in the holder
beneficially owning (other than shares of Common Stock owned through ownership
of Series B Preferred Stock) more than 4.9% of the then outstanding capital
stock of the Company absent 61 day prior written waiver by such holder. Subject
to certain conditions, the Company may elect any time after three years from the
issue date to require conversion of some or all of the Series B Preferred Stock
if the last reported sale price of the Company's Common Stock is at least $27.55
per share for a period of 20 consecutive trading days prior to such election.
Any outstanding balance of Series B Preferred Stock is subject to mandatory
conversion five years from the date of issuance, subject to extension of such
conversion date upon certain events. Subject to certain conditions, the Company
has a right on one occasion to require the holders of the Series B Preferred
Stock to purchase up to an additional 1,000 shares of Series B Preferred Stock
at a purchase price of $10,000 per share during the period between 360 days and
540 days following the original issuance date.

         The holders of the Series B Preferred Stock are entitled to certain
rights, subject to certain limitations, including (i) a right of first refusal
for one year from the date of issuance to purchase a pro rata portion of any
issuance of equity or convertible securities of the Company, (ii) a right to
purchase during the period beginning one year from and ending four years
following the date of original issuance, at the same original issue price up to
50% of the number of shares of Series B Preferred Stock held on November 14,
1997, (iii) certain price-based antidilution rights triggered upon certain
events which would require the Company to issue additional shares of Common
Stock upon conversion of the Series B Preferred Stock, and (iv) a right to
require the Company to redeem some or all of the outstanding shares of Series B
Preferred Stock upon certain events including a change of control or sale of all
or substantially all of the Company's assets, a failure by the Company to timely
file and maintain the effectiveness of a registration statement for the sale of
the shares of Common Stock issuable upon conversion of the Series B Preferred
Stock, an inability to issue shares of Common Stock upon conversion of the
Series B Preferred Stock or a delisting of the Company's Common Stock on the
Nasdaq National Market. A failure to timely register and maintain an effective
registration statement would also result in certain adjustments to the
conversion rate of the Series B Preferred Stock which could have the effect of
increasing the number of shares of Common Stock issuable upon conversion. The
holders of the Series B Preferred Stock have no voting rights, except as
otherwise required under Delaware law or as expressly provided to approve
certain subsequent issuances of Series B Preferred Stock or to change the
rights, preferences or privileges of the Series B Preferred Stock. The Shares
issuable upon conversion of the Series B Preferred Stock are being registered on
a Registration Statement on Form S-3 of which this Prospectus forms a part
pursuant to certain registration rights granted to holders of the Series B
Preferred Stock.

CONVERTIBLE NOTE

         On March 26, 1997, as consideration for a cross-license and settlement
agreement, the Company issued a Convertible Subordinated Promissory Note (the
"Convertible Note") in the principal amount of $15 million to GenPharm
International, Inc., pursuant to the Convertible Note Purchase Agreement (the
"Note Purchase Agreement") of the same date. The Convertible Note bears interest
at a rate of 7% per annum, payable semi-annually, beginning on September 30,
1997. The Convertible Note matures on September 30, 1998. The principal and 






                                      -16-
<PAGE>   17
unpaid accrued interest under the Convertible Note is convertible into shares of
the Company's Common Stock at a conversion price of: (i) $9.00 per share, or
(ii) at the Company's option, 115% of the average closing price of the Company's
Common Stock as reported on the Nasdaq National Market during the 30 trading
days immediately preceding February 28, 1998. The Convertible Note is redeemable
at the Company's option at any time after August 31, 1997, if the closing price
of the Company's Common Stock is at least 130% of the conversion price on at
least 20 of the 30 trading days immediately preceding the date of notice of
redemption. The Note Purchase Agreement contains certain restrictive covenants
that limit the amount of additional debt the Company may incur in excess of $15
million. On October 21, 1997, GenPharm International, Inc. was acquired by
Medarex, Inc. The Shares issuable upon conversion of the Convertible Note are
being registered on a Registration Statement of which this Prospectus forms a
part pursuant to certain registration rights granted to the holder of the
Convertible Note.

WARRANTS

         As of December 1, 1997, the Company had outstanding a warrant to
purchase 750,000 shares of Common Stock at $13.00 per share, with an expiration
date of October 9, 2000. Such warrant was issued pursuant to the Stock Purchase
Agreement between the Company and Hoechst Marion Roussel, Inc. on October 9,
1995 (the "HMR Warrant"). In addition, the Company had outstanding Warrants to
purchase: (i) 624,362 shares of Common Stock at $10.39 per share, with an
expiration date of June 28, 1998; (ii) 36,023 shares of Common Stock at $3.01
per share, with an expiration date of February 28, 1999; (iii) 9,000 shares of
Common Stock at $5.04 per share, with an expiration date of February 28, 1999;
(iv) 56,271 shares of Common Stock at $0.03 per share, with an expiration date
of November 23, 1999; (v) 4,812 shares of Common Stock at $18.18 per share,
with an expiration date of May 24, 1999; and (vi) 7,434 shares of Common Stock
at $18.33 per share, with an expiration date of December 31, 1999. Except for
the HMR Warrant, all such Warrants were assumed by the Company in connection
with the acquisition of Somatix on May 30, 1997 and the Shares issuable upon
exercise of such Warrants are being registered on a Registration Statement on
Form S-3 of which this Prospectus forms a part pursuant to certain registration
rights granted to the holders of such Warrants. Certain of the Warrants are
subject to a net exercise provision by which the holder may exercise the
Warrant by surrendering Shares in lieu of paying the cash exercise price.

                                  LEGAL MATTERS

         The validity of the Shares offered hereby will be passed upon by Wilson
Sonsini Goodrich & Rosati, Professional Corporation, Palo Alto, California,
counsel to the Company.

                                     EXPERTS

         The consolidated financial statements of Cell Genesys appearing in its
Annual Report on Form 10-K/A for the year ended December 31, 1996, have been
audited by Ernst & Young LLP, independent auditors, as set forth in their report
thereon included therein and incorporated herein by reference. Such consolidated
financial statements are incorporated herein by reference in reliance upon such
report given upon the authority of such firm as experts in accounting and
auditing.


















                                      -17-

<PAGE>   18

================================================================================


         NO PERSON IS AUTHORIZED IN CONNECTION WITH ANY OFFERING MADE BY THIS
PROSPECTUS TO GIVE ANY INFORMATION OR TO MAKE ANY REPRESENTATIONS NOT CONTAINED
IN THIS PROSPECTUS, AND, IF GIVEN OR MADE, SUCH INFORMATION OR REPRESENTATIONS
MUST NOT BE RELIED UPON AS HAVING BEEN AUTHORIZED BY THE COMPANY, ANY SELLING
STOCKHOLDER OR BY ANY OTHER PERSON. THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER
TO SELL OR A SOLICITATION OF AN OFFER TO BUY ANY SECURITY OTHER THAN THE SHARES
OFFERED HEREBY, NOR DOES IT CONSTITUTE AN OFFER TO SELL OR A SOLICITATION OF AN
OFFER TO BUY ANY OF THE SHARES OFFERED HEREBY TO ANY PERSON IN ANY JURISDICTION
IN WHICH IT IS UNLAWFUL TO MAKE SUCH AN OFFER OR SOLICITATION. NEITHER THE
DELIVERY OF THIS PROSPECTUS NOR ANY SALE OF OR OFFER TO SELL THE SHARES MADE
HEREUNDER SHALL UNDER ANY CIRCUMSTANCES CREATE ANY IMPLICATION THAT THERE HAS
BEEN NO CHANGE IN THE AFFAIRS OF THE COMPANY SINCE THE DATE HEREOF OR THAT THE
INFORMATION CONTAINED HEREIN IS CORRECT AS OF ANY TIME SUBSEQUENT TO THE DATE
HEREOF.


                                TABLE OF CONTENTS


<TABLE>
<CAPTION>
                                                                              PAGE
<S>                                                                           <C>
Available Information ...............................................          2
Incorporation of Certain
  Documents By Reference ............................................          2
The Company .........................................................          3
Risk Factors ........................................................          4
Use of Proceeds .....................................................         10
Selling Stockholders ................................................         11
Plan of Distribution ................................................         14
Description of Capital Stock ........................................         15
Legal Matters .......................................................         17
Experts .............................................................         17
</TABLE>




                               CELL GENESYS, INC.




                                8,028,569 SHARES

                                       OF

                                  COMMON STOCK






                                   PROSPECTUS















                               January 16, 1998






================================================================================




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