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                                                                    Exhibit 99.1


                                            CONTACTS:

FOR ILEX                                    FOR GENZYME
Craig Tooman (all inquiries)                Sally Curley (investors)
(210) 949-8209                              (617) 768-6140

                                            Maria Foley (media)
                                            (617) 768-6690

FOR IMMEDIATE RELEASE
February 26, 2004

                GENZYME CORPORATION TO ACQUIRE ILEX ONCOLOGY, INC
              ----------------------------------------------------
              TAKES IMPORTANT STEP TO ESTABLISH ONCOLOGY FRANCHISE

     CAMBRIDGE, Mass. and SAN ANTONIO, Texas - Genzyme Corporation (Nasdaq:
GENZ) and ILEX Oncology, Inc (Nasdaq: ILXO) announced today a merger agreement
under which ILEX shareholders will receive shares of Genzyme common stock valued
at $26.00 per share, or approximately $1 billion in equity value. Excluding
amortization, the transaction is expected to be dilutive to Genzyme's near-term
earnings and accretive in 2006.

     With this transaction, Genzyme takes a significant step toward fulfilling
its goal of building an oncology business. ILEX offers a strong franchise with a
growing marketed product, a promising pipeline with two late-stage products and
a first-class clinical development organization. The pipeline significantly
augments Genzyme's long-standing program in oncology, which includes a research
and early development portfolio with particular strengths in antibodies, small
molecules and cell-based therapeutics. The merger also capitalizes on Genzyme's
substantial expertise in

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 2

biologics and targeted therapeutics, and its expanding oncology testing
business. Genzyme is one of the world's top five biotechnology companies, with
ten major marketed products and a global regulatory, manufacturing and
commercial infrastructure. Cancer, unquestionably, represents a significant
unmet medical need that includes well-defined groups of addressable patient
populations. This market is one of great opportunity for Genzyme, which is
dedicated to making a major positive impact on the lives of patients with
serious diseases.

     "This transaction is a very good strategic fit for Genzyme that provides us
with a solid franchise in the important field of oncology," stated Henri A.
Termeer, chairman and chief executive officer, Genzyme Corporation. "Through
this merger, we gain an experienced team that has brought a cancer therapy from
development to market. The combined strength of the ILEX program and Genzyme's
oncology pipeline expertise and infrastructure will provide the foundation for a
sustainable and competitive commercial oncology business."

     "We believe this transaction brings significant value to ILEX shareholders
and recognizes the high quality oncology franchise that we at ILEX have built,"
said Jeffrey Buchalter, president and chief executive officer, ILEX Oncology,
Inc. "Genzyme is a top-tier biotech company with the resources and commitment to
take us to the next level as we continue to expand our markets and, ultimately,
commercialize our own products."

     Genzyme plans to maintain ILEX operations in San Antonio, acknowledging the
impressive work currently underway by the ILEX team.

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 3

A GROWING MARKETED PRODUCT

     ILEX's lead product, CAMPATH(R) (alemtuzumab for injection), is indicated
in the United States for the treatment of B-cell chronic lymphocytic leukemia
(B-CLL). CAMPATH is already having a significant impact in oncology, less than
three years after earning U.S. marketing approval.

     CAMPATH is a humanized monoclonal antibody that binds to a specific target,
CD52, on cell surfaces directing the body's immune system to destroy malignant
cells. The U.S. Food and Drug Administration approved CAMPATH for the treatment
of B-CLL in patients who have been treated with alkylating agents and who have
failed fludarabine therapy. It is the first and only monoclonal antibody
approved by the FDA for the treatment of patients with B-CLL.

     ILEX and its marketing partner, Schering AG (NYSE: SHR), are currently
conducting a number of additional trials with CAMPATH to expand its use. A Phase
III comparison trial with chlorambucil, if successful, will help establish
CAMPATH as a useful therapy for earlier-line B-CLL treatment. ILEX expects to
fully enroll this trial with nearly 300 patients this year. Similarly, ILEX has
initiated a fludarabine combination Phase III trial that will examine the
efficacy of combining CAMPATH with one of the most commonly used agents for CLL.
Two Phase I/II trials of CAMPATH have been initiated in non-Hodgkin's lymphoma
(NHL), one in combination with Rituxan(R) and another as a single agent in
relapsed or refractory low-grade NHL.

     Additionally, CAMPATH is demonstrating its potential to treat other
diseases beyond cancer. A randomized Phase II trial against Rebif(R) in treating
multiple sclerosis should be fully enrolled in 2004 and abstracts from
investigator-sponsored studies of

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 4

CAMPATH in solid organ transplant surgery are expected to be presented at the
American Transplant Congress in May.

     Schering AG holds exclusive worldwide marketing and distribution rights
plus additional exclusive rights in Japan and the Asian Pacific region to
CAMPATH (MABCAMPATH(R) in Europe). In the United States, the product is marketed
by approximately 100 oncology sales representatives of Berlex Laboratories, a
U.S. affiliate of the Schering AG Group.

     ILEX currently fields a 12-member force of medical scientific liaisons to
support the investigational research being done with CAMPATH and other ILEX
pipeline products.

     In 2003, global net sales of CAMPATH reported by Schering AG were $71.7
million, compared to $44.1 million in 2002, a growth rate of 63%. ILEX reported
revenues in 2003 of $34.8 million, primarily driven by the contribution of
CAMPATH. At the end of 2003, ILEX had $198 million in cash and marketable
securities.

STRATEGIC PIPELINE

     ILEX's lead pipeline candidate is clofarabine, a next-generation purine
nucleoside analogue that inhibits both DNA and RNA synthesis. ILEX is currently
investigating clofarabine for use in pediatric and adult acute leukemias, as
well as advanced solid tumors. ILEX has initiated a rolling New Drug Application
(NDA) with the FDA for treatment of relapsed or refractory acute leukemias in
children following receipt of a fast track designation, and Genzyme expects
approval of clofarabine in 2005. ILEX recently reported positive interim results
from Phase II studies for clofarabine in pediatric and adult acute leukemia.
Phase I studies evaluating clofarabine in advanced solid tumors are currently
underway in both intravenous and oral formulations.

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 5

     ILEX holds the rights from Bioenvision, Inc. (Amex: BIV) to develop and
market clofarabine for cancer applications in the United States and Canada.
Bioenvision is responsible for developing clofarabine in the rest of the world.

     ILEX's Phase II pipeline candidate is ILX-651, a synthetic analog of the
natural substance dolastatin that has a unique mechanism of action targeting
tubulin. ILEX is currently enrolling Phase II trials in melanoma and non-small
cell lung cancer after seeing promising activity in Phase I solid tumor trials.
ILEX has exclusive worldwide rights to ILX-651 in cancer.

     ILEX's product portfolio will serve as a solid complement to Genzyme
Oncology's research and development pipeline that is pursuing direct anti-tumor
approaches, anti-angiogenesis and immunotherapy. Genzyme Oncology expects to
report data from its Phase I/II trial of a patient-specific vaccine for kidney
cancer at the June meeting of the American Society of Clinical Oncology. It is
also making substantial progress in its collaboration with the pharmaceutical
division of Kirin Brewery of Japan around its proprietary portfolio of tumor
endothelial markers (TEMs). Genzyme Oncology has generated fully human
antibodies to a number of the TEMs that demonstrate significant activity in in
vitro models of angiogenesis and in in vivo models with tumor-bearing mice.
Kirin has agreed to fund additional research and the collaboration expects to
identify a development candidate this year.

     Genzyme Oncology has also accelerated its activity in approaches to cancer
with a Phase I-II trial of a small molecule that offers promise in liver cancer.
A preclinical program with tumor-targeted compounds is underway and preclinical
work with inhibitors of PRL-3, a gene believed to be implicated in metastatic
colon cancer, is also expected to begin in 2004.

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 6

TRANSACTION TERMS

     Genzyme's business combination with ILEX will take the form of a
stock-for-stock merger and is expected to be completed by the middle of the
year. Under the terms of the agreement, ILEX shareholders will receive shares of
Genzyme General common stock for each ILEX share owned based on an exchange
ratio. This exchange ratio will equal $26.00 divided by the average (rounded to
the nearest cent) of the per share closing prices of Genzyme common stock as
reported by Nasdaq during the 20 trading days ending on the fifth trading day
prior to the closing of the transaction, provided that if this average is
greater than $59.88, then the exchange ratio will be 0.4342, and if this average
is less than $46.58, then the exchange ratio will be 0.5582. Cash will be paid
for fractional shares.

     The tax-free transaction has a total value of approximately $1 billion,
based on ILEX's 39.0 million shares outstanding today and Genzyme's offer price
of $26 per share.

     The transaction has been approved by the boards of directors of both
companies, and is subject to clearance under the Hart-Scott-Rodino Antitrust
Improvements Act and approval of ILEX's stockholders. In association with the
transaction, Genzyme anticipates it may incur charges associated with purchase
accounting which will be detailed following its closing.

ABOUT GENZYME

     Genzyme Corporation is a global biotechnology company dedicated to making a
major positive impact on the lives of people with serious diseases. The
company's broad product portfolio is focused on rare genetic disorders, renal
disease, osteoarthritis and immune-mediated diseases, and includes an
industry-leading array of diagnostic

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 7

products and services. Genzyme's commitment to innovation continues today with
research into novel approaches to cancer, heart disease, and other areas of
unmet medical need. More than 5,300 Genzyme employees in offices around the
globe serve patients in over 80 countries.

ABOUT ILEX

     ILEX Oncology, Inc. is an oncology drug development company. ILEX maintains
a core competency in oncology drug development, with locations in San Antonio,
Texas and Guildford, England. ILEX also conducts research in angiogenesis
inhibition, cell signaling, medical chemistry and nuclear receptor biology at
its laboratories in Boston, Massachusetts and Geneva, Switzerland.

     This press release is not a substitute for the prospectus/proxy statement
Genzyme and ILEX intend to file with the U.S. Securities and Exchange
Commission. Investors are urged to read that document when it becomes available,
because it will contain important information about Genzyme, ILEX, the proposed
business combination and related matters, including detailed risk factors. The
prospectus/proxy statement and other documents filed by Genzyme and ILEX with
the SEC will be available free of charge at the SEC's website (www.sec.gov) and
from Genzyme or ILEX. Requests for copies of Genzyme's SEC filings can be
directed to Genzyme Corporation, 500 Kendall Street, Cambridge, Massachusetts
02142, Attention: Investor Relations. Requests for copies of ILEX's SEC filings
can be directed to Ilex Oncology, Inc., 4545 Horizon Hill Blvd., San Antonio,
Texas 78229, Attention: Investor Relations.

     ILEX, its directors, and certain of its executive officers and employees
may be deemed to be participants in the solicitation of proxies from ILEX's
stockholders in favor of the transaction. Information about the directors and
executive officers of ILEX and

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 8

their ownership of ILEX stock is set forth in the proxy statement for ILEX's
2003 annual meeting of stockholders and other filings with the SEC. Certain
directors, executive officers and employees of ILEX may have direct or indirect
interest in the transaction due to securities holdings, vesting of options and
rights to severance payments. In addition, ILEX's current directors and
officers, after the merger, will be entitled to indemnification by Genzyme and
benefit from insurance coverage for certain liabilities that may arise from
their service as directors and officers of ILEX prior to the merger. Additional
information regarding the participants in the solicitation of ILEX's
stockholders will be included in the prospectus/proxy statement.

     This press release contains forward-looking statements, including
statements about: the potential business combination between Genzyme and ILEX;
the impact of the anticipated transaction on Genzyme's operations and the
potential benefits of the anticipated transaction; plans for expanding the
indications of, and uses for, ILEX's products and product candidates; and
expectations regarding the commencement and completion of, and announcement of
data from, clinical trials conducted by or on behalf of both companies. These
statements are subject to risks and uncertainties that could cause actual
results to differ materially from those projected in these forward-looking
statements. These risks and uncertainties include, among others: the willingness
of ILEX shareholders to vote their shares in favor of the potential business
combination; the receipt of regulatory and third party consents to the extent
required for the transaction; the ability to successfully integrate ILEX's
operations and programs with Genzyme's and the time and resources required to do
so; the actual design, results and timing of preclinical and clinical studies
for both companies' products and product candidates; enrollment rates for both
companies' clinical trials; the actual timing and content of submissions to and
decisions made by regulatory authorities regarding each company's products and
product candidates, including the receipt of marketing approval in the United
States for Clofarabine; the ability to manufacture sufficient quantities of
products for development and commercialization activities and to do so in a
timely and cost-efficient manner; the ability to attract and retain qualified
sales forces; the availability and extent of reimbursement from third party
payers for ILEX's products and product candidates; the scope, validity and
enforceability of patents directed to, and the effect of any third party
proprietary rights on, ILEX's products and product candidates; the accuracy of
Genzyme's and ILEX's information concerning the markets for their respective
products and product candidates, including growth projections; and the risks and
uncertainties described in reports filed by Genzyme and ILEX with the Securities
and Exchange Commission under the Securities Exchange Act of 1934, as amended,
including without limitation the information contained under the heading
"Factors Affecting Future Operating Results" in Genzyme's Quarterly Report on
Form 10Q for the quarter ended September 30, 2003 and in ILEX's Form 10-K for
the year ended December 31, 2002. We caution investors not to place undue
reliance on the forward-looking statements contained in this press release.
These statements speak only as of the date of this press release, and we
undertake no obligation to update or revise the statements, risks or reasons.
All forward looking statements are expressly qualified in their entirety by this
cautionary statement.

     Genzyme(R) is a registered trademark of Genzyme Corporation. CAMPATH(R) and
MABCAMPATH(R) are registered trademarks of ILEX Oncology, Inc. or its
subsidiaries.

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Genzyme Corporation to Acquire ILEX Oncology, Inc. - page 9

Rituxan(R) is a registered trademark of Biogen Idec, Inc. Rebif(R) is a
registered trademark of Ares Trading S.A.

CONFERENCE CALL INFORMATION

     There will be a joint conference call today at 5:30 p.m. ET to discuss the
proposed merger between Genzyme Corporation and ILEX Oncology, Inc. If you would
like to participate, please call 719-457-2658. A replay of this call will be
available from 8:30 p.m. ET on Thursday, February 26 to midnight ET on March 4
by calling 719-457-0820. Please refer to reservation number 740574. This call
will also be webcast live at the investor section of www.genzyme.com

     Morgan Stanley acted as financial advisor and Ropes & Gray LLP acted as
legal counsel to Genzyme. UBS Securities LLC and Fulbright & Jaworski, L.L.P.
acted as financial and legal advisors, respectively, for ILEX.

                                      # # #

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