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Xcyte Therapies and Invitrogen Announce Signing of Asset Purchase
Agreement; Invitrogen to Acquire Xcyte's T Cell Expansion Technology

SEATTLE & CARLSBAD, Calif. Dec 15, 2005 - Xcyte Therapies, Inc.
(Xcyte) (Nasdaq:XCYT) (Nasdaq:XCYTP) and Invitrogen Corporation
(Nasdaq:IVGN) today announced they have entered into a definitive
agreement for Invitrogen to acquire Xcyte's T cell expansion
technology, known as the "Xcellerate Process." The acquired assets
will include intellectual property, the clinical data generated by
Xcyte in the course of six clinical trials of its lead product,
Xcellerated T Cells, as well as raw materials and equipment.

Under the terms of the definitive agreement, upon the closing of the
transaction Invitrogen will make a $5 million cash payment to Xcyte.
In addition, Xcyte is entitled to receive a percentage of any
sublicensing revenue Invitrogen derives from the further licensing
of Xcyte's intellectual property for therapeutic purposes, subject
to certain minimum revenue requirements.

"Xcyte has had a long and productive collaboration with Dynal
Biotech for the production of Xcyte Dynabeads, the proprietary
magnetic bead Xcyte has used to expand and activate T cells for
potential therapeutic indications in oncology and infectious
disease," said Robert L. Kirkman, M.D., Acting President and Chief
Executive Officer of Xcyte. "With Xcyte's decision earlier this year
to pursue a new strategic direction and Invitrogen's acquisition of
Dynal, Invitrogen was our first choice to acquire the Xcellerate
Technology. We are very pleased that they have agreed to do so and
will continue to make this exciting and important approach to
immunotherapy available to the research community."

"To Invitrogen, this transaction represents a cornerstone in a
broader effort to substantially increase our offering to customers
working within immunotherapy. We firmly believe T-cell based therapy
will become an important contributor in the efforts to fight disease
in the future," said Dr. Oystein Amellem, Business Area Manager for
Invitrogen's Dynal Bead Based Separation group. "Expansion of T
cells ex vivo using Dynabeads(R) to which anti-CD3 and anti-C28
monoclonal antibodies are linked may prove useful for the treatment
of cancer, infectious diseases and recently autoimmune diseases. We
are very excited to be able to bring this unique technology to our
customers in the emerging field of cell-based immunotherapy."

The transaction is subject to certain customary closing conditions,
including the approval of Xcyte's stockholders.

Additional Information

Xcyte today separately announced an agreement to acquire all of the
capital stock of Cyclacel Limited, a wholly-owned subsidiary of
Cyclacel Group plc. In conjunction with that transaction, Xcyte
plans to file a registration statement containing a proxy
statement/prospectus with the Securities and Exchange Commission.
Xcyte will also include in its proxy statement/prospectus a
resolution for stockholder approval of the sale of its T cell
expansion technology to Invitrogen Corporation. XCYTE'S STOCKHOLDERS
AND OTHER INVESTORS ARE ENCOURAGED TO READ THE PROXY
STATEMENT/PROSPECTUS, INCLUDING ANY SUBSEQUENT AMENDMENTS OR
SUPPLEMENTS TO THE PROXY STATEMENT/PROSPECTUS, WHEN IT BECOMES
AVAILABLE, AS IT WILL CONTAIN IMPORTANT INFORMATION. Copies of the
proxy statement/prospectus will be available free of charge at the
Securities and Exchange Commission website (www.sec.gov) or from
Xcyte Therapies, Inc., 1124 Columbia Street, Suite 130, Seattle, WA
98104, Attention: Investor Relations, Telephone: 206-262-6200.

Xcyte and its executive officers and directors may be deemed to be
participants in the solicitation of proxies from stockholders of
Xcyte with respect to the proposed transactions. Information
regarding such officers and directors is included in Xcyte's proxy
statement for its 2005 Annual Meeting of Stockholders filed with the
Securities and Exchange Commission on April 29, 2005, and will be
included in the proxy statement/prospectus referred to above. These
documents are or will be available free of charge at the Securities
and Exchange Commission website (www.sec.gov) or from Investor
Relations at Xcyte at the address referred to above.

Forward-Looking Statements

Certain statements contained in this press release are
considered "forward-looking statements" within the meaning of the
Private Securities Litigation Reform Act of 1995, and it is Xcyte's
and Invitrogen's intent that such statements be protected by the
safe harbor created thereby. Forward-looking statements include, but
are not limited to, statements regarding the proposed acquisition by
Invitrogen of Xcyte's T cell expansion technology, the purchase
price payments to be made upon the closing of such transaction and
that Xcyte will issue a proxy statement/prospectus seeking
stockholder approval. Such forward-looking statements are subject to
a number of risks, uncertainties and other factors that could cause
actual results to differ materially from future results expressed or
implied by such forward-looking statements. Potential risks and
uncertainties include, but are not limited to, the risks that
Xcyte's stockholders will not approve the proposed transaction, the
failure to obtain required regulatory approvals, if any, that the
closing of the transaction will be delayed or the transaction will
not close, that the purchase price payment will not be made or that
Xcyte and Cyclacel will not consummate their proposed transaction.
Further information about the risks and uncertainties faced by both
Xcyte Therapies and Invitrogen Corporation is contained in their
respective periodic filings with the Securities and Exchange
Commission.

About Xcyte Therapies

From its inception in 1996 until early July 2005, Xcyte Therapies
devoted substantially all of its efforts to the research and
development of therapies that harness the power of the immune system
to treat cancer and other serious illnesses. Xcyte derived its
therapeutic products from a patient's own T cells, which are cells
of the immune system that orchestrate immune responses and can
detect and eliminate cancer cells and infected cells in the body.
Xcyte used its patented and proprietary Xcellerate Technology to
generate activated T cells, called Xcellerated T Cells, from blood
that is collected from the patient. Activated T cells are T cells
that have been stimulated to carry out immune functions. The
Xcellerate Technology is designed to rapidly activate and expand the
number of the patient's T cells outside of the body. These
Xcellerated T Cells are then administered to the patient.

About Invitrogen

Invitrogen Corporation (Nasdaq:IVGN) provides products and services
that support academic and government research institutions and
pharmaceutical and biotech companies worldwide in their efforts to
improve the human condition. The company provides essential life
science technologies for disease research, drug discovery and
commercial bio-production. Invitrogen's own research and development
efforts are focused on breakthrough innovation in all major areas of
biological discovery, including functional genomics, proteomics, bio-
informatics and cell biology, placing Invitrogen's products in
nearly every major laboratory in the world. Founded in 1987,
Invitrogen is headquartered in Carlsbad, Calif., and conducts
business in more than 70 countries around the world. The company
globally employs approximately 4,800 scientists and other
professionals, and had revenues in excess of $1 billion in 2004.
Information about Invitrogen is available on the Web at
www.invitrogen.com.

Xcyte(R), Xcellerate(R) and Xcellerated T Cells(R) are registered
trademarks and Xcyte Therapies(TM) is a trademark of Xcyte
Therapies, Inc.

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