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FRAZER, Pa., USA and MELBOURNE, AUSTRALIA (March 29, 2011) -- Cephalon, Inc. (Nasdaq: CEPH) and ChemGenex Pharmaceuticals Limited (ASX: CXS) today jointly announced that Cephalon’s wholly-owned subsidiary, Cephalon CXS Holdings Pty Ltd, intends to make a takeover bid for ChemGenex. ChemGenex is a hematology-focused Australian biopharmaceutical company developing its lead product candidate, OMAPRO™, for the treatment of chronic myeloid leukemia (CML) patients. OMAPRO recently completed Phase III clinical trials and ChemGenex plans to file a New Drug Application with the U.S. Food and Drug Administration in the second half of 2011.

The takeover bid has the unanimous support of the ChemGenex Directors who will recommend it to ChemGenex Shareholders and holders of ChemGenex Listed Options in the absence of a superior offer.

In October 2010, Cephalon, through its wholly-owned subsidiary, Cephalon International Holdings, Inc. (Cephalon International) agreed to subscribe for A$15 million of convertible notes issued by ChemGenex and entered into call option agreements with two of ChemGenex’s major shareholders, Stragen International N.V. and Merck Santé S.A.S. ChemGenex shareholders gave advance approval to the conversion of the convertible notes (in accordance with item 7 of section 611 of the Corporations Act) at a general meeting held in December 2010.

Cephalon International has today served notice to convert its convertible notes and exercise its call options to acquire in aggregate 86.4 million ChemGenex Shares. This will result in Cephalon International holding in total 27.57 percent of the issued ChemGenex Shares.

Cephalon’s takeover bid for ChemGenex will comprise two offers:

�?� an offer of A$0.70 cash for each ChemGenex Share (ASX: CXS), cum dividends and other rights; and

�?� an offer of A$0.02 cash for each ASX-listed ChemGenex Option (ASX: CXSOA).

Subject to obtaining advice on the applicable U.S. law requirements, Cephalon will consider extending the share offer to holders of ChemGenex American Depository Receipts (ADRs) if compliance with those requirements is not unduly onerous or impracticable.

The offer price of A$0.70 per ChemGenex Share represents:

�?� a 59 percent premium to the last traded price of ChemGenex Shares;

�?� a 58 percent premium to the one month VWAP of ChemGenex Shares up to the date of this announcement;

�?� a 40 percent premium to the A$0.50 conversion price of the convertible notes issued to Cephalon and approved by the ChemGenex shareholders last year.

"The proposed transaction allows ChemGenex shareholders to realize cash proceeds at a significant premium to recent trading levels and adds an interesting late-stage opportunity to our portfolio," said Kevin Buchi, Chief Executive Officer of Cephalon.

"The ChemGenex Board welcomes Cephalon’s bid which represents an attractive premium to the current and recent trading prices of ChemGenex Shares. In the absence of a superior offer, the Board will unanimously recommend that ChemGenex shareholders accept the offer and realize value for their shares," said Brett Heading, Chairman of ChemGenex.

The takeover offers for ChemGenex Shares and Listed ChemGenex Options will each be subject to certain normal conditions (outlined in Appendices A, B and C), including a 90 percent minimum acceptance condition.

ChemGenex has informed Cephalon that each ChemGenex Director with personal holdings of ChemGenex Shares or Listed ChemGenex Options has confirmed that he intends to accept or procure the acceptance of the offers in respect of those ChemGenex Shares or Listed ChemGenex Options, in the absence of a superior offer.

The total offer value for ChemGenex Shares and Listed ChemGenex Options that Cephalon does not own is approximately A$159 million (US$163 million). The offer values all ChemGenex Shares and options at approximately A$225 million (US$231 million). Cephalon intends to fund the transaction consideration from its available cash on hand.

Cephalon will finalize and mail out its Bidder’s Statement and ChemGenex will finalize and mail out its Target’s Statement as soon as practicable. Holders of ChemGenex Shares and Listed ChemGenex Options should carefully consider those documents before deciding whether to accept into the Cephalon takeover bid.

Cephalon is being advised by Merrill Lynch and Johnson Winter & Slattery.

ChemGenex is being advised by Canaccord Genuity and McCullough Robertson.

Cephalon is a global biopharmaceutical company dedicated to discovering, developing and bringing to market medications to improve the quality of life of individuals around the world. Since its inception in 1987, Cephalon has brought first-in-class and best-in-class medicines to patients in several therapeutic areas. Cephalon has the distinction of being one of the world's fastest-growing biopharmaceutical companies, now among the Fortune 1000 and a member of the S&P 500 Index, employing approximately 4,000 people worldwide. The

company sells numerous branded and generic products around the world. In total, Cephalon sells more than 150 products in nearly 100 countries. More information on Cephalon and its products is available at http:// www. cephalon.com/.

ChemGenex is an oncology focused biopharmaceutical company developing small molecules with new mechanisms of action to treat malignancies with significant unmet medical needs. A New Drug Application is in preparation for submission to the U.S. Food and Drug Administration in H2 2011 for omacetaxine in CML patients who have failed two or more currently approved tyrosine kinase inhibitors. A parallel Marketing Authorisation Application is in preparation for submission to the European Medicines Agency in H2 2011. ChemGenex has established a corporate alliance with Hospira Inc. to develop and commercialize omacetaxine in Europe, the Middle East and parts of Africa, and is seeking to establish commercial partnerships in the rest of the world. ChemGenex plans to commercialize omacetaxine itself in North America. ChemGenex trades on the Australian Stock Exchange under the symbol "CXS". For additional information on ChemGenex Pharmaceuticals, please visit the company’s website at http:// www. chemgenex.com.

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Natalie de Vane Chip Merritt

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