Monrovia, CA – December 9, 2003 – Xencor today announced that its Board of Directors has appointed Harry Stylli, Ph.D., to serve as its new Chief Executive Officer and President, and also as a director of the company, effective November 3, 2003. Bassil Dahiyat, Ph.D., Xencor’s CEO since its founding, will assume the new role of Chief Scientific Officer and remain a director of the company.

“Harry Stylli has an unparalleled record in translating innovative technology into commercial opportunities and drug candidates,“ said Bassil Dahiyat. “His entrepreneurial spirit and strong understanding of the pharmaceutical industry is an excellent fit to carry to success Xencor’s discovery programs and platform for Medicinal Chemistry for Proteins™. I look forward to working with Harry to continue Xencor’s growth as a leading emerging biotherapeutics company.“

Dr. Stylli said, “I am tremendously excited by the opportunities in protein therapeutics that Xencor’s technology has created. Proteins and antibodies have huge growth potential and Xencor’s Protein Design Automation technologies can fuel that growth by creating next generation biotherapeutics that have multiple properties optimized, for example selectivity, potency, stability, safety and pharmacokinetics. The XmAb™ antibodies, which have greatly amplified tumor killing power, exemplify this approach, . My role and that of the entire Xencor team is to leverage our unique capability to create biotherapeutics with superior clinical and commercial potential and to advance our internal candidates ultimately to the market. In the nearer term Xencor will unlock value through selective and creative partnering of our novel drug candidates and discovery capabilities.“

Dr. Stylli most recently was the President and CEO of CovX Pharmaceuticals, a private biotherapeutics company founded in 2001 to exploit novel antibody technology developed at the Scripps Research Institute. Joining CovX shortly after its founding, he developed their business plan, recruited a discovery and development team, created a diversified pipeline of pre-clinical candidates that facilitated their financing.

Prior to CovX, Dr. Stylli co-founded Aurora Biosciences in 1995 and played a key role in raising private capital and in the IPO. In 2001 Dr. Stylli played the pivotal role in the acquisition of Aurora by Vertex for nearly $600 million. As Senior Vice President of Screening Technology and New Ventures, he led the team that created and developed Aurora’s core platform, the Ultra High Throughput Screening System (UHTSS). He also played the key role in generating and negotiating groundbreaking alliances with pharmaceutical companies totaling over $300 million in committed funding. In 1999, Dr. Stylli assumed the role of Senior Vice President of Commercial Development, with responsibility for all business development, marketing and sales. He built a business organization that grew the company’s revenues to approximately $95 million with net profits of $13 million by 2001 and facilitated Aurora’s transition from a platform to an integrated drug discovery company culminating in their acquisition by Vertex. Following the acquisition of Aurora by Vertex, he had PL responsibility as President of Aurora Biosciences and Panvera.

From 1987 to 1995 Dr Stylli held various and increasing responsibilities within the global discovery organization of GSK.

About Protein Design Automation technology

PDA technology combines high performance computing with proprietary molecular biology processes and assays to create broader protein diversity with far greater control and efficiency than other optimization technologies, such as directed evolution and phage display. The technology takes advantage of the information embedded in protein structure to optimize key protein properties, such as binding affinity and selectivity, stability, expression level, safety, and efficacy. This process also creates new intellectual property, continually broadening Xencor’s patent portfolio by generating sets of novel protein sequences that are distinct from naturally occurring proteins.

About XmAb™ technology

Xencor is designing the constant Fc domains of monoclonal antibodies using PDA technology to improve their biochemical and cell biological characteristics, an approach applicable to antibodies against any target antigens. The XmAb platform improves numerous properties of antibodies including enhanced antibody mediated tumor cell killing, improvement of structural stability and reduced immunogenicity. The Company has already created a suite of Fc variants with improved tumor cell killing that can be inserted into any antibody.

About Xencor

Xencor is a pre-clinical stage company that discovers and develops protein therapeutics using its proprietary rational protein design platform. Xencor’s platform applies high performance computing and advanced molecular biology to rapidly discover drug candidates with novel mechanisms and improved safety and efficacy. Xencor is a privately held biopharmaceutical company located in Monrovia, CA. Additional information is available at www.xencor.com.