MONROVIA, CA – June 21, 2005 – Xencor, Inc., a company engineering superior biotherapeutics using its Protein Design Automation® technology, announced today that it has been selected as one of the “Fierce 15“ for 2005 by FierceBiotech, an internationally recognized publication for the biotech industry covering the biotechnology and pharmaceutical markets. Xencor was chosen as one of the top emerging biotechnology companies for the company’s innovative Protein Design Automation (PDA®) technologies, including Xencor’s XmAb™ technology that improves the potency of monoclonal antibodies.

“The beauty of Xencor’s work is that it focuses on the constant region of an antibody, redesigning a segment that helps it more effectively attack a target cell by improving an antibody’s natural immune system recruitment,“ says John Carroll, of FierceBiotech. “Like the other swiftly developing companies on this year’s Fierce 15 list, we’ll be watching Xencor closely in the years ahead.“

“Xencor’s PDA technology has allowed the company to generate an outstanding roster of industrial collaborators and grow an internal pipeline of biotherapeutics, with our lead candidate for the treatment of rheumatoid arthritis, DN-TNF, expected to enter human trials in 2006,“ said Bassil Dahiyat, President and CEO of Xencor. “Xencor is honored to be chosen for this distinction by FierceBiotech, a recognized leader in the industry for advancing an understanding of biotechnology companies and the life science community.“

The Fierce 15 celebrates the spirit of being “fierce“ – championing innovation and creativity, even in the face of intense competition. The list of Fierce 15 companies is available in today’s issue of FierceBiotech and on the FierceBiotech Web site at http://www.fiercebiotech.com.

About FierceBiotech

FierceBiotech, an internationally recognized email newsletter for the biotech industry, provides more than 35,000 executives a must-read briefing on the day’s top stories in the biotech and pharmaceutical industries. Visit www.fiercebiotech.com for additional information.

About Xencor

Xencor, Inc., engineers biotherapeutics including protein therapeutic candidates and XmAb™ antibody drug candidates using its proprietary Protein Design Automation® technology platform. The company is internally advancing several therapeutics against biologically validated targets, including a protein therapeutic drug candidate for the treatment of arthritis and other rheumatic disorders and antibody candidates for the treatment of cancer. With multiple partners such as industry leaders Genentech and Roche, Xencor applies its suite of XmAb antibody Fc domains to improve antibody drug candidates for traits such as potency, affinity, and stability. Xencor also develops therapeutic protein variants in collaboration with major pharmaceutical partners. For more information, please visit www.xencor.com.