Monrovia, CA – February 23, 2009 – Xencor, Inc., an antibody discovery and development company, announced today that it has entered into an antibody optimization collaboration with CSL, Ltd. The collaboration will provide CSL with access to Xencor's XmAb® technology platform to enhance the ADCC effector function of its therapeutic antibodies.

During the research phase of the collaboration, Xencor will provide CSL with the opportunity to utilize the technology broadly across its antibody candidate pipeline. Xencor has granted CSL several commercial licenses to move product candidates into development and ultimately commercialization which incorporate Xencor's XmAb® technology. In addition to an upfront payment, CSL will pay Xencor development milestone payments and product royalties for each product commercialized that incorporates the Xencor technology.

"We are excited to collaborate with our CSL colleagues and enable their antibody products through the use of our XmAb® technology," commented Bassil Dahiyat, Ph.D., Xencor's chief executive officer.

"We are pleased that the market is recognizing the preferred development friendly solution to increasing antibody effector function that Xencor offers," commented James Posada, Ph.D., MBA, Xencor's acting chief business officer.

About XmAb® ADCC technology

The XmAb® platform can increase the potency of therapeutic antibodies by specifically engaging the body's immune system against target antigen cells. A proprietary suite of XmAb® Fc variants allows the selective improvement of antibody cytotoxic properties by enhancing antibody-dependent cell cytotoxicity (ADCC), phagocytosis and/or complement activation. Increased antibody potency has the potential to improve antibody efficacy in a variety of therapeutic areas, including oncology, infectious disease and autoimmune disorders. The technology offers the broadest range of specific Fc receptor activation currently available in the most "development friendly" format which can be used with any production cell line system and does not involve complex manipulations of highly heterogeneous carbohydrate molecules.

About Xencor

Xencor, Inc. engineers superior biotherapeutics using its proprietary Protein Design Automation® technology platform and is a leader in the field of antibody Fc engineering to significantly improve antibody potency and half-life. The company is advancing XmAb® antibody drug candidates optimized for activity against biologically validated targets and its XPro™ protein therapeutic candidate into the clinic. Xencor's product development is led by an antibody candidate, XmAb®2513, in a Phase I clinical trial for the treatment of Hodgkin lymphoma and anaplastic large cell lymphoma, and a protein therapeutic drug candidate, XPro™1595 DN-TNF, for the treatment of inflammatory disease. With multiple partners, such as industry leaders Genentech, Boehringer Ingelheim, MedImmune and Human Genome Sciences, Xencor is applying its suite of proprietary antibody Fc domains to improve antibody drug candidates for traits such as potency and sustained half-life. For more information, please visit www.xencor.com.