Monrovia, Calif. – June 1, 2008 – Xencor, Inc., a company developing protein and antibody therapeutics, announced today that it will present new data from pre-clinical studies evaluating XmAb®5574, an Fc engineered monoclonal antibody targeting the antigen CD19, showing that it elicited complete B cell depletion compared to undetectable B cell depletion seen with an equal dose of a non-Fc engineered anti-CD19 1gG1 analog. Additionally, the pharmacokinetics of XmAb5574 were shown to be indistinguishable from that of the 1gG1 analog. These data will be presented on June 1, 2008 during the American Society of Clinical Oncology (ASCO) 44th Annual Meeting in a poster session entitled, “XmAb®5574: In vitro and in vivo efficacy of an Fc-engineered anti-CD19 monoclonal antibody against lymphoma and leukemia and its effect on B cells in non-human primates.” (Poster: 20G; Abstract: 3078)

“We are thrilled with the data that we have observed to date from the XmAb5574 program because it demonstrates the dramatic impact that Fc engineering has on in vivo efficacy,” said John Desjarlais, Ph.D., Vice President of Research at Xencor. “This crucial element coupled with a favorable pharmacokinetic profile has implications not only for the future of the XmAb5574 program but for the rest of our Fc engineered product pipeline.”

Previous data from the program that was presented last month during the Annual Meeting of the American Association for Cancer Research (AACR) highlighted XmAb5574’s enhanced anti-tumor activity in vitro and in vivo and that it caused sustained B cell depletion in cynomolgus monkeys.

About XmAb®5574

XmAb5574 is a humanized monoclonal antibody that targets the antigen CD19 for treatment of B cell malignancies and autoimmune diseases. XmAb5574 contains a proprietary Xencor XmAb Fc domain that enhances cytotoxic potency and also down regulates B cell activation. CD19 is a pan-B cell surface receptor that is often expressed in NHL and leukemia, making it an ideal target for cancers of the lymphoid origin.

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, Centocor, MedImmune and Human Genome Sciences, Xencor is applying its suite of XmAb 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.