Monrovia, Calif. – December 10, 2007 – Xencor, Inc., a company developing protein and antibody therapeutics, announced today the presentation of results from pre-clinical studies evaluating two humanized monoclonal antibody therapeutics, including the company’s lead candidate, XmAb™2513, during the American Society of Hematology (ASH) 49th Annual Meeting and Exposition, December 8-11, 2007 in Atlanta, GA. Xencor’s XmAb candidates contain proprietary engineered Fc domains that greatly increase anti-tumor potency.

In the studies, XmAb2513 was tested in vivo to evaluate its potential for treating CD30+ diseases such as Hodgkin lymphoma and anaplastic large cell lymphoma. XmAb2513 provided statistically significant reductions in tumor growth and enhancement of survival, and was effective in eliminating established tumors in murine models of disease. In all cases, the treatment was well-tolerated. In vivo studies to evaluate safety and pharmacokinetics showed that single and repeat doses with XmAb2513 were well-tolerated and had suitable exposure and half-life for weekly or biweekly dosing.

“Based on these and other pre-clinical data, we are initiating a Phase I clinical study of XmAb2513 in the coming months,” said Bassil Dahiyat, Ph.D., President and CEO of Xencor. “We also continue to advance our ongoing pre-clinical studies with several other XmAb candidates designed for treatment of blood cancers and autoimmune diseases, such as XmAb5574 for targeting the CD19 antigen in B-cell malignancies and autoimmune diseases.”

In pre-clinical studies of Xencor’s XmAb5574 antibody therapeutic, company scientists observed several direct and indirect (Fc-mediated) mechanisms of antibody-mediated tumor toxicity to evaluate the in vitro efficacy of the antibody against lymphoma and leukemia. The potency of XmAbCD19’s antibody-dependent cell-mediated toxicity (ADCC) increased 10- to 100-fold relative to the native version of the antibody, CD19-IgG1, in a screen of 16 non-Hodgkin lymphoma and leukemia cell lines. ADCC potency and efficacy was also shown to be superior over rituximab, the leading anti-CD20 cancer therapeutic, for both cell lines and tumor cells taken from leukemia and lymphoma patients. XmAb5574 was also potent at blocking tumor growth and killing target cells in murine and primate models. Xencor anticipates initiating pre-clinical toxicology studies with XmAbCD19 in the coming year.

“We are encouraged by the strong results that we continue to observe in pre-clinical testing of our XmAb candidates,” said John Desjarlais, Ph.D., Vice President of Research at Xencor. “Therapeutic treatments that are well-tolerated, targeted and potent enough with the potential to eliminate tumor growths are the focus of our antibody program and what we hope will eventually serve as a much needed alternative for the patients.”

About XmAb™ Antibodies

Xencor’s XmAb engineered Fc domains are designed to enhance the therapeutic properties of monoclonal antibodies and form a leading proprietary position in Fc engineering. Xencor’s Fc domains can be inserted into antibody candidates against any target antigen and may improve one or more important effector functions, including enhanced antibody-mediated tumor cell killing, sustained half-life and increased structural stability. XmAb antibodies are produced using conventional expression and manufacturing processes. Xencor is creating a pipeline of XmAb antibody drug candidates with enhanced potency and pharmaceutical properties.

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. 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, for the treatment of Hodgkin’s disease and T-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 and MedImmune, 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.