Monrovia, Calif. – February 11, 2008 – Xencor, Inc., a company developing protein and antibody therapeutics, has initiated a Phase I clinical trial with its lead product candidate XmAb™2513 in patients with Hodgkin lymphoma (HL) and Anaplastic Large Cell Lymphoma (ALCL). XmAb2513 is a humanized monoclonal antibody that targets the antigen CD30. It is engineered to contain an XmAb™ Fc domain using Xencor’s Protein Design Automation® platform technology to greatly increase its cytotoxic potency.

The open-label, dose escalation study is designed to define the recommended dose of XmAb2513 for subsequent trials, to determine its safety and tolerability, characterize pharmacokinetics and immunogenicity, and to evaluate its antitumor activity in patients with HL and ALCL who have received two or more prior therapeutic treatments.

“We created XmAb2513 entirely with XmAb technology, optimizing its Fc domain and humanizing its variable region resulting in significantly enhanced potency. In pre-clinical studies, XmAb2513 demonstrated superior activity in killing tumor cells and preventing tumor cell growth, and was well tolerated,” said Jeffrey Bloss, M.D., Chief Medical Officer of Xencor. “This is a very promising compound within our pipeline, and we believe it has multiple applications in the treatment of lymphoma and other cancers. Starting clinical testing of the first Xmab in our pipeline is an important milestone towards validating our Fc technology in oncology indications.”

About Hodgkin Lymphoma and T cell Lymphomas

According to the U.S. National Institutes of Health, lymphomas account for about five percent of all cases of cancer in the United States. Of the nearly 500,000 Americans with lymphoma, 142,000 are cases in Hodgkin lymphoma (HL), according to the Lymphoma Research Foundation (LRF). T cell lymphomas account for approximately 15 percent of all non-Hodgkin’s lymphoma (NHL) patients in the United States, according to the LRF. Both are most often treated with a chemotherapy regimen, radiation therapy or a combination of the two. HL, in particular, has a cure rate of 93%, but there is a greater concern with late adverse effects of treatment. Incidence rates with NHL have more than doubled since the 1970s, but survival rates are beginning to increase.

About Xencor, Inc.

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.