Monrovia, Calif. – January 9, 2008 – Xencor, Inc., a company developing protein and antibody therapeutics, announced today that it received a patent from the United States Patent and Trademark Office (US 7,317,091 B1) entitled, “Optimized Fc Variants,“ expanding on the company’s rich patent estate for its proprietary Protein Design Automation® technology. This is the first issuance from a series of patent applications covering Xencor’s XmAb™ antibody Fc engineering technology, which is comprised of a suite of thousands of selectively engineered Fc domains. Xencor’s engineered Fc domains have been shown to improve antibody potency against tumor cells by more than 100-fold, significantly extend in vivo half-life and have been partnered with several large biotechnology and pharmaceutical companies to enhance specific clinical candidates.

“This is an important milestone for the continued implementation of our Fc technology, which will soon enter Phase I clinical testing in our lead clinical candidate, XmAb™2513,“ said Bassil Dahiyat, Ph.D., President and CEO of Xencor. “Our team is actively pursuing a series of additional patents that will comprehensively cover the broad range of applications we have created with our Fc technology.“

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-dependent cell-mediated cytotoxicity (ADCC), extended half-life and selective regulation of immune cells. 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, 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 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, 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.