ROCKVILLE, Maryland and MONROVIA, California – February 7, 2008 – Human Genome Sciences, Inc. (Nasdaq: HGSI) and Xencor, Inc. today announced a collaboration agreement under which Xencor will apply its proprietary XmAb™ humanization and optimization technologies to enhance the pharmacologic properties of monoclonal antibodies developed by HGS that specifically target antigens discovered by HGS.

“The priority focus of HGS continues to be the rapid commercialization of our late-stage compounds – Albuferon® for hepatitis C, LymphoStat-B® for lupus, and ABthrax™ for inhalation anthrax. We also have an exciting mid-stage pipeline led by our oncology program,” said H. Thomas Watkins, President and Chief Executive Officer, HGS. “Today’s announcement demonstrates our continuing commitment to targeted innovation based on HGS discoveries and our extensive intellectual property estate. We look forward to working with Xencor to maximize the therapeutic potential of several important new HGS product candidates.”

Under the terms of the agreement, Xencor will receive an upfront payment and is eligible to receive additional payments including development and commercial milestones, and royalties on any products commercialized under the agreement. HGS will be responsible for all preclinical and clinical development, manufacturing and commercialization. Financial terms were not disclosed.

“This collaboration further validates the breadth of our XmAb technology platform,” commented Bassil Dahiyat, Ph.D., President and CEO of Xencor. “Our proprietary tools optimize the complete antibody structure, including the Fc region to improve potency and half-life and variable region humanization and affinity optimization. We look forward to working with Human Genome Sciences to create novel, best-in-class biotherapeutics that are more effective in treating unmet medical needs while we continue to advance our internal pipeline of XmAb antibody candidates.”

About XmAb™ Technologies

Xencor’s XmAb engineered Fc domains are designed to enhance the therapeutic properties of monoclonal antibodies and can be inserted into antibody candidates against any target antigen to improve one or more important effector functions, including enhanced antibody-mediated tumor cell killing, extended half-life and selective regulation of immune cells. The XmAb™ Fv technology generates high-quality human sequence diversity in antibody variable domains in order to improve affinity, stability and production yield.

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.

About Human Genome Sciences

The mission of HGS is to apply great science and great medicine to bring innovative drugs to patients with unmet medical needs.

The HGS clinical development pipeline includes novel drugs to treat hepatitis C, lupus, anthrax disease, cancer and other immune-mediated diseases. The Company’s primary focus is rapid progress toward the commercialization of its two key lead drugs, Albuferon for hepatitis C and LymphoStat-B (belimumab) for lupus. Phase 3 clinical trials of both drugs are ongoing.

ABthrax (raxibacumab) is in late-stage development for the treatment of anthrax disease, and the Company is on track to begin the delivery in fall 2008 of 20,000 doses of ABthrax to the Strategic National Stockpile under a contract entered into with the U.S. Government in June 2006. Other HGS drugs in clinical development include two TRAIL receptor antibodies for the treatment of cancer. AEG40826, a small-molecule antagonist of IAP (inhibitor of apoptosis) proteins, is expected to enter Phase 1 clinical trials for the treatment of cancer in early 2008. In addition, HGS has substantial financial rights to certain products in the GlaxoSmithKline clinical development pipeline.

For more information about HGS, please visit the Company’s web site at www.hgsi.com. Health professionals or patients interested in clinical trials of HGS products may inquire via the “Contact Us” section of the Company’s web site, www.hgsi.com/products/request.html, or by calling (301) 610-5790, extension 3550.

HGS, Human Genome Sciences, ABthrax, Albuferon and LymphoStat-B are trademarks of Human Genome Sciences, Inc.

HGS Safe Harbor Statement

This announcement contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. The forward-looking statements are based on Human Genome Sciences’ current intent, belief and expectations. These statements are not guarantees of future performance and are subject to certain risks and uncertainties that are difficult to predict. Actual results may differ materially from these forward-looking statements because of the Company’s unproven business model, its dependence on new technologies, the uncertainty and timing of clinical trials, the Company’s ability to develop and commercialize products, its dependence on collaborators for services and revenue, its substantial indebtedness and lease obligations, its changing requirements and costs associated with facilities, intense competition, the uncertainty of patent and intellectual property protection, the Company’s dependence on key management and key suppliers, the uncertainty of regulation of products, the impact of future alliances or transactions and other risks described in the Company’s filings with the Securities and Exchange Commission. In addition, the Company will continue to face risks related to animal and human testing, to the manufacture of ABthrax and to FDA concurrence that ABthrax meets the requirements of the ABthrax contract. If the Company is unable to meet the product requirements associated with the ABthrax contract, the U.S. government will not be required to reimburse the Company for the costs incurred or to purchase any ABthrax doses. Existing and prospective investors are cautioned not to place undue reliance on these forward-looking statements, which speak only as of today’s date. Human Genome Sciences undertakes no obligation to update or revise the information contained in this announcement whether as a result of new information, future events or circumstances or otherwise.

XENCOR CONTACT:
Kim Richards
Porter Novelli Life Sciences
(619) 849-5377
krichards@pnlifesciences.com

HGS CONTACTS:
Jerry Parrott
Vice President, Corporate Communications
301/315-2777

Kate de Santis
Director, Investor Relations
301/251-6003