ROCKVILLE, Maryland – May 17, 2005 – Human Genome Sciences, Inc. (Nasdaq: HGSI) today announced the results of a preclinical study comparing the antiviral efficacy of unmodified interferon alpha with three longer-acting modified forms of interferon alpha, including Albuferon™. The results demonstrate that Albuferon (albumin-interferon alpha) exhibits more antiviral activity at clinically achieved serum levels than standard interferon alpha or the modified interferons, Pegasys (peginterferon alfa-2a) and Peg-Intron (peginterferon alfa-2b).

The data were presented in Chicago at Digestive Disease Week 2005 in a poster presentation entitled, “Albuferon Exhibits Efficient Anti-HCV Activity in Cell Culture.” ^{1, 2} The data show that at clinically achieved patient serum C_max concentrations (peak blood levels) Albuferon exhibited greater antiviral activity than either Pegasys or Peg-Intron. Albuferon showed similar anti-HCV (anti-hepatitis C virus) activity in the human liver cells and non-liver cells, suggesting that the anti-HCV activity is not limited to hepatic cells. The magnitude of interferon-stimulated gene expression at 6 hours and 48 hours was comparable for all three modified interferons at the drug concentrations evaluated.

David C. Stump, M.D., Executive Vice President, Drug Development, said, “The in vitro data presented at Digestive Disease Week are consistent with emerging clinical and preclinical evidence and provide additional support for our continued evaluation in Phase 2 trials of Albuferon’s potential to play an important role in the treatment of patients with chronic hepatitis C. We recently reported the positive results of a Phase 2 study of this novel therapeutic protein in patients with chronic hepatitis C who were naïve to interferon alpha-based treatment regimens.³ A Phase 2 study of Albuferon in combination with ribavirin is underway in patients who failed to respond to previous treatment.⁴ Soon, we expect to initiate a larger Phase 2b trial of Albuferon in combination with ribavirin in treatment-naive patients. Approximately half of genotype 1 HCV patients fail to respond to treatment regimens based on the pegylated interferons, and chronic hepatitis C represents a significant unmet medical need. It is our hope that Albuferon will one day help address this need.”

Hepatitis C is an inflammation of the liver caused by the hepatitis C virus. It is the most common chronic blood-borne infection in the developed world, and afflicts approximately four million people in the United States. The hepatitis C virus is transmitted primarily through significant or repeated exposures to infected blood. Intravenous drug use and sexual contact with infected persons account for the majority of new hepatitis C infections. When detectable levels of the hepatitis C virus in the blood persist for at least six months, a person is diagnosed as having chronic hepatitis C. The current standard of care for treating chronic hepatitis C is combination therapy consisting of pegylated interferon and ribavirin, an antiviral drug.^5-10 Most patients currently receive pegylated interferon once weekly, with daily doses of ribavirin. A significant need exists for more convenient treatment options with fewer side effects.

Clinical and preclinical results to date demonstrate that Albuferon is well tolerated, with adverse events that are transient and mostly mild to moderate in severity, and exhibits a robust antiviral activity, with a pharmacokinetic profile that supports dosing at intervals of two to four weeks.^{3-4, 11-15} Albuferon is a novel, long-acting form of interferon alpha. Recombinant interferon alpha is approved for the treatment of hepatitis C, hepatitis B and a broad range of cancers. Human Genome Sciences modified interferon alpha to improve its pharmacological properties by using the company’s proprietary albumin fusion technology.

For more information about Albuferon, see www.hgsi.com/products/albuferon.html. Health professionals interested in more information about trials involving Human Genome Sciences products are encouraged to inquire via the Contact Us section of the Human Genome Sciences web site, www.hgsi.com/products/request.html, or by calling (301) 610-5790, extension 3550.

Human Genome Sciences is a company with the mission to treat and cure disease by bringing new gene-based protein and antibody drugs to patients.

HGS, Human Genome Sciences and Albuferon are trademarks of Human Genome Sciences, Inc.

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 planned 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. 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.

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Footnotes:

1. Liu C, Zhu H, Xu Y, Nelson DR, et al. Albuferon™ exhibits efficient anti-HCV activity in cell culture. Digestive Disease Week (DDW) 2005, Chicago. Abstract #S920.
2. Digestive Disease Week is sponsored jointly by the American Association for the Study of Liver Diseases, the American Gastroenterological Association, the American Society for Gastrointestinal Endosopy, and the Society for Surgery of the Alimentary Tract.
3. Bain V, et al. A Phase 2 study to assess antiviral response, safety, and pharmacokinetics of Albuferon in IFN a-naïve subjects with genotype 1 chronic hepatitis C. 40th Annual Meeting of the European Association for the Study of the Liver (EASL), Paris. April 14, 2005. Oral presentation. (Abstract #18.)
4. (HGSI Press Release) Human Genome Sciences Initiates Phase 2 Clinical Trial of Albuferon in Combination with Ribavirin in Treatment-Experienced Hepatitis C Patients. November 30, 2004.
5. Strader DB, Wright T, Thomas DL, and Seeff, LB. AASLD practice guideline: diagnosis, management, and treatment of hepatitis C. Hepatology 2004 April; 39 (4): 1147-1171.
6. Hadziyannis SJ, Sette H, Morgan TR, et al. Peginterferon-α2a and ribavirin combination therapy in chronic hepatitis C. Ann Intern Med 2004; 140:346-355.
7. Shiffman ML, Di Bisceglie AM, Lindsay KL, et al. Peginterferon alfa-2a and ribavirin in patients with chronic hepatitis C who have failed prior treatment. Gastroenterology 2004; 126:1015-1023.
8. Zeng N, Cohen CK, Schwartz P, Rai R. Treatment of HCV infected patients, including non-responders to PEG-IFN alfa-2b/RBV, with PEG-IFN alfa-2a/RBV: the Johns Hopkins experience. Digestive Disease Week 2004 Internet Conference Report. Abstract #1233.
9. Zeuzem S. Heterogeneous virologic response rates to interferon-based therapy in patients with chronic hepatitis C: who responds less well? Ann Intern Med 2004; 140:370-381.
10. Management of Hepatitis C: 2002. National Institutes of Health Consensus Development Conference.
11. Balan V, et al. Albuferon -- A novel therapeutic agent for hepatitis C: results of a Phase 1/2 study in treatment-experienced subjects with chronic hepatitis C. 55th Annual Meeting of the American Association for the Study of Liver Diseases, Boston. November 2, 2004. Oral presentation (Abstract #265).
12. (HGSI Press Release) Human Genome Sciences Reports Positive Results of Phase 1/2 Clinical Trial of Albuferon in Chronic Hepatitis C. November 2, 2004.
13. Balan V, et al. Molecular profiles of drug response in HCV infected patients during the first four weeks of therapy for chronic hepatitis C virus with pegylated interferon containing regimens or Albuferon. 54 th Annual Meeting of the American Association for the Study of Liver Diseases, Boston. October 25, 2003.
14. Moore P, Balan V, et al. Modulation of interferon specific gene expression by Albuferon in subjects with chronic hepatitis C and correlation with anti-viral response. 40 th Annual Meeting of the European Association for the Study of the Liver (EASL), Paris. April 14, 2005. (Abstract #447).
15. Osborn B, Olsen H, Nardelli B, et al. Pharmacokinetic and Pharmacodynamic Studies of a Human Serum Albumin-Interferon-a Fusion Protein in Cynomolgus Monkeys. J Pharmacol Exp Ther 2002 Nov; 303: 540-548.

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