ROCKVILLE, Maryland -- June 25, 2003 -- Human Genome Sciences, Inc. (Nasdaq: HGSI) today will report on the progress of its products in clinical development during a presentation at BIO 2003, the annual convention of the Biotechnology Industry Organization (BIO), in Washington, D. C. The Company is advancing a number of its products to their next stage of clinical development in 2003. In a separate press release issued earlier today, Human Genome Sciences announced that it has received clearance from the U. S. Food and Drug Administration (FDA) to begin a clinical trial of ABthrax™, a novel drug for the prevention and treatment of anthrax infections, in healthy adults.¹ (A webcast of today's Human Genome Sciences presentation may be accessed at www.hgsi.com.)

David C. Stump, M.D., Senior Vice President, Drug Development, said, "We are pleased to be able to proceed with a clinical trial to evaluate the safety, tolerability, and pharmacology of ABthrax in healthy adults. Positive results from such a human study, along with our preclinical proof of efficacy data, would support the further development of ABthrax as a new means to prevent and treat anthrax infections. We believe that ABthrax has the potential to provide significant protection when given prophylactically, and may lessen the natural progression of anthrax infection and increase survival if given as a post-exposure treatment. ABthrax also may prevent and treat infections by antibiotic-resistant strains of anthrax."

"In the second half of 2003, we expect to achieve significant additional product milestones, including the initiation of Phase 2 clinical trials of LymphoStat-B™ for the treatment of lupus and rheumatoid arthritis, and the initiation of Phase 1 clinical trials of a human monoclonal antibody to TRAIL Receptor-2 for the treatment of cancer. We also expect to report the results of clinical trials for a number of our products. As we continue our clinical development progress, we maintain our priority focus on cancer and immunology, as well as our later-stage product candidates."

Human Genome Sciences Receives FDA Clearance to Initiate Clinical Trial of ABthrax for the Prevention and Treatment of Anthrax Infections

Human Genome Sciences announced earlier today that it has received clearance from the FDA of its Investigational New Drug (IND) application to begin enrolling healthy adult volunteers into a Phase 1 placebo-controlled, dose-escalation clinical trial to evaluate the safety, tolerability and pharmacokinetics of ABthrax, a novel drug for the prevention and treatment of anthrax infections.¹ ABthrax (human monoclonal antibody to Bacillus anthracis protective antigen) was discovered and developed by Human Genome Sciences. It has been shown to be effective in protecting against anthrax in multiple experimental models in animals.² Large-scale development and manufacture of ABthrax is dependent on government funding, either under existing authority or under proposed Project Bioshield legislation.

Most anthrax fatalities are caused by the irreversible effects of the anthrax toxins. Research has shown that protective antigen is a central component of the anthrax toxins that contribute to the progression of anthrax infection at the cellular level.³ ABthrax specifically recognizes and blocks the binding of protective antigen to cell surfaces and prevents the anthrax toxins from entering and killing the cells. Results from preclinical studies conducted to date demonstrate that a single dose of ABthrax administered prophylactically increases survival significantly in both rabbits and nonhuman primates exposed by inhaling anthrax spores.² Human Genome Sciences plans to develop ABthrax for use as a prophylactic and therapeutic drug to prevent and treat anthrax infections.

Under the Bioterrorism Act of 2002, the FDA specified the evidence required to demonstrate the efficacy of new drug and biological products used to counter biological agents, when traditional efficacy studies in humans are not feasible.⁴ According to the guidelines set forth in the Bioterrorism Act, successful studies in relevant animal models will be considered sufficient to establish efficacy for licensure and marketing approval. ABthrax is effective in preventing the lethal effects of anthrax infection in two relevant models, rabbits and nonhuman primates. According to the guidelines, human clinical trials will be required to establish safety, tolerability, and pharmacology, but not efficacy.

Update on Progress of Other Human Genome Sciences Products in Clinical Development

Dr. Stump also presented an overview of the progress of Human Genome Sciences' other products in clinical development: (1) repifermin for chronic venous ulcers and cancer therapy-induced mucositis; (2) BLyS™ for common variable immunodeficiency and immunoglobulin-A deficiency; (3) LymphoRad™ ¹³¹ for cancer; (4) TRAIL-R1 mAb for cancer; (5) LymphoStat-B™ for lupus and rheumatoid arthritis; (6) TRAIL-R2 mAb for cancer;⁵ (7)Albutropin™ for growth hormone deficiency; (8) Albuferon™-alpha for hepatitis C and cancer; and (9) Albuleukin™ for cancer.

Human Genome Sciences focuses its internal product development efforts on novel human protein and antibody drugs discovered through genomics-based research, and on new improved long-acting versions of existing protein drugs created using its proprietary albumin fusion technology. Human Genome Sciences relies on collaborations for the development of gene therapy products, small molecule drugs, and diagnostic products discovered using its genomics-based technology. The Company's partners have four additional drugs in clinical development that they discovered using Human Genome Sciences' technology.

Novel Protein Drugs

Repifermin: Repifermin (keratinocyte growth factor-2, KGF-2) is a novel human protein discovered by Human Genome Sciences that stimulates the repair of injured skin and mucosal tissues. Human Genome Sciences is developing repifermin as a potential treatment for chronic venous ulcers and cancer therapy-induced mucositis.

In April 2003, Human Genome Sciences announced that Phase 2 clinical trial results demonstrate that systemically administered repifermin is well tolerated and shows efficacy in treating cancer therapy-induced mucositis.⁶ A second Phase 2 clinical trial of repifermin for the treatment of cancer therapy-induced mucositis was initiated in September 2001 and is ongoing.⁷ Based on data emerging from studies conducted in healthy volunteers, the ongoing clinical trial was amended to add a cohort of patients who will receive a new high dose of repifermin at 75 mcg/kg. A total of approximately 90 patients will be enrolled in the study, and it is anticipated that results will be available before the end of 2003.

In late 2002, the Company completed randomization of 352 patients into a double-blind, placebo-controlled Phase 2b clinical trial of topically administered repifermin for the treatment of chronic venous ulcers.⁸ Human Genome Sciences expects to complete the treatment and follow-up phase of the Phase 2b protocol and to have results available in the fall of 2003.

BLyS: BLyS (B-lymphocyte stimulator) is a novel human protein discovered by Human Genome Sciences that is being developed as a potential treatment for patients with immunodeficiencies. Results from a Phase 1 clinical trial to evaluate the safety and pharmacology of BLyS in patients with common variable immunodeficiency were reported in January 2003.⁸ Results demonstrate that BLyS is safe and well tolerated. An ongoing Phase 1 clinical trial evaluating BLyS for the treatment of immunoglobulin-A (IgA) deficiency has completed enrollment.⁹ Results are expected during the second half of 2003.

LymphoRad ¹³¹: LymphoRad¹³¹ is a radioiodinated form of B-lymphocyte stimulator (BLyS), a novel human protein discovered by Human Genome Sciences.¹⁰ Data were presented from ongoing Phase 1 clinical trials evaluating the safety, pharmacology, and dosimetry of LymphoRad¹³¹ in patients with multiple myeloma and non-Hodgkin's lymphoma. As expected based on preclinical studies, imaging data from patients administered LymphoRad¹³¹ to date show that the drug is targeting B cells and B-cell tumors. Enrollment is expected to continue throughout 2003 and into 2004.

Novel Antibody Drugs

LymphoStat-B: Human Genome Sciences is developing LymphoStat-B, a human monoclonal antibody to B-lymphocyte stimulator (BLyS), as a potential treatment for patients with autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis.

Results from a Phase 1 clinical trial of LymphoStat-B in systemic lupus erythematosus patients were announced in April 2003.¹¹ The results demonstrate that LymphoStat-B is well tolerated and biologically active in patients with lupus. As expected based on preclinical research, results show that LymphoStat-B significantly reduces the levels of circulating B (CD 20) cells, the precursor cells to those that produce the body's normal and abnormal antibodies. The Company has met with its clinical investigators and the FDA, and plans to advance LymphoStat-B to Phase 2 clinical trials in lupus soon. The Phase 2 study in patients with lupus will be designed as a placebo-controlled, double-blind trial to evaluate different doses of intravenously administered LymphoStat-B. LymphoStat-B has received a Fast Track Product designation for the treatment of systemic lupus erythematosus from the FDA.

Human Genome Sciences also plans to initiate Phase 2 clinical trials of LymphoStat-B for the treatment of rheumatoid arthritis during the second half of 2003. The Phase 2 study in patients with rheumatoid arthritis will be designed as a placebo-controlled, double-blind trial to evaluate different doses of intravenously administered LymphoStat-B in rheumatoid arthritis patients who have failed at least one disease-modifying treatment.

TRAIL-R1 mAb: TRAIL Receptor-1 agonistic human monoclonal antibody (TRAIL-R1 mAb) is a novel anticancer drug that specifically recognizes and binds to the TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) Receptor-1 protein. The TRAIL-R1 protein was discovered by Human Genome Sciences, and is found on the surface of a number of solid tumor and hematopoietic cancer cells. Human Genome Sciences is currently conducting Phase 1 clinical trials to evaluate the safety and pharmacology of TRAIL-R1 mAb in patients with advanced solid tumors and/or hematological malignancies, and expects to have results available in 2004.¹²

TRAIL-R2 mAb: The TRAIL Receptor-2 protein was discovered by Human Genome Sciences. TRAIL Receptor-2 agonistic human monoclonal antibody (TRAIL-R2 mAb) specifically recognizes the TRAIL Receptor-2 protein, found on the surface of a number of solid tumor and hematopoietic cancer cells. Binding of TRAIL-R2 mAb to TRAIL Receptor-2 triggers programmed cell death, or apoptosis. Results of preclinical studies to date show that TRAIL-R2 mAb specifically binds to TRAIL Receptor-2, induces apoptosis, and has anti-tumor activity in a broad range of tumor types, both as a single agent and in combination with chemotherapy.¹³ The Company hopes to begin Phase 1 clinical trials to evaluate the safety and pharmacology of TRAIL-R2 mAb for the treatment of cancer in 2003.

Albumin Fusion Drugs

Albutropin: Results from a Phase 1 clinical trial of Albutropin in adults with growth hormone deficiency demonstrate that Albutropin is well tolerated and, as expected based on preclinical results, remains in the blood substantially longer than is reported for recombinant native human growth hormone.¹⁴ The Phase 1 results also show that Albutropin is biologically active and capable of restoring levels of insulin-like growth factor-1, a robust surrogate marker for the biological activity of human growth hormone, to within the normal range in a dose-dependent manner.

Dr. Stump said, "Based on our successful completion of Phase 1, we have submitted proposed trial designs to the FDA for Phase 2 studies of Albutropin. Before the initiation of these trials, the FDA has requested that we develop supplemental immunoassays for the detection of antibodies to human growth hormone. These supplemental assays are in addition to what has been required to date for the development of Albutropin. This will delay the initiation of further clinical trials of Albutropin."

Albuferon-alpha: Results from a Phase 1 clinical trial of Albuferon-alpha demonstrate that Albuferon-alpha is well tolerated, has a prolonged half-life, and is biologically active in adults with chronic hepatitis C.¹⁵ The Company is continuing to evaluate Albuferon-alpha at higher doses, in single-dose and repeat-dose cohorts, under an amended protocol designed to seek the maximum biological response that can be achieved at a tolerable dose. Interim data continue to demonstrate that Albuferon-alpha is well tolerated, has a prolonged half-life and is biologically active in patients with chronic hepatitis C who have failed previous interferon-alpha treatments.

Albuleukin: Albuleukin is a novel long-acting form of interleukin-2 that the Company is developing as a potential treatment for a broad range of cancers. Human Genome Sciences is conducting Phase 1 clinical trials to evaluate the safety and pharmacology of Albuleukin in patients with solid tumor cancers.¹⁶ Results are expected in 2004.

Dr. Stump said, "Human Genome Sciences has an exceptionally rich development pipeline of novel clinical and advanced preclinical compounds that meet major unmet medical needs, such as cancer and lupus. There is much work yet to be done, but we are pleased with our productivity to date, and I believe that the quantity and quality of our product candidates position us well for continuing focus and progress in the months and years ahead."

For additional information on Human Genome Sciences, please visit our web site at www.hgsi.com. Health professionals interested in clinical studies involving HGSI 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 us at (301) 610-5790, extension 3550.

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

HGS, Human Genome Sciences, ABthrax, Albuferon, Albuleukin, Albutropin, BLyS, LymphoRad and LymphoStat-B are trademarks of Human Genome Sciences, Inc. All other trademarks and trade names are the property of their respective owners.

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. (HGSI Press Release) Human Genome Sciences Receives FDA Clearance To Initiate Human Trial Of Novel Drug To Prevent And Treat Anthrax Infections. June 25, 2003.

2. (HGSI Press Release) Human Genome Sciences Develops New Means To Prevent And Treat Anthrax Infections. March 18, 2003.

3. Inglesby TV, O'Toole T, Henderson DA, et al. Anthrax as a Biological Weapon, 2002: Updated recommendations for Management. JAMA May, 2002. 287(17): 2236-2252.

4. Public Health Security And Bioterrorism Preparedness And Response Act Of 2002: Section 123. http://www.fda.gov/oc/bioterrorism/PL107-188.html.

5. The Company has said that it expects to initiate Phase 1 clinical trials in mid-2003.

6. (HGSI Press Release) Results Of Phase 2 Repifermin Clinical Trial Demonstrate Safety And Efficacy In Patients With Cancer Therapy-Induced Mucositis. April 2, 2003.

7. (HGSI Press Release) Human Genome Sciences Initiates New Clinical Trial For Treatment Of Mucositis In Multiple Myeloma Patients. September 4, 2001.

8. (HGSI Press Release) Human Genome Sciences Reports Progress In Clinical Trials Of Five Drugs At JP Morgan H&Q Conference. January 6, 2003.

9. (HGSI Press Release) Human Genome Sciences Announces Trial For Treatment Of Immunoglobulin-A Deficiency. September 19, 2001.

10. (HGSI Press Release) Human Genome Sciences Announces Clearance Of Investigational New Drug Application For LymphoRad¹³¹, A New Anticancer Drug For The Treatment Of B-Cell Tumors. May 14, 2002.

11. (HGSI Press Release) Results Of Phase 1 Clinical Trial Demonstrate That LymphoStat-B™ Is Safe And Biologically Active In Patients With Systemic Lupus Erythematosus. April 21, 2003.

12. (HGSI Press Release) Human Genome Sciences Initiates Trial of Novel Anticancer Drug. April 30, 2002.

13. (HGSI Press Release) Human Genome Sciences Reports Results Of Preclinical Studies Of Albuleukin™ And Trail-R1 And Trail-R2 Agonistic Human Monoclonal Antibodies At The Society For Biological Therapy. November 11, 2002.

14. (HGSI Press Release). Results of Phase 1 Albutropin™ Trial Demonstrate Safety, Prolonged Half-Life and Biological Activity. October 8, 2002.

15. (HGSI Press Release) Interim Results of Phase 1 Albuferon™-alpha Clinical Trial Demonstrate Safety, Prolonged Half-Life, and Biological Activity in Patients Infected with Hepatitis C. November 4, 2002.

16. (HGSI Press Release) Human Genome Sciences Initiates Trial Of Albuleukin™, A Recombinant Human Protein For Treating Solid Tumor Cancers. January 7, 2002.

CONTACTS:
David C. Stump, M.D.
Senior Vice President, Drug Development
301/309-8504
Jerry Parrott
Vice President, Corporate Communications
301/315-2777
Kate de Santis
Director, Investor Relations
301/251-6003