Human Genome Sciences, Inc. (Nasdaq: HGSI) today issued the following statement in response to the GlaxoSmithKline plc (GSK) announcement regarding expiration of the Hart-Scott-Rodino waiting period: “The HGS Board of Directors has rejected GSK’s unsolicited $13.00 per share offer, after concluding unanimously that the GSK offer is inadequate, does not reflect the value inherent in HGS and is not in the best interests of our stockholders. We announced on April 19 that our Board has authorized the exploration of strategic alternatives in the best interests of stockholders, including a potential sale of the Company. This process continues to be active and fully underway. We invited GSK to participate in this process, but GSK declined and instead commenced its unsolicited tender offer, which seeks to circumvent, disrupt and prematurely end our strategic review process to the disadvantage of HGS stockholders. We are committed to completing this process as expeditiously as possible. The HGS Board of Directors recommends that HGS stockholders reject GSK’s tender offer and not tender any of their shares to GSK.” ABOUT HUMAN GENOME SCIENCES Human Genome Sciences exists to place new therapies into the hands of those battling serious disease. For more information about HGS, please visit the Company’s web site at www.hgsi.com. Health professionals and patients interested in clinical trials of HGS products may inquire via email to firstname.lastname@example.org or by calling HGS at 1-240-314-4430. HGS, Human Genome Sciences and BENLYSTA are trademarks of Human Genome Sciences, Inc. Other trademarks referenced are the property of their respective owners. SAFE HARBOR STATEMENT This announcement includes statements that are forward-looking. These forward- looking statements include those regarding our expectations for BENLYSTA, darapladib, albiglutide and raxibacumab, among others. These forward-looking statements are based on our current intentions, beliefs and expectations regarding future events. We cannot guarantee that any forward-looking statement will be accurate. Investors should realize that if underlying assumptions prove inaccurate or unknown risks or uncertainties materialize, actual results could differ materially from our expectations. Investors are, therefore, cautioned not to place undue reliance on any forward-looking statement. Any forward-looking statement speaks only as of the date of this announcement, and, except as required by law, we do not undertake to update any forward-looking statement to reflect new information, events or circumstances.