Human Genome Sciences has received an unsolicited proposal from GlaxoSmithKline to acquire HGS for $13.00 per share in cash.

The HGS Board of Directors, in consultation with independent financial and legal advisors, has carefully reviewed and considered the GSK offer and has determined that the offer does not reflect the value inherent in HGS.

HGS also announced that its Board of Directors has authorized the exploration of strategic alternatives in the best interests of shareholders, including, but not limited to, a potential sale of the Company.

HGS has retained Goldman, Sachs & Co. and Credit Suisse Securities (USA) LLC to assist in this process, with Skadden, Arps, Slate, Meagher & Flom LLP and DLA Piper LLP (US) serving as legal counsel.

GSK has been invited to participate in this process and HGS has requested additional information regarding investigational products in GSK’s clinical pipeline to which HGS has substantial financial rights, including darapladib, currently in Phase 3 development for the treatment of cardiovascular disease, and albiglutide, currently in Phase 3 development for the treatment of type 2 diabetes.

HUMAN GENOME SCIENCES ANNOUNCES UNSOLICITED OFFER FROM GLAXOSMITHKLINE; HGS BOARD OF DIRECTORS AUTHORIZES EXPLORATION OF STRATEGIC ALTERNATIVES

Board believes $13 unsolicited offer does not reflect value inherent in Company

Goldman Sachs and Credit Suisse retained to assist with exploration of strategic alternatives

Additional information requested from GSK

ROCKVILLE, Maryland – April 19, 2012 – Human Genome Sciences, Inc. (Nasdaq: HGSI) today announced that it has received an unsolicited proposal from GlaxoSmithKline plc (GSK) to acquire HGS for $13.00 per share in cash.

The HGS Board of Directors, in consultation with independent financial and legal advisors, has carefully reviewed and considered the GSK offer and has determined that the offer does not reflect the value inherent in HGS.

HGS also announced today that its Board of Directors has authorized the exploration of strategic alternatives in the best interests of shareholders, including, but not limited to, a potential sale of the Company. HGS has retained Goldman, Sachs & Co. and Credit Suisse Securities (USA) LLC to assist in this process, with Skadden, Arps, Slate, Meagher & Flom LLP and DLA Piper LLP (US) serving as legal counsel.

GSK has been invited to participate in this process and HGS has requested additional information regarding investigational products in GSK’s clinical pipeline to which HGS has substantial financial rights, including darapladib, currently in Phase 3 development for the treatment of cardiovascular disease, and albiglutide, currently in Phase 3 development for the treatment of type 2 diabetes.

There can be no assurance that any transaction will occur or if so on what terms. HGS does not intend to discuss the status of its evaluation unless and until a specific transaction has been approved.

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 clinicaltrialsinfo@hgsi.com 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.