Shares of Human Genome Sciences (HGSI) skyrocketed 100% Thursday morning after the biotechnology company rejected a $2.6 billion buyout bid from British drug maker GlaxoSmithKline (GSK).

Human Genome Sciences said the unsolicited $13-a-share offer from Glaxo “does not reflect the inherent value” of the Rockville, Md. company even though it represents an 81% premium on its Wednesday closing price.

Yet Human Genome didn’t close the door to an eventual deal, saying it is seeking more information from Glaxo and exploring strategic alternatives on its own with help from investment banks Goldman Sachs (GS) and Credit Suisse (CS).

“There can be no assurance that any transaction will occur or if so on what terms,” Human Genome said in a statement, adding it doesn’t plan to talk about the status of the talks unless a deal is reached.

Founded more than 20 years ago, Human Genome was one of the first companies to use genes to develop new drugs.

The two companies have a long history together as they began a research partnership in 1993 and have teamed up more recently on a lupus drug.

Glaxo, which is based in the U.K., confirmed it approached Human Genome with the takeover bid. The company defended the offer price, saying it reflects “full and fair value.”

“It also eliminates substantial execution risk for Human Genome Sciences shareholders and delivers immediate and certain value that is superior to what we believe Human Genome Sciences can reasonably expect to create as a standalone company,” Glaxo CEO Sir Andrew Witty said in a statement.

Glaxo said is prepared to enter into talks immediately and projected a tie-up could generate $200 million in cost synergies by 2015 and add to the company’s bottom line beginning in 2013.

Human Genome said it has requested additional information from Glaxo related to products in the British company’s clinical pipeline like carviovascular drug darapladib and diabetes treatment albiglutide. Human Genome said it has “substantial” financial rights to both drugs.

Wall Street is clearly betting Human Genome will receive a higher bid than the one unveiled by Glaxo. Shares of Human Genome were recently up 100.14% to $14.35, trading well above the $13 offer price.

Analysts at Baird maintained their “buy” rating on Human Genome, predicting the next offer for the biotech company will be about $19, according to Dow Jones Newswires.

Human Genome’s shares have swung in a very wide range over the past year, trading as high as $30 in late April 2011 and as low as $6.51 in December.

U.S.-listed shares of Glaxo hit a 52-week high on the news, recently trading up 1.01% to $46.84.

"We are disappointed that Human Genome Sciences has rejected our offer without discussion and are confident that our offer is in the best interest of shareholders of both companies,” Witty said.

Lazard (LAZ) and Morgan Stanley (MS) are serving as Glaxo’s financial advisors.

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