GlaxoSmithKline (NYSE: GSK) says it has secured U.S. government approval to buy Human Genome Services (Nasdaq: HGSI), but the Maryland firm says it will continue to fight the hostile takeover.

In a press release, GSK said “the waiting period under the Hart-Scott-Rodino Antitrust Improvement Act” had expired, thus “GSK has clearance under the U.S. anti-trust laws to acquire HGS, and the tender offer condition with respect to the expiration of the applicable waiting period under the HSR Act has been satisfied.”

GSK has offered $13 in cash for each HGSI share. The deal is valued at $2.6 billion. The offer is set to expire at midnight on June 7.

HGSI has adopted a “poison pill” defense and said it would continue to fight GSK’s takeover.

“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,” HGSI said in a statement.

“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.”

GSK operates its U.S. headquarters in RTP.