The price remains unchanged at $13 a share under the longer tender, which will now expire at 5 p.m. New York time on 29 June, Britain's biggest drugmaker said on Friday. The initial tender period ran out on 7 June.
People familiar with the situation previously told Reuters that GSK was set to extend its tender offer - a direct appeal to Human Genome shareholders over the heads of management - while it begins a process to replace the entire HGS board with its own nominees.
The British company has already started reaching out to executives in the drug industry as well as finance and governance experts who could be nominated as independent directors of the 12-member board.
Sources said on May 30 that GSK intended to seek approval from Human Genome shareholders to replace the board under a "consent solicitation" process, which could come in the next few weeks. No details on the process were given on Friday.