The biotech’s board of directors announced late today that a sale is being explored after Carl Icahn, who recently bought a stake, and other investors expressed interest in a deal. Who are those others? A Biogen spokewoman tells Bloomberg News that unnamed drugmakers have expressed interest.

The move comes after the Federal Trade Commission two months ago cleared Icahn to increase his stake in the biotech, prompting a run-up in the stock as investors speculated Icahn would launch a takeover. Icahn, of course, often presses for a sale when he thinks a stock is undervalued. He bought shares in MedImmune before helping to force a $15 billion sale to AstraZeneca.

“The rumor that the company has been in play for a long time has been confirmed by this,” Geoff Porges, an analyst at Sanford C. Bernstein, tells Bloomberg. “Icahn is saying he’s at the table and he’s willing to put a bid in.” Investors love Icahn’s interest. Biogen rose $12.47, or 18 percent, to $81.90, its biggest one-day gain since rising 20 percent in February 2004, the wire service notes.

Biogen last month forecast sales and revenue growth higher than analysts had estimated, citing increased demand for Tysabri. The company expects 100,000 patients to be using Tysabri by the end of 2010, a figure that suggests sales of the drug could reach $2.8 million, according to JP Morgan Securities analyst Geoffrey Meacham, the wire reports.