(Reuters) -Sage Therapeutics ( SAGE ) said on Monday that its board has unanimously rejected the takeover offer it received from Biogen.

The company said Biogen's offer to buy all outstanding shares of Sage that it does not already own for $7.22 apiece, "significantly undervalues Sage and believes it is not in the best interest of shareholders."

Biogen has a 10.2% stake in Sage Therapeutics ( SAGE ).

Sage said it has initiated a process to explore strategic alternatives, but has not set a timetable for the review process.