The purchase values each share of Sirna at $13, more than double its $6.45 closing price. The stock has a 52-week high of $8.52.
Sirna is working on developing a new class of medicines based on RNA interference technology. Therapeutics of this type selectively catalyze the destruction of the RNA transcribed from an individual gene.
One area in which RNA interference shows great promise is in cancer research, Merck said.