Merck & Co. said Monday that it is acquiring San Francisco-based Sirna Therapeutics, in a transaction worth $1.1B. Sirna Therapeutics develops medicines based on RNA interference (RNAi) technology. Merck will acquire 100 percent of the equity of Sirna for $13 per share in cash. Sirna's RNAi therapeutics are aimed at treating cancer and other conditions. The firm was backed by Oxford Bioscience Partners, Sprout Group, Venrock Associates, and others.