SAN RAFAEL (TheStreet) -- BioMarin Pharmaceuticals is expanding its reach into the market for rare disease therapies with a deal to buy Prosensa Holding for up to $840 million, the companies announced Monday. The deal is a gamble of sorts for Biomarin, which is betting that its experience and expertise in securing approvals for so-called "orphan disease" drugs can help Prosensa win approval for drisapersen, an experimental treatment for Duchenne muscular dystrophy, despite mixed results from clinical trials.