PTC Therapeutics Inc. is already established as a player in rare diseases, working with Roche Holding AG to develop and market Evrysdi (risdiplam) to treat certain patients with spinal muscular atrophy. With Evrysdi now approved in the U.S. and Europe, and Translarna (ataluren) approved in Europe for Duchenne muscular dystrophy, South Plainfield, N.J.-based PTC is approaching a crucial juncture with its first gene therapy product.