DMPK and myotonic dystrophy type 1: Myotonic dystrophy type 1 (DM1) is caused by a highlystructuredRNA repeat expansion, r(CUG)exp, harbored in the 3′untranslated region (3′ UTR) of dystrophia myotonica proteinkinase (DMPK) mRNA and drives disease through a gain-of-functionmechanism.