DMPK and myotonic dystrophy type 1: Myotonic dystrophy type 1 (DM1) is a progressive, dominantly inherited, multisystem disease caused by an expanded and unstable trinucleotide CTG repeat localized to the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene on chromosome 19q13.3 [4].