Myotonic dystrophy type 1 (DM1; Steinert’s disease, MIM#160900) is the most common muscular dystrophy in adulthood due to an unstable CTG expansion in the 3′ untranslated region of the myotonic dystrophy protein kinase (DMPK) gene (MIM#605377). The gene discussed is DMPK; the disease is muscular dystrophy.