Myotonic dystrophy type 1 (DM1) is caused by expansion of a CTG triplet repeat within the 3′ untranslated region (UTR) of DMPK, whereas myotonic dystrophy type 2 (DM2) results from expansion of a CCTG repeat within the first intron of ZNF9[2], [3]. Here, DMPK is linked to myotonic dystrophy type 1.