The DM1 expansion mutation is bidirectionally transcribed and located as a (CTG)n repeat in the 3′ untranslated region (UTR) of DMPK and as a (CAG)n repeat in an alternatively spliced intron of the DM1 locus antisense RNA (DM1-AS) gene. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.