Myotonic dystrophy type 1 [DM1, (OMIM 160900)] is an autosomal dominant muscular dystrophy that results from a trinucleotide CTG repeat expansion (50–4000 copies) in the 3′ UTR of the dystrophia myotonica protein kinase gene (DMPK) [1, 2]. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.