DMPK and myotonic dystrophy type 1: DM1 is caused by expansion of an unstable CTG trinucleotide repeat in the 3′ untranslated region of the dystrophia myotonica protein kinase (DMPK) gene located on chromosome 19q13.3 (Brook et al., 1992; Fu et al., 1992; Mahadevan et al., 1992).