There are two clinically similar but genetically distinct types: DM type 1 (DM1, also known as Steinert’s disease; MIM #160900), caused by an unstable expansion of a CTG trinucleotide repeat in the noncoding region of the dystrophia myotonic-protein kinase gene (DMPK) [2], and DM type 2 (DM2; MIM #602668), caused by a tetra-nucleotide repeat CCTG expansion in the zinc finger 9 (ZNF9) gene [3]. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.