The more common form in the United States, DM type 1 (DM1), has a mutation frequency of 1 in 2,100 and is caused by an unstable expanded CTG repeat in the 3′-UTR of the DM1 protein kinase (DMPK) gene, with 50 to more than 4,000 repeats in affected individuals and 5 to 38 repeats in those unaffected (2, 3). The gene discussed is DMPK; the disease is myotonic dystrophy type 1.