DM1 is caused by an expansion of cytosine-thymine-guanine (CTG) trinucleotide repeats in the 3′-untranslated region of the DMPK gene and typically presents with more severe symptoms and a higher prevalence than DM2, which is caused by tetranucleotide repeat expansion located in the CNBP gene. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.