DMPK and Myotonia: Myotonic dystrophy type 1 (DM1) is an autosomal dominant disorder characterized by muscle weakness, myotonia, cataracts, and cardiac conduction defects; it is associated with expansions of cytosine-thymine-guanine (CTG) repeats in the 3′ untranslated region of the myotonic dystrophy protein kinase (DMPK) on chromosome 19 (reviewed in [1]).