Over 200 mutations in the human skeletal muscle ClC-1 gene (CLCN1) on chromosome 7 have been linked to myotonia congenita, which can be inherited in an autosomal recessive (Becker type) or autosomal dominant (Thomsen type) manner (8–11, 15, 87, 88). Here, CLCN1 is linked to Thomsen and Becker disease.