Patients homozygous for sodium channel mutations causing paramyotonia congenita (SCN4A, Ile1393Thr), hypokalemic periodic paralysis (SCN4A, Arg1132Gln) and myotonia congenita (CLCN1, Gly190Ser, Ile556Asn, Ala313Thr, Ile556Asn) display much more severe clinical features than patients heterozygous for these mutations (Plassart-Schiess et al. 1998; Arzel-Hézode et al. 2010; Shalata et al. 2010). Here, CLCN1 is linked to Thomsen and Becker disease.