Mutations in the SCN4A gene (sodium channel, voltage-gated, type IV, alpha subunit) encoding the human skeletal muscle NaV1.4 channel cause five different skeletal muscle disorders: potassium-aggravated myotonia (PAM), paramyotonia congenita (PMC), hyperkalemic periodic paralysis (hyperPP), hypokalemic periodic paralysis (HypoPP), and a form of congenital myasthenic syndrome (CMS) [22]. Here, SCN4A is linked to myotonia fluctuans.