SCN4A/Nav1.4 is predominantly expressed in skeletal muscles, and mutations in Nav1.4 are associated with multiple skeletal muscle disorders including myotonia, paramyotonia congenita (PC), hyper or hypokalemic periodic paralysis (hyperPP or hypoPP), and congenital myasthenic syndrome (CMS). The gene discussed is SCN4A; the disease is congenital myasthenic syndrome.