Mutations in the sodium channel gene SCN4A encoding the Nav1.4 voltage-gated sodium channel are well-known causes of the skeletal muscle channelopathies: paramyotonia congenita (PMC), other forms of myotonia, and periodic paralyses (hyperkalemic periodic paralysis [HyperPP], normokalemic, and hypokalemic periodic paralysis). Here, SCN4A is linked to hyperkalemic periodic paralysis.