Several allelic disorders of skeletal muscle are caused by mutations of SCN4A that encodes the pore-forming α subunit of the voltage-gated sodium channel (NaV1.4).1 Missense mutations with gain-of-function changes (GOF; too much inward Na+ current) are found in hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita, and several variants of sodium channel myotonia.2 Leaky channels resulting from mutations of arginine residues in the voltage sensor domain cause hypokalemic periodic paralysis (HypoPP) type 2.3,4 These traits are all dominantly inherited. The gene discussed is SCN4A; the disease is hyperkalemic periodic paralysis.