The SCN4A gene encodes for a component (i.e., the alpha subunit four) of the voltage-gated sodium channel integral membrane protein, which form a pore in the cytoplasmic membrane conducting sodium ions through the membrane and is responsible, so far of: (a) a group of related muscular disorders, including hyperkalemic periodic paralysis [HYPP; MIM # 170500], paramyotonia congenita [PMC; MIM # 168300]; and (b) a group of disorders classified as potassium-aggravated myotonia [MIM # 608390], and hypokalemic periodic paralysis type 2 [HOKPP2; MIM # 613345]. Here, SCN4A is linked to acetazolamide-responsive myotonia.