SCN4A and hypokalemic periodic paralysis: The definitive diagnosis of HypoPP lies on genetic testing with mutations in the genes known to cause HypoPP: CACNA1S, which encodes the pore-forming subunit of the skeletal muscle voltage-gated calcium channel Cav1.1 (HypoPP1), or SCN4A (HypoPP2) (Jurkat-Rott et al., 1994; Ptáček et al., 1994; Jurkat-Rott et al., 2000).