SCN4A and hypokalemic periodic paralysis: Nearly all mutations causing familial HypoPP are found in voltage-gated ion channels of skeletal muscle, such as the hCaV1.1 calcium channel (encoded by CACNA1S) in type I HypoPP, or the homologous hNaV1.4 sodium channel (encoded by SCN4A) in type II HypoPP1,2.