SCN4A and hyperkalemic periodic paralysis: The ion channel genes involved are the skeletal muscle calcium channel gene CACNA1S encoding Cav1.1 (HypoPP), the skeletal muscle sodium channel gene SCN4A encoding Nav1.4 (HypoPP and HyperPP) and the inward rectifying potassium channel gene KCNJ2 encoding Kir2.1 (ATS), which is expressed in skeletal muscles but also present in other tissues explaining the multisystem involvement in Andersen-Tawil syndrome.