Missense mutations at arginine residues in the S4 segments of the voltage sensor domains of sodium or calcium channels account for almost all cases of HypoPP,3 and have a common functional defect, the gating pore leakage current, caused by a mutation-induced anomalous ion conduction pathway.4 Since R1460 is an arginine in the S4 segment of domain IV in NaV1.4 (figure 1B), we tested whether R1460Q or R1460W mutant channels have a detectable gating pore leakage current. The gene discussed is SCN4A; the disease is hypokalemic periodic paralysis.