Mutations of NaV1.4 responsible for hyperkalemic periodic paralysis increase NaPIC (Cannon et al., 1991; Cannon and Strittmatter, 1993a), which appears to play a central role in triggering the depolarization that underlies attacks of transient weakness (Lehmann-Horn et al., 1987; Jurkat-Rott et al., 2010; Cannon, 2015). The gene discussed is SCN4A; the disease is periodic paralysis.