SCN4A and periodic paralysis: Moreover, voltage-clamp fluorescent recordings to observe the Nav1.4 VSDs revealed that DIII and DIV VSD immobilization is correlated with the onset of inactivation (Cha et al., 1999); some mutants in DIII VSD were shown to impair fast inactivation and cause channelopathies, e.g., a mutation of R1135H in DIII S4 of Nav1.4 significantly enhanced entry into inactivation and prolonged recovery to cause hypokalemia periodic paralysis (Groome et al., 2014).