SCN4A and paramyotonia congenita of Von Eulenburg: One mutation of Nav1.4 causing paramyotonia congenita (I693T [34]) is located in the S4–S5 linker of domain II, only 10 amino acids N-terminal to the amino acid substitution in L858F in Nav1.7 [7], and is located at corresponding position to the IEM mutation Nav1.7/I848T [15].