SCN4A and periodic paralysis: The first Na+ channelopathy identified in humans was the hyperkalemic form of periodic paralysis (HYPP or HyperPP, OMIM#170500) 30 years ago (Figure 1): missense mutations in SCN4A, the gene encoding the pore-forming subunit of Nav1.4 channels, were reported as the cause of this familial form of transient muscle weakness.