The rapid membrane depolarization associated with action potentials in vertebrate nerve and muscle is largely a function of Na+ ion diffusion through voltage-gated sodium channels (Hille, 1992). The first cloned sodium channel α subunit cDNA was isolated from the electric organ of Electrophorus electricus (Noda et al., 1984). Related sodium channel cDNAs have been isolated from a variety of invertebrates including cnidarians, arthropods, molluscs, and urochordates (Loughney et al., 1989; Okamura et al., 1994; Dyer et al., 1997; Spafford et al., 1998). Recent isolation and characterization of the genes encoding mammalian sodium channels have clarified the evolutionary history of the sodium channel gene family. The association of sodium channel mutations with neurological disorders has focused attention on this gene family (Bulman, 1997; Meisler et al., 1997; Wallace et al., 1998).