The gene encoding the principal voltage-dependent sodium channel expressed in adult human skeletal muscle (SCN4A) has recently been linked to the pathogenesis of human hyperkalemic periodic paralysis and paramyotonia congenita. We report the cloning and nucleotide sequence determination of the normal product of this gene. The 7,823 nucleotide complementary DNA, designated hSkM1, encodes a 1,836 amino acid protein that exhibits 92% identity with the tetrodotoxin-sensitive rat skeletal muscle sodium channel alpha subunit, but lower homology with either the human heart sodium channel or with other sodium channels from immature rat muscle or rat brain. Specific hSkM1 RNA transcripts are expressed in adult human skeletal muscle but not in heart, brain, or uterus. The SCN4A gene product, hSkM1, is the human homologue of rSkM1, the tetrodotoxin-sensitive sodium channel characteristic of adult rat skeletal muscle. This structural information should provide the necessary backdrop for identifying and evaluating mutations affecting the function of this channel in the periodic paralyses.