We have identified four putative human sodium channel gene sequences, 55 bp each, using the polymerase chain reaction (PCR) on total human placental DNA with primers specific for the cDNA sequence of the rat brain sodium channel Iα (Scn1a) gene. One of these sequences was extended bidirectionally by genomic inverse-PCR to obtain a 1.6-kb fragment. Sequencing of this 1, 556-bp fragment showed a 282-bp complete exon, which has 95% and 94% homology at the nucleotide and amino acid levels, respectively, with the rat Scn1a gene. We putatively assign this sequence as belonging to the gene coding the α-subunit of a human brain type I sodium channel (SCN1A). PCR on human × rodent somatic cell hybrids with primers derived from SCN1A localized this gene to chromosome 2. Fluorescence in situ hybridization to human metaphase chromosomes sublocalized the gene to chromosome band 2q24.