Benign Familial Neonatal Convulsions (BFNC) is an epileptic disorder with an autosomal dominant mode of transmission. It has been shown that about 80% of BFNC pedigrees are linked to a genetic defect on chromosome 20q13.3. A candidate gene for the epilepsies, the gene coding for the alpha4 subunit of the nicotinic cholinergic receptor (CHRNA4), has previously been localized on chromosome 20. Here we report a single point mutation converting a serine codon to a stop codon in the exon 5 of CHRNA4, in one BFNC family. Identification of CHRNA4 as the defective gene in 20q-BFNC represents the first example of a human idiopathic epilepsy caused by a mutation directly affecting a neurotransmitter receptor in the central nervous system.