Generalized idiopathic epilepsies (IGE) cause 40% of all seizures and commonly have a genetic basis. One type of IGE is Benign Familial Neonatal Convulsions (BFNC), a dominantly inherited disorder of newborns. A submicroscopic deletion of chromosome 20q13.3 which co-segregates with seizures in a BFNC family has been identified. Characterization of cDNAs spanning the deleted region identified a novel voltage-gated potassium channel, KCNQ2, which belongs to a new KCNQ1-like class of potassium channels. Nine other BFNC probands were shown to have KCNQ2 mutations including three missense mutations, three frameshifts, two nonsense mutations, and one splice site mutation. A second gene, KCNQ3, was found in a separate BFNC family in which the mutation had been localized to chromosome 8. A missense mutation was found in this gene in perfect cosegregation with the BFNC phenotype in this latter family. This demonstrates that defects in potassium channels can cause epilepsy. Furthermore, some members of one of the BFNC families with a mutation in KCNQ2 also exhibited rolandic epilepsy and one individual with juvenile myoclonic epilepsy has a mutation in an alternative exon of KCNQ3.