Most epilepsies are characterized as "idiopathic" because of the lack of a known cause. Nevertheless, recently, there has been significant progress in the molecular genetics of idiopathic epilepsy. Mutations in gene-encoding ion channels were found to be the underlying disorder in all idiopathic epilepsies with a known molecular basis. Missense mutations in the voltage-gated sodium channel a1 subunit gene (SCN1A) were firstly identified in patients with generalized epilepsy with febrile seizures plus additional symptoms (GEFS + ). Subsequently, mutations of SCN1A were also found in patients with severe myoclonic epilepsy of infancy (SMEI) or Dravet syndrome, and in patients with borderline SMEI (SMEB), a milder form of Dravet syndrome. We describe a case of a new missense de novo mutation of SCN1A in a child with the clinical features of borderline SMEI syndrome.