SCN1A and Dravet syndrome: The proportion of proband with an SCN1A gene linked seizure disorder is decreased in affected parents and in the proband severity of the disease increases causing mostly SCN1A gene-related severe myoclonic epilepsy in infancy (SCN1A SMEI), intractable childhood epilepsy with generalized tonic-clonic seizures (ICE-GTC) and Dravet syndrome (DS) that may be the result of a pathogenic de novo mutation.