Severe myoclonic epilepsy of infancy (SMEI, Dravet syndrome), which is mainly caused by the <i>SCN1A</i> mutation, is a severe epileptic encephalopathy that manifests in infancy and leads to intractable seizures and developmental impairment. The gene discussed is SCN1A; the disease is encephalopathy, progressive, early-onset, with brain edema and/or leukoencephalopathy.