A neonatal/infantile epilepsy syndrome characterized by the onset of non-self-limiting seizures and developmental regression or delay in infants/neonates. This condition is typically caused by genetic mutations that disrupt normal brain development, affecting both cognitive and motor development that is not responsive to typical seizure treatments. The disease is neonatal/infantile-onset epilepsy syndrome with developmental and epileptic encephalopathy (MONDO_0800490, a Monarch Disease Ontology term). Also known as: NIE-SDE.