The recurrent pathogenic variant <i>KCNC1-</i>p.Ala421Val (A421V) is a cause of developmental and epileptic encephalopathy characterized by moderate-to-severe developmental delay/intellectual disability, and infantile-onset treatment-resistant epilepsy with multiple seizure types, including myoclonic seizures. The gene discussed is KCNC1; the disease is Seizure.