Pathogenic variants in SMC1A are often dominant-negative and cause Cornelia de Lange syndrome (CdLS) with growth retardation and typical facial features; however, rare SMC1A variants cause a developmental and epileptic encephalopathy (DEE) with intractable early-onset epilepsy that is absent in CdLS. This evidence concerns the gene SMC1A and developmental and epileptic encephalopathy.