The onset of severe, drug‐resistant seizures in early childhood is characteristic of the rare epileptic disorders Lennox‐Gastaut syndrome, Dravet syndrome, and CDKL5 deficiency disorder and is frequently observed in the rare genetic conditions tuberous sclerosis complex and Rett syndrome. The gene discussed is CDKL5; the disease is encephalopathy, progressive, early-onset, with brain edema and/or leukoencephalopathy.