Mutations in the X-linked cyclin-dependent kinase-like 5 gene (CDKL5) are associated with CDKL5 deficiency disorder (CDD [MIM300203]) [1], a neurodevelopmental disease characterized by early infantile epileptic encephalopathy, autism spectrum disorders, and intellectual disability [2–6]. Here, CDKL5 is linked to early-infantile DEE.