CDKL5 and Intellectual disability: Patients with RTT are classified into the early seizure (Hanefeld) variant and the congenital (Rolando) variant; these classes may have cyclin-dependent kinase-like 5 (CDKL5) [70,77] or fork-head box G1 (FOXG1) mutations [70,78,79], respectively [80,81,82], suggesting that patients with features of RTT without MECP2 mutations may have mutations in other genes, including those associated with epileptic encephalopathies, intellectual disability (ID), or autism spectrum disorder [83].