Among them, individuals with early seizure onset variant RTT [15,16], who manifest epilepsy before regression, have mutations in the cyclin-dependent kinase-like 5 gene (CDKL5), and patients with congenital RTT, who show gross early abnormal development, have molecular defects in the forkhead box G1 gene FOXG1 [17]. The gene discussed is CDKL5; the disease is epilepsy.