Generally, almost 95% of RTT patients carry mutations in the MeCP2 gene, and recent findings demonstrated that two additional genes, cyclin-dependent kinase like 5 (CDKL5) (Evans et al., 2005) and fork head box G1 (FOXG1), can also be involved in the pathogenesis of this syndrome (Mencarelli et al., 2010). The gene discussed is CDKL5; the disease is Rett syndrome.