Indeed, 90–95% of individuals with typical RTT and 70% of atypical cases are mutated in MECP2. Additionally, genetic changes in the X-linked Cyclin-Dependent Kinase-Like 5 (CDKL5; OMIM #300203) (Evans et al., 2005) or the Forkhead box G1 (FOXG1; OMIM #164874) (Philippe et al., 2010) genes have been related with atypical and rarely classic forms of RTT (Neul et al., 2010). Here, MECP2 is linked to Rett syndrome.