In the vast majority of cases (~ 90%), sporadic functional loss of the methyl-CpG binding protein 2 (MeCP2) gene causes RTT, with mutations in CDKL5, FOXG1, CTNNB1 and WDR45 genes [20–22] contributing to the rarer atypical or variant RTT phenotypes. This evidence concerns the gene MECP2 and Rett syndrome.