Loss-of-function mutations of the X-linked methyl-CpG binding protein 2 (MeCP2) gene is the major cause (approximately 90 %) of classical cases of RTT while cyclin-dependent kinase-like 5 (CDKL5) and forkhead box protein G1 (FOXG1) gene mutations represent the remaining 10 % of the cases [2]. This evidence concerns the gene CDKL5 and Rett syndrome.