Furthermore, the clinical diagnosis of Rett syndrome (RTT OMIM 3127520) is advantaged by the definition of stringent clinical criteria, which upon revision [12] allow classification and distinguishing classical RTT from both early onset epilepsy and congenital atypical RTT, the latter two mainly ascribed to disease causing variants in the CDKL5 and FOXG1 genes. This evidence concerns the gene CDKL5 and Rett syndrome.