CDKL5 and atypical Rett syndrome: 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.