Thus, Rett-syndrome-like phenotypes (i.e. Rett syndrome in males) caused by MECP2 mutations concomitant with KSM were observed in 5 cases (0.1%); two cases exhibited fragile X syndrome (FMR1 repeat expansions were molecularly confirmed) with KSM; a case of CDKL5 mutation (C532C > T) causing CDKL5-deficiency demonstrated KSM; single cases of KSM were concomitant with Gilbert and Williams-Beuren syndromes (cytogenetically/molecularly confirmed). This evidence concerns the gene CDKL5 and fragile X syndrome.