MECP2 and atypical Rett syndrome: Rett syndrome (RTT), the second commonest cause of severe mental retardation in the female gender, is represented by a devastating neurodevelopmental disorder with a wide phenotypical heterogeneity which is caused in the overwhelming majority of cases by loss-of-function de novo sporadic mutations in the X-linked gene encoding the methyl-CpG binding protein 2 (MeCP2) [6].