Rett syndrome (RTT) [1,2] is a significantly disabling neurodevelopmental disorder primarily, but not exclusively, affecting girls and women, that is caused in the majority of cases (>96%) by pathogenic loss of function genetic variants in the X-linked gene methyl-CpG-binding protein 2 (MECP2) gene [3,4,5]. This evidence concerns the gene MECP2 and atypical Rett syndrome.