MECP2 and Down syndrome: Extensive information on the spectrum of clinical features and disease progression in RTT has been acquired from the US Natural History Study (NHS) of RTT and RTT-related disorders, which enrolled people with RTT and disorders with clinical and genetic relationships to RTT: MECP2 duplication syndrome (MDS); CDKL5 deficiency disorder (CDD); and FOXG1 syndrome (FS).