MECP2 and Rett syndrome: Huppke and Gärtner classified the male patients with mutations in MECP2 into three groups: (1) boys presenting severe encephalopathy and infantile death with MECP2 mutations seen in RTT girls; (2) boys with RTT, who are mosaic or have a 47, XXY karyotype and harbor mutations already seen in girls with RTT; and (3) boys with less severe neurological and/or psychiatric manifestations.