MECP2 and Intellectual disability: Dosage-variation on functional MECP2 during human neurodevelopment have been described in RTT, predominantly affecting females (Amir et al., 1999), but also affecting males in MDS (D’Mello, 2021; Pascual-Alonso et al., 2021) and in cases of somatic mosaicism and intellectual disability associated with RTT (Orrico et al., 2000; Kudo et al., 2002; Topçu et al., 2002; Venâncio et al., 2007).