MECP2 and Rett syndrome: Many severe RTT-causing missense mutations in MECP2, such as R111G and R106W, cluster in the methyl-CpG binding domain (MBD) (Nan et al. 1993) and abolish MeCP2 binding to methylated cytosines (Goffin et al. 2012; Heckman et al. 2014; Brown et al. 2016; Johnson et al. 2017).