RTT is primarily caused by loss-of-function mutations in methyl-CpG-binding protein 2 (MECP2) (Amir et al., 1999), the gene encoding MeCP2, a transcriptional repressor that binds to methylated CpG sites in promoter regions of DNA (Lewis et al., 1992; Nan et al., 1997). Here, MECP2 is linked to Rett syndrome.