MECP2 and Rett syndrome: RTT is known to be caused in the overwhelming majority of cases by sporadic de novo loss-of-function mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene [6] encoding methyl-CpG-binding protein 2 (MeCP2), a nuclear protein that binds to methylated CpGs and regulates gene expression [7, 8].