MECP2 and X-linked intellectual disability: Duplications within Xq28 involving MECP2 account for 1% of X-linked intellectual disability (del Gaudio et al., 2006; Friez et al., 2006; Lugtenberg et al., 2006; Meins et al., 2005; Van Esch et al., 2005; Cox et al., 2003) and triplications encompassing MECP2 lead to a more severe phenotype (del Gaudio et al., 2006).