MECP2 and Intellectual disability: These include Pelizaeus–Merzbacher Disease (PMD), caused by duplications of the proteolipid 1 protein (PLP1; MIM #300401) and developmental delay with intellectual disability caused by duplications of the methyl-CpG-binding protein 2 gene (MECP2; MIM #300005) [Lee and Lupski, 2006; Stankiewicz and Lupski, 2010].