Among the pathogenic X-linked duplications, the most prominent and well-studied is that of the Methyl-CpG-binding Protein 2 gene (MECP2; OMIM: 300005), located at Xq28, causing severe X-linked intellectual disability (XLID) [10], and its loss of function causes Rett syndrome (OMIM: 613454) [11, 12]. The gene discussed is MECP2; the disease is Rett syndrome.