A well-known XLID microduplication region is Xq28, in which there is duplication of X-linked genes such as methyl-CpG-binding protein 2 (MECP2; MIM *300005) a key gene involved in Rett syndrome, a neurodevelopmental disorder that affects mostly girls Xq28 duplication syndrome is an important aspect of XLID. This evidence concerns the gene MECP2 and neurodevelopmental disorder.