Most of the mutations in MECP2 are hypomorphic, thus leading to partial or complete “loss of function” of the protein; however hypermorphic mutations, associated with duplication of portions of Xq28 spanning the MECP2 locus, have been related to a neurodevelopmental disorder called MeCP2 duplication syndrome (MDS) (van Esch et al., 2005). Here, MECP2 is linked to X-linked syndromic intellectual disability.