MECP2 and Neurodevelopmental delay: Accordingly, MECP2 mutations have been demonstrated to cause various clinical conditions (HGMD), such as Rett syndrome (678 different mutations), MECP2 duplication syndrome (85), X-linked ID (52), neurodevelopmental delay (35), brain abnormalities (26), atypical Rett syndrome (22), and psychomotor developmental delay (14).