Mutations in the X-linked MECP2 gene, encoding the methyl-CpG binding protein 2 (MeCP2), cause a broad spectrum of neuropsychiatric diseases including Rett syndrome (RTT), an early-onset neurodevelopmental disorder that represents the most common genetic cause of severe intellectual disability in girls worldwide (Chahrour and Zoghbi, 2007). The gene discussed is MECP2; the disease is Intellectual disability.