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). This evidence concerns the gene MECP2 and atypical Rett syndrome.