The significant reduction in MeCP2 expression in 79% of autism, 100% of Angelman syndrome, 75% of Prader-Willi syndrome, and 60% of Down syndrome cases [57] compared to age-matched controls suggests that altered MeCP2 expression contributes to abnormal postnatal brain development and to an abnormal course of neuron maturation in neurodevelopmental disorders [68]. This evidence concerns the gene MECP2 and Down syndrome.