Of note, alterations in the expression of MeCP2 are known to be responsible of neurodevelopmental diseases in humans: sporadic, loss-of-function mutations in the gene coding for MeCP2 result in Rett syndrome (Amir et al., 1999), while a double dosage of MeCP2 causes a severe developmental delay and ID, with even mild over-expression having a robust effect (Delobel et al., 1998; Van Esch et al., 2005). This evidence concerns the gene MECP2 and atypical Rett syndrome.