Canonical examples of such broad regulators are the RNA-binding protein FMR1, which underlies fragile X syndrome, and the chromatin modulator MeCP2, whose loss or gain respectively causes Rett syndrome or MECP2-duplication syndrome (Amir et al., 1999; Pieretti et al., 1991; Verkerk et al., 1991). This evidence concerns the gene MECP2 and atypical Rett syndrome.