FMR1 and fragile X syndrome: Two examples of diseases that are accompanied by extensive changes in chromatin marks are fragile X syndrome (FXS), caused by the expansion of CGG repeats in the FMR1 gene located on the X chromosome (8–10), and Friedreich’s ataxia (FRDA), caused by a homozygous GAA repeat expansion in the first intron of the FXN gene (11).