FMR1 and fragile X syndrome: This is consistent with the finding that in FXS repressive chromatin marks such as trimethylation of histone H3 at lysine 9 (H3K9me3) and trimethylation of histone H4 at lysine 20 (H4K20me3) associate with exon 1 of FMR1, which contains the CGG repeats, but do not associate with the promoter region (Kumari and Usdin 2010).