FMR1 and fragile X syndrome: Treatment of FXS cells with inhibitors of class I, II, and IV histone deacetylases (HDAC) including 4-phenylbutyrate, sodium butyrate (NaB), trichostatin A (TSA), romidepsin, and vorinostat have been shown to be ineffective at reactivating the silenced FMR1 gene; however, a synergistic effect of 5-aza-dC and some of these inhibitors has been reported [120,121].