Firstly, the inactivation of the FMR1 gene is associated not only with DNA demethylation but also with increased levels of deacetylation of histones H3 and H4 (Pietrobono et al., 2005; Li et al., 2018), suggesting that other epigenetic modifications, in addition to DNA methylation, play a critical role in the pathogenesis of FXS. Here, FMR1 is linked to fragile X syndrome.