FMR1 and fragile X syndrome: In addition to DNA methylation and hypoacetylated histones, the silenced FMR1 gene in FXS patient cells is associated with the marks of facultative heterochromatin, histone H3 di-methylated at lysine 9 (H3K9me2) and histone H3 tri-methylated at lysine 27 (H3K27me3), as well as the marks of constitutive heterochromatin, histone H4 tri-methylated at lysine 20 (H4K20me3) and histone H3 tri-methylated at lysine 9 (H3K9me3) [6,7,8].