Figure 5: Transcriptionally active euchromatin has high levels of histone acetylation and enriched trimethylation of H3K4, H3K36, or H3K79 residues. Conversely, transcriptionally repressed heterochromatin is enriched in trimethylated H3K9, K3K27, and H4K20 and has reduced histone acetylation, mediated by HDAC activity. Heterochromatinization of euchromatin loci is induced by the binding of heterochromatin protein 1 (HP1) to methylated H3K9 and mediated by corepressor proteins such as retinoblastoma protein (pRb) and KAP1. Demethylation of specific histone residues is mediated by a number of histone demethylase enzymes, including LSD1 and Jumonji C-domain proteins (the latter mentioned above in relation to IDH mutations).