Figure 4: Histone tail modifications include methylation, acetylation, phosphorylation, ADP-ribosylation, and ubiquitination. Of these modifications, methylation and acetylation have the most influence on chromatin structure. Histone acetylases (HATs) catalyze acetylation of the histone tails, and histone deacetylases (HDACs) reverse acetylation. Histone methylation can involve mono-, di-, or trimethylation of arginine and lysine residues of one of the highly conserved histone units.