Histone methylation involves the addition of methyl groups to lysine or arginine residues on histones, catalyzed by HMTs. Unlike acetylation, methylation can either activate or repress transcription depending on the specific site and type of methylation. For example, trimethylation of histone H3 at lysine 4 (H3K4me3) is associated with active transcription, while trimethylation at lysine 27 (H3K27me3) is linked to gene repression. Enzymes known as histone demethylases can remove these methyl groups to reverse the modification.
