Histone methylation is achieved by HMTs. They can be classified into three groups: SET domain and non-SET domain lysine methyltransferases, and arginine methyltransferases. All of these use SAM as a coenzyme to transfer methyl groups to lysine or arginine residues of substrate proteins. There are three distinct states of lysine methylation (i.e., mono-, di-, and tri-methylated) (Varier and Timmers 2011). Histone methylation can result in transcriptional activation or repression, depending on the position of the lysine that is modified (Berger 2007). For example, methylation of H3K4,1 H3K36, and H3K791 is associated with active transcription, whereas methylation of H3K9, H3K27, and H4K20 generally indicates silenced chromatin. Histone demethylation is achieved by a group of enzymes collectively known as HDMs.
