Post-translational modifications of proteins are essential for the regulation and function of several known biological processes。 There are many ways that proteins can be post-translationally modified and a familiar post-transcriptional modification of Lysine includes methylation。 Lysine can be methylated once（me1)， twice（me2) or three times（me3) by lysine methyltransferases。 The transfer of methyl groups from S-adenosyl methionine to histones is catalyzed by enzymes known as histone methyltransferases。 Histones which are methylated on certain residues can act epigenetically to repress or activate gene expression。
