Protein methylation is a post-translational modification (PTM) process, in which highly specific enzymes called methyltransferases are responsible for the addition of methyl groups to a targeted molecule and S-adenosyl methionine (SAM) as the primary donor of methyl group. Protein methylation commonly occurs on arginine, lysine, histidine, proline, and carboxyl groups. Protein methylation plays an important role in modulating cellular and biological processes, including transcriptional regulation, RNA processing, metabolism and signal transduction.