The biological importance of the methylation and demethylation of lysine and arginine side-chains is of increasing interest from both basic science and pharmaceutical perspectives. Along with other post-translational modifications, including acetylation, phosphorylation and ubiquitination, the dynamic methylation of the tails of the histone H3 and H4 proteins plays central roles in the regulation of transcription. PcG are histone methyltransferase (HTMase) proteins discovered in Drosophila melanogaster as the product of genes that are required to prevent inappropriate expression of homeotic (Hox) genes.  Histone methylation can occur on lysine or arginine aminoacidic residues and is carried out by two enzymes: lysine methyltransferases (HKMTs or PKMTs) and arginine methyltransferases (HRMTs or PRMTs), respectively. HKMT3 family methylates H3-K36 and H4-K20 through SET2 and NSD1 activities, respectively. HKMT6 comprises EZH1 and EZH2 enzymes that act on H3-K27. 
They functions as the catalytic subunit of polycomb repressive complex 2 (PRC2). HKMTs constitute an important, novel drug target class for the development of small-molecule drug therapies against a number of serious human diseases. About nine human arginine methyltransferases (HRMT 1-9) have been characterized and are classified in four different typology, depending on the type of methylation they accomplish. HRMT type I (HRMT1, HRMT2, HRMT3, HRMT4/CARM1, HRMT6 and HRMT8) catalyze an asymmetric reaction transferring two methyl groups from SAM to one guanidine nitrogen giving the asymmetric N,N -dimethylarginine.

References

1.Zagni C, et al. Curr Med Chem. 2013;20(2):167–185.