Methylation been discovered on a variety of lysine residues in various histones: K4 (utilizing the only-letter amino-acid code for lysine), K9, K27, K36 and K79 in histone H3, K20 in histone H4, K59 within the globular domain of histone H4 2 and K26 of histone H1B 3 A number of proteins accountable for the methylation of specific residues have been characterized, and all however one of these comprises a SET domain