Methylation been discovered on a spread of lysine residues in numerous histones: K4 (using the one-letter amino-acid code for lysine), K9, K27, K36 and K79 in histone H3, K20 in histone H4, K59 within the globular area of histone H4 2 and K26 of histone H1B three A number of proteins responsible for the methylation of particular residues have been characterized, and all however one of these incorporates a SET domain