G9a，also known as Euchromatic histone-lysine N-methyltransferase 2 (EHMT2)，is a member of a family of histone lysine methyltransferases，each of which contains a conserved catalytic SET domain originally identified in Drosophila Su[var]3-9，Enhancer of zeste，and Trithorax proteins. Recombinant G9a can mono-，di- and tri-methylate histone H3 on Lys9 and Lys27 in vitro. However，in vivo G9a forms a complex with GLP，a G9a-related histone methyltransferase，and together these proteins function as the major euchromatic histone H3 Lys9 mono- and di-methyltransferases，creating transcriptionally repressive marks that facilitate gene silencing. G9a methylates itself on Lys165，a modification that regulates the association of HP1 repressor proteins with the G9a/GLP complex.
