G9a，also known as Euchromatic histone-lysine N-methyltransferase 2 （EHMT2），is a member of a family of histone lysine methyltransferases. 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. The G9a/GLP complex also contains Wiz，a zinc finger protein?that is required for G9a/GLP?hetero-dimerization and complex stability. Wiz contains two CtBP?co-repressor binding sites，which mediate?the association of the G9a/GLP with the CtBP?co-repressor complex. In addition，G9a and GLP?are components of other large transcriptional?co-repressor complexes，such as those involving E2F6 and CDP/cut. G9a interacts with DNMT1，and both proteins?are required for methylation of DNA and histone H3 （Lys9） at replication foci，providing a functional?link?between histone H3 Lys9 and CpG?methylation during DNA?replication. G9a activity is critical for meiotic prophase progression，as mutant mice deficient in germ?line G9a show a large loss of mature gametes. In addition，G9a facilitates increased global?levels of di-methyl?histone H3 （Lys9） during hypoxic stress and increased G9a expression is associated with hepatocellular carcinoma.
