Histone methylation is the modification of certain amino acids in a histone protein by the addition of one, two, or three methyl groups. Histone methylation has been suggested to be a complex, yet more durable and stable posttranslational modification relative to histone acetylation, and can occur in various states: mono-(me), di-(me2), or trimethylation (me3), with each methylation event having unique effects on gene transcription.