Histone methylation is an epigenetic modification resulting in a change in gene expression. It is a post-translational modification where between one and three methyl groups are added to lysine or arginine residues in a histone tail by the enzyme histone methyltransferase. Note that the number of methyl groups added is significant and may have a different effect on gene expression. Whereas other post-translational modifications such as histone acetylation have a set effect on gene expression, histone methylation can either repress or activate gene expression. This is because methylation does not immediately result in a change in gene expression, but recruits proteins which either stimulate or inhibit transcription, by creating binding sites.