An example: The Lysine 9 (K9) residue of the mammalian Histone 3 protein's tail is trimethylated (all H's on Nitrogen are replaced with CH3) by an epigenetic enzyme called Setdb1, a histone methyltransferase. Trimethylation of H3K9 results in profound gene silencing (heterochromatin) because the histones tightly wrap DNA and join closer together to close any gaps and exposed DNA that could be transcribed by RNA polymerase.