The di- and tri-methylation of lysine-21 of histone H4 (H4K20) are performed by SUV420H1 (KMT5B), SUV420H2 (KMT5C) and possibly SMYD3 (KMT3E) (Foreman et al. 2011). Trimethylation is associated with heterochromatin formation and gene repression (Schotta et al. 2004). Monomethylation is performed by SETD8 (KMT5A) (Yin et al. 2005).