Histone lysine methyltransferases. Histone methylation is catalyzed by lysine (K) or arginine (R) methyltransferases that add methyl groups to specific amino acid residues within the histone H3 or H4 tails or globular domains. Several distinct families of lysine methyltransferases (HKMTs) are shown at the top, with the site of methylation in the histones indicated by the arrow. Histone binding proteins are shown below the histones, including chromodomain-containing proteins (indicated in blue) and tudor domain– or MBT domain–containing proteins that recognize the H4K20 methyl mark (53BP1 and L3MBTL1, indicated in orange). Many of the methyltransferases shown have been linked to cancer. The protein arginine methyltransferases (PRMTs) are not shown.