Histone methyltransferases are a group of enzymes that catalyze the methylation of histone lysine and arginine by adding methyl groups to specific histone arginine or lysine residues. Histone methyltransferaes can be classified into 3 classes, including SET domain lysine methyltransferases, non-SET domain lysine methyltransferases and arginine methyltransferases (PRMTs), all of which use S-adenosylmethionine as a cosubstrate for the transfer of the methyl group. Aberrant histone methylation has been associated with a wide range of human cancers (such as hematological malignancies), which leads to the development of novel cancer chemotherapies targeting cancer-associated histone methyltransferases (more than 20 lysine methyltransferases and 9 arginine methyltransferases in humans).