Histone methyltransferases (HMT) are enzymes, histone-lysine N-methyltransferase and histone-arginine N-methyltransferase, which catalyze the transfer of one to three methyl groups from the cofactorS-Adenosyl methionine to lysine and arginine residues of histoneproteins. These proteins often contain an SET (Su(var)3-9, Enhancer of Zeste, Trithorax) domain, however the recently discovered HMT Dot1 lacks the characteristic SET domain.