What is histone methyltransferase?

Histone methyltransferase is a class of histone-modifying enzymes that catalyzes the transfer of methyl groups to histone proteins. The attachment of methyl groups occurs predominantly at specific lysine or arginine residues on histones H3 and H4. Histone modifications play a crucial role in regulating gene expression. Methylated histones can either activate or repress transcription depending on the site of methylation. 

Two examples of histone methyltransferases are histone-lysine N-methyltransferase and histone-arginine N-methyltransferase.