The histone methyltransferase KMT1A / SUV39H1 is the mammalian homologue of the Drosophila PEV modifier Su(var)3-9 protein. SUV39H1 contains the SET and chromo domains. In eukaryotic cells, SUV39H1 and the methyl-lysine binding protein HP1 interact to repress transcription at heterochromatic sites. SUV39H1 methylates histone H3 on lysine 9, which generates a binding site for HP1 proteins that plays a role in heterochromatin formation and gene silencing.