SUV39H1 (UniProtKB/Swiss-Prot entry O43463) is a histone methyltransferase that specifically trimethylates lysine 9 of histone H3, thereby playing an essential role in transcriptional gene silencing and heterochromatin formation as well as many other regulatory processes. It’s also a component of the eNoSC (energy-dependent nucleolar silencing) complex, which mediates silencing of rDNA in response to changes in the intracellular energy status. Loss of function may cause chromosome instability. SUV39H1 is also able to methylate histone H1.