Chaetocin, an antitumor antibiotic, is a thiodioxopiperazine natural product produced by Chaetomium species. Specific inhibitor of the lysine-specific methyltransferase SU (VAR)3-9 both in vitro and in vivo. It displays potent antimyeloma activity in IL-6-dependent myeloma cell lines. Its antimyeloma activity appears to be due to induction of oxidative stress and consequent apoptosis.