A potent anti-tumor agent used as a selective and irreversible inhibitor of the 20S proteasome; inhibits proteasome activity in cell growth assays (IC50 = 4 nM) and demonstrates potent cytotoxicity against B16-F10, HCT116, and Moser solid tumor cells, as well as P388 and K562 leukemia cells (IC50s = 2-44 nM)