An irreversible inhibitor of the chymotrypsin-like β5 subunit of the constitutive 20S proteasome (IC50 = 5.2 nM) and the β5i subunit of the immunoproteasome 20Si (LMP7; IC50 = 14 nM) with minimal cross reactivity to other proteases; induces cell cycle arrest and apoptosis in human cancer cell lines including multiple myeloma, lymphoma, and various solid tumors (IC50s = 2.4-20 nM).