Salinosporamide A is a natural proteasome inhibitor. It is a β-lactone-γ-lactam that irreversibly inhibits all three protease activities within the proteasome (IC50s = 3.5, 28, and 430 nM for β5 (chymotrypsin), β2 (macropain), and β1 (C5) subunits, respectively), both in vitro and in vivo. 2,3Salinosporamide A has potential applications in cancer therapy, particularly in combination with other chemotherapeutics.