VR23 is a potent and selective inhibitor of trypsin-like proteasomes (IC50 = 1 nmol/L), chymotrypsin-like proteasomes (IC50 = 50-100 nmol/L), and caspase-like proteasomes (IC50 = 3 μmol/L). The primary molecular target of VR23 was β2 of the 20S proteasome catalytic subunit. Notably, VR23 was structurally distinct from other known proteasome inhibitors and selectively killed cancer cells by apoptosis, with little effect on noncancerous cells. VR23 was effective in vivo in controlling multiple myelomas and metastatic breast cancer cells, in the latter case also enhancing the antitumor activity of paclitaxel while reducing its side effects.