Bortezomib (PS-341) is a reversible and selective proteasome inhibitor, and potently inhibits 20S proteasome (Ki=0.6 nM) by targeting a threonine residue. Bortezomib disrupts the cell cycle, induces apoptosis, and inhibits NF-κB. Bortezomib is the first proteasome inhibitor anticancer agent. Anti-cancer activity.