Cell-permeable. A potent and selective inhibitor of the 20 S proteasome. Binds irreversibly to the active site N-terminal threonine residue of the catalytic β-subunit of the 20S proteasome, thereby inhibiting its chymotrypsin and trypsin-like activities. Induces neurite outgrowth in Neuro 2A neuroblastoma cells and has been reported to induce apoptosis in human monoblast U937 cells. Also inhibits NF-kB activation (IC₅₀ = 10 µM).