HMB-val-ser-leu-VE is a tripeptide bearing a C-terminal vinyl ester which acts as a potent, selective inhibitor of the trypsin-like activity of the 20S proteasome. It inhibits the tryptic-like, chymotryptic-like, and post acidic activities of the purified 20S proteasome with IC50 values of 0.33 μM, >10 μM, and >10 μM, respectively.{13343} HMB-val-ser-leu-VE is cell permeable and stable (t1/2 ~6 hours) in in vitro assays.