A cell-permeable inhibitor of calpain I (Ki = 190 nM), calpain II (Ki = 220 nM), cathepsin B (Ki = 150 nM), and cathepsin L (Ki = 0.5 nM). Inhibits neutral cysteine proteases and the proteasome (Ki = 6 µM). Can initiate apoptosis in HL-60 cells but blocks dexamethasone-induced apoptosis in thymocytes and cycloheximide-induced apoptosis in metamyelocytes. Inhibits cyclin B degradation and arrests the cell cycle at G1/S and at meta-/anaphase.