Z-Gly-Pro-Phe-Leu-CHO (Z-GPFL-CHO) is a tetrapeptide aldehyde serving as a selective and potent proteasome inhibitor, demonstrating inhibition constants (Ki) of 1.5 µM, 2.3 µM, and 40.5 µM for branched-chain amino acid-preferred, small neutral amino acid-preferred, and chymotrypsin-like activities, respectively, and an inhibitory concentration (IC50) of 3.1 µM for peptidyl-glutamyl peptide hydrolyzing activity [1].