Potent inhibitor of cytosolic (class 1) aldehyde dehydrogenase (ALDH) enzymes (IC50 values 0.057 µM, 1.2 µM, 3.0 µM, 1.2 µM, 0.16 µM, and 13 µM for inhibition of ALDH1A1, ALDH1A2, ALDH1A3, ALDH1B1, ALDH2, and ALDH5A1, respectively). DEAB was also found to be an excellent substrate for ALDH3A1, and an irreversible inhibitor of ALDH7A1 (KI value 100 µM). Low turn-over rates and/or covalent bonding of the ALDH substrate DEAB are the cause of its inhibitory effect on the enzymes. At the time of development DEAB was found to be a potent inhibitor of cytosolic ALDH1 but not mitochondrial ALDH2. Commonly used as “selective” inhibitor of ALDH isoenzymes in cancer stem cell biology.