BMS-538305 hydrochloride is a potent and selective inhibitor of dipeptidyl peptidase IV (DPP-IV), exhibiting strong biological activity by forming a reversible covalent complex with the enzyme. BMS-538305 hydrochloride demonstrates its mechanism of action through the covalent attachment between the nitrile carbon of the inhibitor and the enzyme residue S630, as unveiled by the X-ray crystal structure analysis.