Phe-Pro-Arg-CMK (FPRCK). Extremely potent and selective irreversible inhibitor of thrombin (Kobs/[I] = 107M-1S-1). Reacts with thrombin in a 1:1 stoichiometry. Can also be used to inhibit tissue plasminogen activator, urokinase, Factor VIIa, and Factor XIa.