Fibrinopeptides A and B (FPA and FPB) are short amino acid sequences located at the N-terminus of fibrinogen A and B, respectively. Fibrinopeptide B is an N-terminal modified peptide with pyroglutamylation, a common post-translational process. In one of the final steps in the coagulation pathway, thrombin converts soluble fibrinogen to fibrin by first cleaving off FPA to form fibrin I, which spontaneously polymerizes. Thrombin then releases fibrinopeptide B (FPB) from the beta-chains of the fibrin I subunits to form fibrin II polymers, which associate and cross-link to form a semi-solid network, while the fibrinopeptides remain soluble in plasma. Fibrin formation associated with active thrombosis leads to significantly higher plasma levels of FPB and thus measurement of plasma FPB levels is a more sensitive and specific serologic marker for acute thrombosis.