Serpins are a superfamily of proteins classified into 16 clades (A–P). Their name, serpin, is named for their ability to inhibit serine proteases like chymotrypsin or trypsin. Some are capable of cross-class inhibition of proteases from the subtilisin, papain and caspase families. Some serpins lack protease inhibitory activity and serve other roles, such as hormone transporters, molecular chaperones or catalysts for DNA condensation. Serpin F2, alpha-2 antiplasmin, is a member of serpin superfamily. It circulates at approximately 70 μg/mL, and is primarily synthesized and secreted by the liver although production of the serpin in kidney and brain has been observed. The main physiological function of this serpin is the regulation of fibrin clot formation by inhibiting plasmin, which occurs when Serpin F2 forms a 1:1 stable complex with plasmin. It has unique biochemical characteristic during the fibrin clot formation. Serpin F2 can be incorporated into fibrin clot by crosslinking of glutamine residue at position 14 of the molecule and a lysine residue in fibrin α-chain. As a result, a residual level of Serpin F2 in plasma may be decreased. Clinical studies indicate that an increase in Serpin F2 level is associated with an increase in both venous and arterial thrombotic risk, whereas Serpin F2 deficiency is associated with hemophilia-like bleeding symptoms.