– SERPIN G1 is a member of the serpin family, The C-terminal serpin domain is similar to other serpins, and this part of C1-INH provides the inhibitory activity. SERPIN G1 is involved in the inhibition of the complement system to prevent spontaneous activation. SERPIN G1 may play a potentially crucial role in regulating important physiological pathways including complement activation, blood coagulation, fibrinolysis and the generation of kinins. SERPIN G1 prevents the proteolytic cleavage of later complement components C4 and C2 by C1 and MBL. SERPIN G1 is a very efficient physiological inhibitor of FXIIa, plasma kallikrein and fXIa, and could inhibit chymotrypsin and kallikrein. It forms a proteolytically inactive stoichiometric complex with the C1r or C1s proteases in the C1 complex of classical pathway of complement. Activation of the C1 complex is under control of the C1-inhibitor.