Serpin G1, also known as C1-inhibitor （C1-INH）, is a crucial regulator of the complement, coagulation, fibrinolytic, and contact systems. The levels of protease/C1-INH complexes in the blood correlate with the in vivo activation of these four cascade-like plasma enzyme systems. C1-INH inhibits the activated forms of C1r and C1s, which are part of the first complement component, thereby regulating complement activation. Synthesized in the liver, deficiencies in C1-INH are associated with hereditary angioneurotic edema （HANE）.