A 77-kDa subcomponent of complement C1, encoded by gene C1S, is a serine PROTEASE existing as a proenzyme (homodimer) in the intact complement C1 complex. Upon the binding of complement c1q to antibodies, the activated complement c1r cleaves C1s into two chains, A (heavy) and B (light, the serine protease), linked by disulfide bonds yielding the active C1s. The activated C1s, in turn, cleaves complement c2 and complement c4 to form C4b2a (CLASSICAL C3 CONVERTASE).