C1r (Complement C1r subcomponent) is a protein that in humans is encoded by the C1R gene. C1r B chain is a serine protease that combines with C1q and C1s to form C1, the first component of the classical pathway of the complement system. Along with C1s and C1q complex it forms C1, the first component of the serum complement system. C1 is a calcium-dependent tri-molecular complex of C1q, C1r and C1s in the molar ration of 1:2:2. C1r is a dimer of identical chains, each of which is activated by cleavage into two chains, A and B, connected by disulfide bonds.