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. Selective cleavage of Lys(or Arg)-|-Ile bond in complement subcomponent C1s to form the active form of C1s (EC 3.4.21.42). C1 is a calcium-dependent trimolecular 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. The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains. Complement component C1r deficiency [MIM:216950] leads to the failure of the classical complement system activation pathway (C1 deficiency). Individuals with C1 deficiency are highly susceptible to infections by microorganisms and have greater risk in developing autoimmune diseases such as systemic lupus erythematosus (SLE). The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase S1 family.