Complement C1r subcomponent (EC 3.4.21.41, activated complement C1r, C overbar 1r esterase, C1r) is a protein involved in the complement system of the innate immune system. In humans, C1r is encoded by the C1R gene. C1r along with C1q and C1s form the C1 complex, which is the first component of the serum complement system. C1r is an enzyme that activates C1s to its active form, by proteolytic cleavage.