C1s (Complement component 1S) is a protein that in humans is encoded by the C1S gene. It is inhibited by SERPING1. C1s B chain is a serine protease that combines with C1q and C1r to form C1, the first component of the classical pathway of the complement system. The protein cleaves C4, which eventually leads to the production of the C4b-C2a form of C3-convertase.