Complement component 1S (EC 3.4.21.42, C1 esterase, activated complement C1s, complement C overbar 1r ) is a protein involved in the complement system.
C1s cleaves C4, which eventually leads to the production of the C4b-C2a form of C3-convertase.
Complement C1s subcomponent is a protein that in humans is encoded by the C1S gene.
C1S has been shown to interact with C1R.