Complement regulator C4b-binding protein (C4BP) and the anticoagulant vitamin K-dependent protein S form a high affinity complexin human plasma. C4BP is composed of sevena-chains and a uniqueb-chain, each chain comprising repeating complement control protein (CCP) modules. The binding site for protein S mainly involves the first of the threeb-chain CCPs (CCP1). However, recently it has been suggested that CCP2 of theb-chain also contributes to the binding of proteinS.