Degradation of FVa and FVIIIa by APC. Both FVa and FVIIIa are cleaved and inhibited by APC in surface-bound reactions also involving cofactors to APC. Protein S and APC interact on the membrane and are sufficient to inhibit FVa, whereas the regulation of the FVIIIa additionally involves FV, which in this situation serves as cofactor to APC. Illustration by Marie Dauenheimer.