APC-independent anticoagulant activity of protein S in plasma from healthy controls, protein S–deficient patients, and carriers of the prothrombin G20210A mutation. The APC-independent anticoagulant activity of protein S (pSR) was determined with the factor Xa–based assay in plasma from 60 healthy controls, 16 protein S–deficient patients, and 46 prothrombin G20210A carriers as described in “Patients, materials and methods.” Factor V Leiden carriers are indicated by open circles. The mean value, represented as a horizontal bar, was 1.62 for the control population, 1.31 for the protein S–deficient patients, and 1.47 for the prothrombin G20210A carriers.