APC resistance assay. Varying concentrations of APC were added either to pseutarin C (○; 8 nM) or bovine FXa-FVa (•; FXa 42 nM, FVa 2 nM) complex diluted in 50 mM Tris-HCl buffer (pH 7.5) containing 100 mM NaCl, 5 mM CaCl₂, and 0.5 mg/mL BSA. The reaction mixture was incubated for 30 minutes at room temperature. Prothrombin was added to a final concentration of 2.8 μM and thrombin formed was assayed using thrombin-specific chromogenic substrate S-2238 as described in “Materials and methods.” Each point represents an average of 2 independent experiments each carried out in triplicates. Error bars indicate SEM.