F10 and systemic lupus erythematosus: To assess whether APS and SLE/APS- IgG may be binding to different epitopes on the active site of FXa, we selected different APS (n = 5) and SLE/APS- (n = 5) IgG on the basis of similar FXa binding and examined their individual and combined effects (at a final concentration of 200 μg/ml) upon FXa enzymatic activity.