Histones reduce the demand for FXa to initiate clot formation. (A) Thrombin generation from FXa activation of prothrombin in the presence of different concentrations of FXa ± H4 (50 µg/mL). (B) Typical thrombin generation curve in recalcified FX-deficient PPP ± FXa (500 pmol/L) ± histones (50 µg/mL). (C) Clot formation of recalcified (12.5 mmol/L) FX-deficient plasma ± H4 (50 µg/mL) supplemented with 500, 1000, or 5000 pM FXa.