Histones enhance clot formation in hemophilia. (A) FXa concentrations (pM) following recalcification of normal, FVIII-deficient, and FIX-deficient PPP. (B) Histone H4 (50 μg/mL) enhances thrombin generation in FVIII- and FIX-deficient plasma. (C) Histone H4 (50 μg/mL) (solid lines) induced clot formation in FVIII and FIX-deficient plasma following recalcification. There was no clot formation in the deficient plasmas (dotted lines), and normal plasma clot time (red) is shown for comparison. (D) FVIII-deficient mice were anesthetized and infused with histone H4 (5 mg/kg) retro-orbitally with and without intraperitoneal infusion of nonanticoagulant heparin (500 µg/mice) to neutralize histones. One hour after infusion, bloods were taken for dilute aPTT assays. Clot times (seconds) for each individual mouse (dots), from 5 mice per group, are presented along with mean values for each group (lines). *P < .05 (analysis of variance) compared with the absence of histones; ^#P < .05 compared with histones alone.