Effect of recombinant factor IX mutants at 10% plasma levels on the average time to hemostasis (ATTH) after saphenous vein incision in the hemophilia B mouse. Wild-type or Factor IX gene–ablated C57BL/6 mice were subjected to incision of the distal saphenous vein, and repeat bleeding times were determined with serial clot disruptions for 30 minutes, as described in “Saphenous vein hemostasis and thrombosis models.” The median ATTH (solid lines) for wild-type and Factor IX gene–ablated C57B6 mice was 51 seconds and more than 1800 seconds, respectively. Factor IX gene–ablated C57BL/6 mice were injected with recombinant factor IX wild-type, R170A, or R233A at doses calculated to result in 10% normal plasma levels before saphenous vein incision. Injection of factor IX into the gene-ablated mice shortened the median ATTH for factor IX wild-type and R170A to 111 seconds and 34 seconds, respectively (P ≤ .008, Mann-Whitney test). Median ATTH for factor IX R233A was 1304 seconds (not significant). The y-axis for ATTH represents a log scale.