F.IX clotting activity and antigen determination in C57Bl/6 hemophilia B mice following liver delivery by tail-vein injection of AAV-F.IX or R338A vectors. (A) F.IX-specific activity was calculated by dividing the clotting activity by the antigen levels and expressed as unit per milligram. Average of specific activity of liver-synthesized F.IX at expression levels ranging from 100 to 4000 ng/mL following injection of AAV-F.IX-R338A (▪; n = 9) compared with F.IX-WT (□; n = 13). Bars represent mean values ± SD. (B) Graphic data plot showing individual test samples for F.IX-specific activity determination in animals receiving F.IX-WT (□) or F.IX-R338A (). P value was calculated by Student t test. Trendlines were derived from linear regression analysis.