 Recent preliminary clinical data of a hemophilia B gene transfer study which is also being conducted by Spark Therapeutics shows all study participants achieving therapeutic factor IX activity levels average of maintaining factor IX activity levels around 30 of normal with no confirmed bleeds, after receiving Spark gene transfer, with the approach of using the novel bio-engineered recombinant adeno-associated viral rAAV vector carrying a high specific activity of a factor IX gene. The approach being tested in this clinical research study uses a further modified novel AAV vector with a stronger attraction to the human liver to deliver the human factor VIII hFVIII gene into liver cells so that they can produce factor VIII protein. 