The purpose of this study is to determine the safety of giving a normal factor IX gene to treat individuals who have an abnormal or no factor IX gene. Recruitment will be limited to adults (≥ 18 years) with a confirmed diagnosis of hemophilia B (HB), resulting from a missense mutation in the coagulation factor IX (FIX) gene or a nonsense mutation that has not been associated with an inhibitor. Only subjects who have no evidence of active hepatitis or anti-hFIX antibodies, and who have been treated/exposed to Factor IX concentrates for at least ten years and have had an average of 3 bleeding episodes per year requiring FIX administration will be enrolled. Patients will be recruited within the United States for treatment at St. Jude Children's Research Hospital, and patients will be recruited in England and other countries for treatment in London by our British collaborators.