A prior clinical gene transfer study of intrahepatic adeno-associated virus (AAV) vector administration of the factor IX gene (AAV2-hFIX16) to adult subjects with severe hemophilia B enrolled several subjects from 2001 through 2004. These subjects received various doses of AAV2-hFIX16 manufactured by Avigen, Inc. (Alameda, CA). They were monitored up to at least one year following vector administration for evaluation of adverse events and for indices of efficacy. At the time study enrollment was suspended by Avigen, Inc., and IND 9398 under which the study was conducted was transferred to The Children's Hospital of Philadelphia (May 2005), there were no guidelines for long-term follow-up of the trial subjects. However, in an unrelated study conducted in France, of pediatric subjects administered integrating retrovirus vectors for X-linked severe combined immune deficiency (X-SCID), several leukemias developed years after transfer of the γc common chain gene. The underlying mechanism was attributed to proviral integration near, and upregulation of, proto-oncogenes such as LMO-2, coupled with robust expression of the γc common chain, inducing unregulated cellular proliferation. In response to these events, and in concert with the NIH and gene transfer scientific community, the U.S. Food and Drug Administration (US FDA) issued guidelines for the 'long-term follow-up of gene transfer subjects'. Since AAV vectors have a low rate of genomic integration, the risks of integration are low, but the long-term effect of the persistence of vector particles in humans is unknown. The investigators plan to enroll available subjects from the prior intrahepatic AAV gene transfer study to this observational study to follow them for up to 15 years after gene transfer.