Several subjects enrolled in a multi-site, gene transfer clinical study to evaluate the intrahepatic administration of AAV2-hFIX16 vector for the treatment of severe hemophilia B between 2001 and 2009. As the US FDA has established guidelines for the long-term follow-up (LTFU) of subjects receiving investigational gene therapy products, this protocol seeks to characterize the clinical outcome and the type and seriousness of adverse events following the AAV gene transfer. The primary study tools will consist of annual history/physical examination and blood tests, as well as periodic liver ultrasound, to characterize clinical outcomes. Where possible, data will be obtained for up to 15 years following hepatic AAV2-hFIX16 gene transfer.