Inclusion Criteria: 1. Male 2. Age ≥18 years 3. Subjects with congenital hemophilia B, classified as severe or moderately severe, and are currently on factor IX prophylaxis 4. \>150 previous exposure days of treatment with factor IX protein Exclusion Criteria: 1. History of factor IX inhibitors 2. Positive factor IX inhibitor test at screening 3. Select screening laboratory value \>2 times upper limit of normal 4. Positive human immunodeficiency virus (HIV) test at screening, not controlled with anti-viral therapy 5. Active infection with hepatitis B or C virus at screening 6. History of Hepatitis B or C exposure, currently controlled by antiviral therapy at the end of the lead-in phase 7. Previous gene therapy treatment 8. Receipt of an experimental agent within 60 days prior to screening 9. Current participation or anticipated participation within one year after study drug administration in this trial in any other interventional clinical trial involving drugs or devices