A Trial That Evaluates Disease Characteristics in Hemophilia B Adult Male Participants Receiving Prophylaxis With Standard of Care Factor IX Protein (FIX) Replacement Therapy

Key Inclusion Criteria: 1. Previous experience with FIX therapy (≥50 documented exposure days to a FIX protein product such as recombinant, plasma-derived or extended half-life FIX product) with a current stable prophylaxis regimen for \>2 months prior to enrollment and intention to use FIX replacement therapy for the duration of the study 2. No known hypersensitivity to FIX replacement product 3. Willing to be contacted about a potential future clustered regularly interspaced short palindromic repeats (CRISPR)-based Factor 9 (F9) gene insertion clinical trial in which they may have the opportunity to screen for enrollment Key Exclusion Criteria: 1. History of any coagulation disorder; requires anticoagulant therapy 2. Lack of adherence with documentation of bleeds and/or prophylaxis replacement therapy administration in the opinion of the investigator, based on medical history 3. History of FIX inhibitor (clinical or laboratory-based assessment) on 2 or more occasions, as defined in the protocol 4. Bethesda inhibitor titer greater than the upper limit of normal (ULN) at screening 5. Any detectable pre-existing antibodies to the Adeno-associated virus serotype 8 (AAV8) capsid; as measured by an assay at prescreening, as defined in the protocol 6. Is positive for hepatitis B or C at screening, as defined in protocol 7. If any of the following pre-existing diagnoses are documented: * Cholestatic liver disease * Liver cirrhosis * Portal hypertension; or * Splenomegaly; or * Hepatic encephalopathy 8. History of arterial or venous thrombo-embolic events, as defined in the protocol 9. History of clinically significant cardiovascular, respiratory, hepatic, renal (including nephrotic syndrome), gastrointestinal (including protein-losing enteropathy), endocrine, hematological (including thrombophilia), psychiatric, or neurological disease, as assessed by the investigator that may confound the results of the study or poses an additional risk to the participant by study participation 10. Previously received of any AAV-gene based therapy with a marketed gene therapy or in a clinical trial or intent to receive approved or investigational AAV-gene based therapy during the study period NOTE: Other Inclusion/Exclusion Protocol Defined Criteria Apply