This study is focused on males who have Hemophilia B and who need regular preventive treatment with factor IX protein (FIX) replacement therapy to prevent and also to control their bleeding events. The aim of the study is to gather at least 6 months of information on bleeding events for each individual participant while they continue to use their usual FIX replacement therapy. There is no experimental treatment being tested in this study. The study is informational, and part of a larger program to understand and treat Hemophilia B with a potential experimental new therapy in the future. There is no obligation to agree to taking part in this future study.

The study is looking to answer several other research questions to help understand each participant's individual disease characteristics, including:

• How often to use FIX replacement therapy, both on a regular basis (prophylaxis) and as needed to treat bleeding events
• Measurement of FIX activity (factor IX is a clotting factor) by different laboratories using different types of tests in Hemophilia B participants
• Possible complications from the FIX replacement therapy the patient receives (usual standard of care will continue to be used)
• How quality of life is affected by Hemophilia B
• How joint health is affected by Hemophilia B
• How often the participant visits the emergency room, urgent care center, physician's office, hospital, or has a telemedicine visit as a result of bleeding events
• Whether the body makes antibodies (a protein produced by the body's immune system) against the FIX replacement therapy you receive, which could make the drug less effective or could lead to side effects

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

Male with moderately severe to severe Hemophilia B with medical history of FIX functional activity/ FIX:C (≤2% or <0.02 IU/mL) or a documented genotype known to produce severe hemophilia B

Male patients with Hemophilia B with a history of FIX:C ≤2% or a documented genotype known to produce severe hemophilia B on current FIX Replacement Therapy prophylaxis

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf