A Study to Investigate the Safety and Effectiveness of a Coagulation Factor IX Gene Insertion Therapy (REGV131-LNP1265) in Pediatric, Adolescent and Adult Participants With Hemophilia B

Key Inclusion Criteria:

1. Confirmed diagnosis of severe or moderately severe hemophilia B with medical historyof FIX functional activity (≤2% or <0.02 IU/mL) or documented genotype known toproduce severe hemophilia B

2. Currently taking FIX prophylaxis and previous experience with FIX therapy, asdefined in the protocol

3. Participation in the lead-in period of this interventional study OR a separatelead-in study (R0000-HEMB-2187 [NCT05568459]) for at least 6 months for ABR datawhile taking FIX prophylaxis, as defined in the protocol

Key Exclusion Criteria:

1. History of FIX inhibitor (clinical or laboratory-based assessment) on 2 or moreoccasions

2. Bethesda inhibitor titer greater than the upper limit of normal (ULN) at screening

3. Detectable pre-existing antibodies to the adeno-associated virus serotype 8 (AAV8)capsid; as measured by enzyme-linked immunosorbent assay (ELISA) at prescreening (orfinal lead-in visit, if applicable).

4. Any significant underlying liver disease such as: cholestatic liver disease, livercirrhosis, portal hypertension, splenomegaly, hepatic encephalopathy

5. Evidence of advanced liver fibrosis, as defined in the protocol

6. Evidence of cirrhosis and/or portal hypertension as assessed by abdominal ultrasoundat screening or measured within 6 months prior to the screening visit

7. History of arterial or venous thrombo-embolic events, as defined in the protocol

8. History of hypersensitivity to corticosteroids or known medical condition thatrequires chronic administration of corticosteroids

9. Previously received any AAV gene-based therapy or intends to receive approved orinvestigational AAV-based gene therapy other than REGV131-LNP1265 during the studyperiod

NOTE: Other Inclusion/Exclusion Protocol Defined Criteria Apply