Efficacy and Safety of CSL222 (Etranacogene Dezaparvovec) Gene Therapy in Adults With Hemophilia B With Pretreatment Adeno-associated Virus Serotype 5 (AAV5) Neutralizing Antibodies (Nabs)

Inclusion Criteria: Has congenital hemophilia B with known severe or moderately severe FIX deficiency (≤ 2% of normal circulating FIX) for which the participant is on continuous routine FIX prophylaxis Has 2 consecutive detectable AAV5 NAb titer results between Screening and Visit L-Final using a validated AAV5 NAb assay (based on central laboratory results) Has > 150 previous exposure days to FIX replacement therapy Has been on stable FIX prophylaxis for at least 2 months before Screening Has demonstrated capability to independently, accurately, and in a timely manner complete the eDiary during the Lead-in Period, as judged by the investigator Acceptance to barrier contraception protection for 1 year starting the day of CSL222 treatment Able to provide informed consent after receipt of verbal and written information about the study Investigator believes that the participant (or the participant's legally acceptable representative[s]) understands the nature, scope, and possible consequences of the study and is able to adhere to the study procedures. Exclusion Criteria: History of FIX inhibitors or positive FIX inhibitor test at Screening or Visit L (lead-in period)-Final (based on central laboratory results) Screening and Visit L-Final laboratory values that meet the definition of Severe Hepatic Impairment per Common Terminology Criteria for Adverse Events (CTCAE) (based on central laboratory results) ALT > 2 × the upper limit of normal (ULN) at Screening and Visit L-Final (based on central laboratory results) Any condition other than hemophilia B resulting in an increased bleeding tendency Any uncontrolled or untreated infection (human immunodeficiency virus [HIV], hepatitis C, etc.) or any other significant concurrent, uncontrolled medical condition evaluated by the investigator to interfere with adherence to the clinical study protocol or with the degree of tolerance to CSL222. Thrombocytopenia, defined as a platelet count below 50 × 10^9/L, at Screening and Visit L-Final (based on central laboratory results) Known history of allergy to corticosteroids or known medical condition that would require chronic administration of steroids. Known uncontrolled allergic conditions or allergy / hypersensitivity to any component of the CSL222 excipients Previous gene therapy treatment Receipt of an experimental agent or device within 60 days before Screening until the end of the study. Note: Other protocol pre-specified exclusion criteria may apply.