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: * Considered legally an adult, as defined by country regulations. * Has congenital hemophilia B with known severe or moderately severe FIX deficiency (less than or equal to \[\<=\] 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 greater than (\>) 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 adhere to contraception guidelines. * 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 Prescreening, Screening or Visit L-Final (based on central laboratory results). * Screening or Visit L-Final laboratory values (based on central laboratory results) of total bilirubin \> 2 × the upper limit of normal (ULN) (except if caused by Gilbert's syndrom…