An Observational Cohort Study to Characterize the Effectiveness and Safety of HEMGENIX® in Patients With Hemophilia B

Inclusion Criteria: * HEMGENIX Cohort: * \- Treatment with commercial HEMGENIX. * \- Have provided signed written informed consent within 3 months before or within 6 months after HEMGENIX treatment, or within 6 months of when the study is initiated at the participating site. * FIX Prophylaxis Cohort: * \- Adult patients (≥ 18 years) with hemophilia B who have consented and enrolled in ATHN Transcends Hemophilia Cohort (or a similar registry) and are receiving FIX prophylaxis therapy. Exclusion Criteria: * HEMGENIX Cohort: * \- The patient population that will be observed in this study must not have been treated with etranacogene dezaparvovec in a clinical trial.