This observational, post-authorization, long-term follow-up study aims to investigate the short and long-term effectiveness and safety of HEMGENIX in patients with hemophilia B. The study will also include a cohort of patients with hemophilia B treated with FIX prophylaxis to enable interpretation of relevant efficacy and safety findings of HEMGENIX.

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.

Patients with hemophilia B who have been treated with HEMGENIX and are enrolled in either the sponsor's study or the ATHN Transcends study NCT04398628 (Hemophilia Cohort, Gene Therapy Outcomes Arm), and patients who receive routine prophylaxis treatment with FIX replacement therapy that are enrolled in the ATHN Transcends study (Hemophilia Cohort, Natural History Arm), or a similar registry. All patients will provide signed informed consent required for participation.

CSL will consider requests to share Individual Patient Data (IPD) from systematic review groups or bona-fide researchers. For information on the process and requirements for submitting a voluntary data sharing request for IPD, please contact CSL at clinicaltrials@cslbehring.com.

Applicable country specific privacy and other laws and regulations will be considered and may prevent sharing of IPD.

If the request is approved and the researcher has executed an appropriate data sharing agreement, IPD that has been appropriately anonymized will be available.

IPD requests may be submitted to CSL no earlier than 12 months after publication of the results of this study via an article made available on a public website.

Requests may only be made by systematic review groups or bona-fide researchers whose proposed use of the IPD is non-commercial in nature and has been approved by an internal review committee.

An IPD request will not be considered by CSL unless the proposed research question seeks to answer a significant and unknown medical science or patient care question as determined by CSL's internal review committee.

The requesting party must execute an appropriate data sharing agreement before IPD will be made available.