AAV Gene Therapy Screening/Observational Protocol (ECLIPSE)

This is a prospective study to collect baseline disease characteristics in patients with HB that are required to establish eligibility for treatment with a novel AAV gene therapy in a subsequent Freeline gene therapy treatment study. Participants providing consent will attend an enrolment visit to complete eligibility evaluations, collect health and demographic information and receive instruction for completing the study diary. Participants will then complete a diary recording bleeding episodes and FIX usage until they enrol into a gene therapy treatment study. A blood sample will be drawn at a convenient timepoint during the study to assess the participant's AAV neutralising antibody (NAb) status. No treatment intervention will occur as part of this study.