Liver Biopsy In Haemophilia Gene Therapy

Inclusion criteria: 1. Male and aged 18 to 80 years old 2. Patients who were enrolled and treated in one of the following clinical trials at Royal Free Hospital: * AGT4HB (EudraCT No 2005-005711-17) - FIX AAV gene therapy trial (Sponsor: St Jude Children's Research Hospital) * GO-8 (EudraCT No 2016-000925-38) - FVIII AAV gene therapy trial (Sponsor: UCL) * FLT180a-01 (EudraCT: 2017-000852-24) - FIX AAV gene therapy trial ((Sponsor: UCL) \[now enrolled in long term follow up study FLT180a-04 (EudraCT No 2017-005080-40) (Sponsor: Freeline Therapeutics Ltd) 3. Patients with endogenous FVIII:C/FIX:C expression at \>1% any time after gene transfer, associated with normal prothrombin (PT) and thrombin times (TT) as determined in a coagulation assay. Exclusion Criteria: 1. Patients with a platelet count measured at \<140 x109/L 2. Any condition that, in the opinion of the investigator or Sponsor would prevent the patient from fully complying with the requirements of the study and/or would influence or interfere with evaluation and interpretation of subject safety or efficacy result. 3. Patients with abnormal kidney function (estimated GFR \<50ml/min) 4. Patients with a known allergy to iodine-based intravenous contrast agents 5. Patients with a known allergy to local or general anaesthetic 6. Patients with a known reaction to FVIII/FIX concentrate infusions 7. Presence of FVIII or FIX inhibitor (done within 14 weeks of biopsy) 8. Evidence of any bleeding disorder not related to haemophilia A or B 9. Patients unable and unwilling to provide and sign an informed consent.