Inclusion criteria:  Male and aged 18 to 80 years old  Patients who were enrolled and treated in one of the following clinical trials:  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) Patients with endogenous FVIII:C/FIX:C expression at >1% after gene transfer that is stably maintained for >6 months and associated with normal prothrombin (PT) and thrombin times (TT) as determined in a coagulation assay  Exclusion Criteria:  Patients with a platelet count measured at <140 x109/L 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. Patients with abnormal kidney function (estimated GFR <50ml/min) Patients with a known allergy to iodine-based intravenous contrast agents Patients with a known allergy to local or general anaesthetic Patients with a known reaction to FVIII/FIX concentrate infusions Presence of FVIII or FIX inhibitor (done within 14 weeks of biopsy) Evidence of any bleeding disorder not related to haemophilia A or B Patients unable and unwilling to provide and sign an informed consent.