This Phase I/II clinical study will evaluate the safety and efficacy of valoctocogene roxaparvovec in patients with severe haemophilia A and inhibitors to FVIII. Part A of the study will involve subjects who have active inhibitors to FVIII, and Part B involving subjects with a prior history of inhibitors.

Inclusion Criteria:

1. Males ≥ 18 years of age with hemophilia A and documented prior residual FVIII activity ≤ 1 IU/dL including, but not limited to, at the time of detected inhibitors, at the time of signing the informed consent.

2. History of a positive inhibitor result with the first positive result at least 12 month prior to Screening.

   Part A: Demonstrated no immunological tolerance to exogenous FVIII. Part B: Demonstrated tolerance to exogenous FVIII and negative FVIII inhibitor screening titer < 0.6 BU.

3. Prophylactic or on-demand hemophilia therapy in the last 12 months. Bleeding, inhibitor & hemophilia therapy Hx over previous 12 months.

4. Sexually active participants must agree to use an acceptable method of effective contraception. Participants must agree to contraception use for at least 12 weeks post-infusion.

5. Willing to abstain from consumption of alcohol for at least the first 52 weeks following BMN 270 infusion.

Exclusion Criteria:

1. Detectable pre-existing antibodies to the AAV5 capsid.
2. Any evidence of active infection or any immunosuppressive disorder; patients with HIV infection and undetectable viral load are not excluded.
3. Currently undergoing, or plan to receive during the study, immune tolerance induction therapy or prophylaxis with FVIII (Part A only).
4. Significant renal dysfunction or liver dysfunction, infection or history of hepatic malignancy.
5. Evidence of any bleeding disorder not related to hemophilia A.

Biological males only