Hemophilia A (HA) is an X-linked bleeding disorder caused by mutations in the coagulation factor VIII (FVIII) gene.1 Severe HA (FVIII <1 IU/dL)2 is associated with bleeding in joints and soft tissues, leading to disabling arthropathy.3 Standard of care for severe HA—regular prophylaxis with exogenous FVIII—does not prevent breakthrough bleeding.3,4. The gene discussed is F8; the disease is hemophilia A.