The virus vector doesn't implant in the bone marrow. Hemophilia is caused by deficiency of Factor VIII (Hemophilia A) or Factor IX (Hemophilia B). The factor is produced in the liver and they use a virus that specifically targets the liver to get the gene in there. I agree though that transformational is a superlative and there have been other gene therapies that showed in initial promise for Hemophilia but the effects fade out over a few years.