Gene therapy can potentially fulfill this need, because (1) hemophilia is an inherited monogenic disease, (2) its symptoms can be improved with a 1 % increase of in vivo coagulation activity, and (3) the patient’s phenotype can be improved through a stable supply of therapeutic proteins, such as FVIII and FIX, instead of having cells producing the coagulation factors. The gene discussed is F9; the disease is hemophilia.