In hemophilia B patients the body's genes for factor IX are defective so the cells either make factor IX molecules that do not work properly, or they make no factor IX at all. Gene therapy would insert correct factor IX genes into the body by one of two basic methods. An in vivo approach introduces new genes directly into the body under condition which allow the genes to be taken up by various cells. Alternatively, an ex vivo approach introduces the new genes into cells in the laboratory, and those cells are then introduced into the body. There are advantages and disadvantages to both methods.
Viral vectors are one type being studied by a number of researchers.
When a virus infects a cell it inserts its genes into the cell which causes the cell to produce more viruses. By genetic engineering the viral genes can be replaced with, for instance, a factor IX gene. The problem, however, is that the factor IX production often only lasts a few weeks. It appears that the body's immune system see these transformed cells as harmful and eliminates them as though they were infected with normal viruses. Researchers continue to look for better vectors to overcome this problem.