What are the problems with gene therapy?

1)the alleles may not go into every target cell, which are cells that need the new non-faulty cell 2)the alleles may be inserted into the chromosomes in random places, rather than in the required position, so they do not work properly 3)some treated cells may be replaced naturally by the patient's own untreated cells, as cells are frequently replaced through the process of mitosis during growth and repair