I thought it was interesting to note that out of 15 SCID (severe combined immune deficiency) patients cured by retroviral gene therapy, 2 developed leukemia. This as a direct integration of the virus into a pro-oncogene, the interesting thing is that both patients had an insert in the same gene! (LMO-2) This poses questions on the safety of gene therapy. Ofcourse, not all virusses integrate themselves into the genome (like adeno, or adeno-associated virusses) but those other ones bring severe immunological risks with them.. I wonder, wouldn't it be possible to target a virus to a specific 'junk region' of the genome? I am not sure how virusses integrate themselves, they probably depend on regions with less dense histone packing..