Another approach is to look for rare kinds of adenovirus with less antigenicity, or to which humans are unlikely to have antibodies from previous exposure to common colds. But humans might develop antibodies to the previously-unknown adenoviruses. On the other hand, oncoretro- viruses cannot invade cells that are not reproducing at the moment. They like to enter blasts. The liver doesn't have many at any one time unless the organ is immature or unless it is stimulated, as with a partial hepactectomy or other less drastic means.
Muscle is well-supplied with myoblasts.
A retrovirus cannot carry many copies of a gene so its productivity, once it gets in place, is limited, very limited. It integrates in the genome of NON-dividing cells, a great advantage. Those parts of the virus that make it pathogenic are removed for the most part, but full security from infectivity is still an uncertain goal. It is modified with different promoter regions. Liver cells are easily targeted through portal vein injections, or muscle cells by direct injection.