>>Using different viral envelope proteins that recognize different receptors can vary the range of cells that can be transduced, but still does not provide much specificity. The difficulty is that, because retroviral vectors cannot be generated at a high titer, it is not possible to get a large number of vector particles to the desired cell type in vivo. The viral particles would bind to many cells they encounter and therefore, would be diluted out before reaching their target.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3722627/
