Cell-delivered gene therapy is making an impact on a range of diseases (1–17). To date, successful treatments have generally been in conditions involving genetic deficiencies/abnormalities, for which introduction of a normal gene allele has been corrective (1–12, 18). Such an approach requires a vector containing the normal allele to overcome the mutant or lacking gene. The vector of choice for cell-delivered gene therapy is often a lentivirus that integrates and expresses introduced therapeutic genes in host target cells and their…