Currently, the most efficient gene transfer methodologies harness the capacity of engineered viruses, such as retroviruses, to bypass natural cellular barriers to exogenous nucleic acid uptake. We have chosen to use retroviruses for several reasons. First, their derivation is easy. Second, unlike Adenovirus- mediated gene delivery, expression from retroviruses is long-term, as adenoviruses do not integrate. Adeno-associated viruses have limited space for genes and regulatory units, as well as there being some contro versy as to their ability to integrate (21). Thus, retroviruses offer the best current compromise in terms of long-term expression (10), genomic flexibility, and stable integration, among other features. The main advantage being of course for retroviruses is that their integration into the host genome allows for their stable transmission through cell division. This ensures that in cell types which undergo multiple inde pendent maturation steps, such as hematopoietic cell progression, the retrovirus construct will remain resident and continue to express (10, 11)