Experimental schema. CD34⁺ progenitors were isolated from human bone marrow by immunomagnetic selection. Cells were transduced by the LN retroviral vector packaged with the GALV pseudotype envelope protein by the PG13 cell line. All transductions were performed in the presence of IL-3 (10 ng/mL), IL-6 (50 U/mL), and SCF (50 ng/mL). Transductions were performed in suspension culture (SNS; supernatant, no stroma), in suspension with addition of 100 ng/mL FLT3 ligand (SNS + FL), or with the support of an irradiated allogeneic bone marrow stromal cell monolayer (supernatant on stroma [SOS]). After transduction, methylcellulose-based CFU assays with or without G418 were performed to assess the extent of gene transfer into colony-forming progenitors. The remainder of each sample was cotransplanted with IL-3–producing primary human marrow stromal cells into a cohort of sublethally irradiated bnx mice. Mice were harvested 7 to 8 months after transplantation, and the extent of human hematopoietic cell engraftment and vector marking in all tissues was assessed.