Assessment of transduction efficiency. Nonadherent cells were harvested after 4 days of coculture on irradiated retroviral-producing fibroblasts, incubated with biotinylated G-CSF and a cocktail of lineage-restricted antisera, and analyzed by flow cytometry. The binding of biotinylated G-CSF by lineage⁺cells (upper-right quadrant) was seen even in the presence of a 100-fold molar excess of nonlabeled G-CSF (data not shown) and therefore represents nonspecific binding. Transduction efficiency was assessed by determining the percentage of lineage⁻ cells that specifically bound G-CSF (upper left quadrant). No specific G-CSF binding was detected in cells transduced with the empty retroviral vector (neo). Nonviable cells were excluded from the analyses. Shown are representative results of one of five experiments.