Cotransduction and selective expansion of K562 cells with varying MOIs. K562 cells were cotransduced with a low MGMT : GFP virus ratio or equal amounts of each virus at a higher total MOI. Single- and dual-positive AGT and GFP expression levels were evaluated prior to (a) and after selection with 10 μM BG and 15 μM BCNU (b). Total AGT and total GFP values are the sum of the corresponding single and dual-expressing populations. Fold enrichment of dual-positive populations after drug selection is indicated for each transduction. (c) Cotransduced K562 cultures from panel 3A were undiluted or diluted into a 50-fold excess of untransduced K562 cells and treated twice with 10 μM BG and 15 μM BCNU. The fraction of dual-positive cells in the total AGT-expressing population remains constant after stringent selection conditions. Error bars indicate ±SEM, .