Protection of murine progenitors from the combined toxicity of TMTX plus NBMPR using a DHFR retroviral vector. Murine BM cells were first transduced with either a retroviral vector expressing the L22Y-DHFR gene (L22Y) or, as a control, the human multidrug resistance 1 gene (MDR). Cells were subsequently cultured in medium containing dialyzed serum, 150 nmol/L TMTX, 100 μmol/L hypoxanthine (hypo), and 1 μmol/L thymidine (Thd), in the presence or absence of 1 μmol/L NBMPR. Equal volumes of cells were washed and plated in methylcellulose after 4 days of suspension culture to determine the myeloid progenitor content. Progenitor survival is expressed as a percentage of progenitors per volume relative to TMTX-free conditions. Each bar represents the mean ±1 SD from three independent experiments.