Expansion kinetics for total cells and drug-resistant progenitors after retroviral transduction. Cells were maintained in liquid suspension cultures with addition of IL-3, IL-6, and SCF. A typical cell expansion is shown for cells from either MDR1 (•) or DHFR (○) cocultures (left). No significant difference in cell expansion was noted between these two groups. Cells were removed on day 0, 6, and 12 for clonogenic progenitor assay in methylcellulose. Selective concentrations of Taxol or trimetrexate were used to determine MDR1 or DHFR expressing drug-resistant progenitor cells, respectively. The progenitor population was found to expand at a similar rate as the total cells.