FIG. 5.

The number of gene transfer events is proportional to the concentration of retrovirus and the number of target cells. NIH 3T3 cells were plated at various densities (1.0 × 10⁴, 2.0 × 10⁴, 4.0 × 10⁴, and 8.0 × 10⁴ cells/well; six-well plate) and were transduced the next day (3.5 h) with 1.0 ml of lacZ retrovirus of different dilutions (1:250, 1:500, 1:1,000, and 1:2,000) in the presence of Polybrene. At the start of transduction, the number of target cells was counted in parallel wells. Two days later, the number of CFU was measured by X-Gal staining. The number of CFU is plotted as a function of virus dilution (A). CFU increases as virus concentration increases. The number of CFU is plotted as a function of cell number at the start of transduction (B). The number of CFU increases as cell number increases.