Titer of MLV/HIV-1 pseudotyped vector particles released from producer cells. A: NIH3T3-CD4/X4 cells (1 × 105) were incubated with 1, 0.1 or 0.01 ml supernatant from producer cells in a total volume of 1 ml. For titer determination, the number of GFP+ cells (upper left) was determined. B: Titers were calculated by measuring the percentage of GFP-positive cells after transduction. Values are the average of 5 experiments.