FACS analysis of NGFR expression in retrovirally transduced TF1 cells. (A) Histograms of TF1 cells expressing the NGFR transgene after transduction with the LINGFR, MINGFR, or MINT vectors. TF1 cells were transduced with equal volumes of amphotropic viral supernatants for 4 hours (as described in Materials and Methods) and NGFR expression was analyzed by FACS 4 days later after staining for cell-surface NGFR with a FITC-conjugated anti-NGFR antibody. The profile of nontransduced TF1 cells (thin lines representing 0.5%) was overlaid to highlight transduced cell NGFR⁺ populations. Approximately 62%, 59%, and 40% of TF1 cells were transduced with the LINGFR, MINGFR, and MINT vectors, respectively. (B) Kinetics of NGFR expression in TF1 cells after transduction with the MINT vector. After 4-hour exposure to MINT vector supernatant, TF1 cells were either processed immediately (day 0) or cultured for 1, 2, 3, or 4 days and then stained for NGFR expression. Based on FACS histograms as shown in (A), the percentages of NGFR-expressing cells and relative levels of NGFR expression in transduced cells were plotted as a function of time in culture. The relative intensity of NGFR expression is defined by increased mean fluorescence intensity (MFI) normalized by the MFI of nontransduced cells.