Retroviral titration. NIH 3T3, HeLa, and A549 cells were plated the day before infection and cultured overnight with serial dilutions of the virus stocks, as described in Materials and Methods. The target cell numbers were counted at the beginning of the infection in one replicate well. (A) FACS analysis of cells 4 days after infection. The percentage of NTP+ cells is indicated in each quadrant. (B) Southern blot analysis of genomic DNA extracted from NIH 3T3 and A549 cells 7 days after infection. The DNA was digested with Nhe I and analyzed with a radiolabeled NTP probe. Samples corresponding to the 1, 10−1, 10−2, and 10−3 dilutions are loaded from left to right. On the left are two samples obtained from clones bearing one and two copies of the vector. EB, endogenous bands.
