Oku-BL and PTLD1 tumor cells differ in their dependence on EBV for proliferation. (A) The growth of tumor cell populations was measured over time as EBV was evicted. Dashed lines represent cells in which dnEBNA1 remain induced; solid lines, cells in which dnEBNA1 is uninduced. (Left graph) PTLD1 cells (clone P5-1 only shown) transduced with a control vector (gray lines) or a Bcl-XL expression vector (orange lines). (Right graph) Oku-BL cells (clone O2-4 only shown). The percentage of EBV-negative Oku-BL cells was determined by fluorescence in situ hybridization analysis. The average number of doublings from at least 3 independent experiments plus or minus SD is shown. (B) The percentage of cells in each stage of the cell cycle was determined by propidium iodide staining and fluorescence-activated cell sorting analysis. Cells transduced with a Bcl-XL expression vector were analyzed 20 days (Oku-BL) or 12 days (PTLD1) after induction of dnEBNA1 (the cell populations marked in orange). The average of 3 independent experiments ± SD is shown.