Apoptosis in response to ATRA or camptothecin. TF1-neo cells (top panels) and TF1 cells expressing the type L (middle panels) or S (lower panels) PML-RARα isoforms were plated at 1 × 10⁵ cells/mL in complete medium (+GM-CSF) and cultured with vehicle alone (left panels), ATRA (10⁻⁶ mol/L, 96 hours; middle panels), or camptothecin (0.15 mol/L, 16 hours; right panels). Apoptosis was evaluated by flow cytometry after staining cells with an FITC-conjugated Annexin-V antibody (1 μ/mL) and propidium iodide (2.5 μg/mL) as discussed in Materials and Methods. Black dots, viable cells; blue dots, early stage (live) apoptotic cells; red dots, late stage (dead) apoptotic cells. The percentages of events in each quadrant are shown.