Flow cytometric analysis of fragmented DNA ends labeled with fluoresceinated digoxygenin-conjugated dUTP in a terminal transferase (TdT)-catalyzed reaction (TUNEL; abscissa) as a function of cellular DNA content (ordinate). One-half of each sample was labeled in the absence of TdT enzyme (−TdT) to determine the background level of FITC fluorescence; the remaining half of the sample was labeled in the presence of TdT (+TdT) to fluorescently tag fragmented DNA ends. Cells to the right of the vertical line in the (+TdT) plots have free DNA ends specifically labeled by TdT, indicating apoptotic cell death. In this analysis, Jurkat cells treated for 6 hours with the toxic chemotherapy drug VP-16 showed a substantial number of cells (35%) undergoing apoptosis. In contrast, bone marrow cells cultured on naive NIH 3T3 cells (Mock) had 4% apoptotic cells, whereas unsorted GATA-2irGFP–transduced cells had 3% apoptotic cells (data not shown). Both sorted GFP-expressing and nonexpressing cell populations for each vector, as indicated, contained less than 1% apoptotic cells. Similar results were obtained in another experiment.