P-gp expression and the corresponding rhodamine-123 efflux activity. The two left flow cytometer contour plots (a) reflect P-gp expression as measured by staining of blast cells with antibody 4E3.16. Cells in the lower right quadrant are positive for 4E3.16, whereas cells in the left lower quadrant are negative for 4E3.16 because they lack fluorescence above background. The two right flow cytometer cytograms (b) show cellular rhodamine-123 fluorescence after 1 hour of cell incubation at 37°C in CD34-positive blast cells. A shift of rhodamine-123 fluorescence from the right upper to the left upper quadrant reflects cellular efflux of rhodamine-123. Patient #1 showed P-gp expression in a small subpopulation of blast cells and lacked measurable rhodamine-123 efflux activity despite CD34 expression. In patient #2, P-gp was detected on >90% of blast cells and these blast cells showed marked rhodamine-123 efflux activity, which was inhibitable by PSC 833 (data not shown).