Introduced PU.1 promotes apoptosis of the leukemia cells. (A-B) Flow cytometric profiles showing that wt but not mutant PU.1 provokes apoptosis in the deleted lines, as assessed by the binding of annexin V (B), and that inclusion of GM-CSF in the medium (top panels only) is protective, allowing more differentiated (CD11b⁺) cells to develop or survive (A). Dotted lines indicate staining with isotype control antibodies. (C) Apoptotic cells (brown, arrows) visualized on cytospins by the TUNEL assay for DNA degradation. Cells shown in the top panel only had been cultivated in M-CSF.