Differentiated cells (DMSO [1.3%] and 1 μmol/L RA for 3 to 4 days) and undifferentiated cells were analyzed in parallel for nuclear morphology in cytospin preparations and classified as promyelocytes (PM), myelocytes and metamyelocytes (M + MM), and band and segmented cells (B + S). Results are expressed as the mean percentage of cells showing the corresponding morphological features in four independent experiments. The ability to mount a respiratory burst was assessed by NBT reduction and given as percentage of cells containing blue intracellular formazan deposits after 30 minutes of stimulation by 0.2 μg/mL PMA at 37°C. Cell-surface expression of CD11b was determined by FACS analysis and expressed as MFI. The differentiation of wild-type and transfected HL-60 cells was identical, and results from both cell types were combined in the statistical analysis of the data. Values are mean ± SD of the number of experiments shown in parentheses.