Effect of the MEK inhibitor on CD34 and CD41 cell surface expression. Representative histograms from 1 of at least 4 experiments performed with blood from different donors are shown for each marker. Cells from control cultures (filled histograms) or cultures containing 6 μmol/L PD98059 (open histograms) were harvested at the indicated days and stained separately with either anti-CD34 antibody coupled to R-PE or anti-CD41 antibody coupled to FITC. In some experiments, labeling was performed with FITC-anti-CD34 and R-PE-anti-CD41 antibodies and similar results were obtained. For each marker, the percentage of positive cells (or the MFI for CD41 at days 10 and 13) are indicated: gray labeling, controls; black labeling, PD98059-treated samples.