Flow cytometry analysis of cell-surface CD33 and CD34 expression on myeloid progenitor cells. On the day of infection, magnetic bead–enriched, nonadherent progenitor cells were stained with either isotype control antibody, anti-CD33 antibody, or anti-CD34 antibody. Expression is shown as a histogram, with the percentage of cells expressing cell-surface CD33 or CD34 above isotype control staining indicated.