Downregulation of surface PrP^C in retinoic acid–induced granulocytic differentiation of HL-60 cells. HL-60 cells were cultured in the presence of 1 mol/L all-trans retinoic acid for 0, 1, 3, and 6 days, then stained for flow cytometry analysis with MoAb 3F4. Basal level of PrP^C staining in undifferentiated cells (solid line) is shown relative to an IgG2b isotype control (in gray) (A). The progressive decrease of PrP^C staining during differentiation (in black) is shown relative to basal level (B through D).