CD33 expression in the cell lines. The indicated cell lines were stained with FITC-labeled anti-CD33 antibody (thick lines), or control antibody (thin lines), and analyzed by flow cytometry. The percentage of positive cells (after substraction of background) and mean fluorescence intensities (in parentheses) are indicated in each panel. (A) HL-60 line. (B) KG-1 line. (C) NB-4 line. (D) THP-1 line.