Two-color flow cytometry analysis of the cell surface expression of the CXCR4 receptor on bone marrow CD34⁺cells and megakaryocytes. Nonadherent light-density bone marrow cells were labeled simultaneously with a PE-conjugated anti-CXCR4 MoAb (12G5) and either an FITC-conjugated anti-CD34 (left panel) or anti-CD61 (right panel) MoAb. Double staining with PE- or FITC-labeled normal isotype mouse IgG was used as the negative control (data not shown). The figure in each quadrant represents the percentage of cells.