Determination of CXCR4 expression by flow cytometry. The graph displays pseudocolor dot plots of 2 different AML samples with either low CXCR4 surface expression on CD34⁺/CD117⁺ AML cells (top row) or high CXCR4 expression (bottom row). The left-hand boxes display the relative size (forward scatter) and granularity (sideward scatter) characteristics of the patients' samples. The second column of boxes displays the relative fluorescence intensities of the AML cell samples stained with fluorescence-labeled anti-CD34 and anti-CD117 antibodies, as displayed on the horizontal and vertical axes, respectively. This allows for gating on the CD34⁺/CD117⁺ AML cells, as indicated by the gate drawn around the CD34⁺/CD117⁺ population. The percentage of CD34⁺/CD117⁺ cells is indicated above each of the populations. The third column of boxes displays relative fluorescence intensities for CD34 and CXCR4, as indicated on the horizontal and vertical axes. The relative proportion of CXCR4-positive and CXCR4-negative cells is displayed next to each of these populations. The right-hand boxes display overlay histogram plots that depict the relative CXCR4 fluorescence intensity of the CD34⁺/CD117⁺ AML cells (red-shaded histogram) in comparison with the isotype control stain (blue line). The mean fluorescence intensities are displayed next to each histogram. Moreover, the mean fluorescence intensity ratio (MFIR) for CXCR4 is displayed.