CXCR4 is expressed on CD34⁺cells and mediates SDF-1–induced responses. (A) Flow cytometric analysis of CXCR4 expression on CD34⁺ cells from bone marrow (BM), umbilical cord blood (UCB), G-CSF mobilized peripheral blood (MPB) cells, or cyclophosphamide plus G-CSF (CY/G-CSF) MPB. Low-density cells were stained with CD34 FITC, CD45TC, and either a control mouse antibody IgG2a (empty histograms) or anti-CXCR4 MoAb (filled histograms), revealed by goat anti-mouse PE antibody, as described in Materials and Methods. The analysis shows the staining of a gated population of low SSC, CD34⁺, CD45^dull cells. (B) Inhibition of SDF-1–dependent chemotaxis by anti-CXCR4 antibody. Migration of BM CD34⁺ cells in response to SDF-1 (1.5 and 0.3 μg/mL, respectively) and in the presence of 10 μg/mL of either 12.G5 anti-CXCR4 MoAb (▪), or an isotype-matched control mouse IgG (□).