Analysis of CXCR-4 expression by flow cytometry. The results are shown as fluorescence histograms (solid, CXCR-4 expression; line, respective IgG control). CD34⁺ hematopoietic progenitor cells from four patients (CD34⁺ no. 1 through 4) were positive for CXCR-4, while CD34⁺ cell lines (KG1, KG1a, Kasumi-1, MOLM-1) expressed only low levels. In contrast, variable CXCR-4 expression was found in primary leukemic cells (AML #1 through #12). The cell line HL60, peripheral blood B lymphocytes, and B lymphoma cell lines (OCI-Ly8, DOHH-2) were brightly positive for CXCR-4.