To evaluate the in vivo tumor forming ability of CXCR4 and CXCR7-expressing cells, a single line was chosen to evaluate tumor formation of the various sub-populations of cells, i.e. the CXCR4+, CXCR7+, CXCR4+/CXCR7+ (double positive), and CXCR4−/CXCR7− (double negative) sub-populations. The gene discussed is ACKR3; the disease is neoplasm.