Serial dilution experiments of intraperitoneal inoculation in immunodeficient mice revealed that CXCR4+ small cells also possessed a higher tumorigenic ability (tumor development in 4/5 and 1/5 mice upon inoculation of 105 and 104 cells, respectively) compared with CXCR4- small cells (1/10 and 0/10 mice following 105 and 104 cells, respectively) (Fig 3Bi). This evidence concerns the gene CXCR4 and neoplasm.