The finding that the cancer cells in the ex vivo culture expressed a far lower level of CXCR4 than the cells in the orthotopic tumor in vivo (Fig 5C) suggests that stress-promoting factors other than a long period of time of culture and a highly increased cell density are needed for cells to express a higher expression level of CXCR4. The gene discussed is CXCR4; the disease is neoplasm.