CXCR4 and neoplasm: The most stringent stem cells feature is the tumor-forming assay; to this aim 1 × 103 and 1 × 104 OVCAR-5 and OVCAR-4 derived cells sorted as CD133−CXCR4−, CD133+CXCR4−, CD133−CXCR4+ and CD133+CXCR4+ were inoculated in nude mice.