CXCR4 and cancer: Still, given promising anti-tumor activities in vitro and in animal tumor models in vivo [1, 2, 5, 10, 12, 15], compounds that target this interaction (foremost plerixafor) are in clinical investigation for the treatment of patients with diverse solid malignancies [13],3 thus raising interest in the role of CXCR4 signaling in rare cancers such as Ewing sarcoma.