At the same time, analyses of CXCR7 protein expression in Ewing sarcoma cell lines (including CXCR4-low lines) point out very limited total protein [49] and surface [21] expressions compared to CXCR4, suggesting that our findings in both CXCR4-high and -low cell lines are not primarily mediated by CXCR7; which nevertheless will be an important area of study in this context. This evidence concerns the gene ACKR3 and Ewing sarcoma.