CXCR4 and Ewing sarcoma: Because CXCR4 expression of Ewing sarcoma cell lines had been found heterogeneous within cell populations and highly dynamic in response to serum deprivation and space constraints [20], we performed our analyses at a constant confluence of 70–80% and in both standard (10% serum) (Fig. 3) and serum-free growth conditions (Additional file 1).