CXCR4 and neoplasm: The chemokines CCL2 and CXCL12 (previously known as monocytic chemotactic protein 1 (MCP-1) and stromal-derived factor-1 (SDF-1), respectively) are crucial for the attraction of CCR2+ and/or CXCR4+ MDSCs, Treg cells and TANs as well as for the polarization of macrophages towards tumor-supportive TAMs (15, 16).