CCL2 and neoplasm: One common mechanism of radiation induced myeloid cell tumor infiltration is increased homing through the upregulation of chemokine expression in tumor cells, such as CCL2, CCL5, and HIF-1 induced stromal-derived factor 1α (SDF-1α), which interacts with CCR2, CCR5, and CXCR4 (163, 167–170).