CCL2 and breast cancer: Homing and migration of MSC to tumor sites has been proved to be mediated by monocyte chemotactic protein-1 (MCP-1 or CCL2) secreted by primary breast cancers [39], or stromal cell-derived factor 1 (SDF-1), a small chemotactic cytokine that activates leukocytes and is often induced by proinflammatory stimuli such as TNF-α or IL-1 [40] in response to prostate, colorectal, and breast cancer in vitro [41].