CXCL12 and neoplasm: The tumor microenvironment plays a major role in activating circulating EPCs and mediating neovascularization and stressors in the tumor microenvironment such as hypoxia, glucose deprivation, and reactive oxygen species upregulate transcription of angiogenic factors like VEGF, SDF-1, MCP-1, and erythropoietin in EPCs [12,20,21].