CXCL12 and neoplasm: Stressors, including hypoxia, glucose deprivation and reactive oxygen species, are activated in the tumor microenvironment and result in the upregulation of the transcription of angiogenic factors, including vascular endothelial growth factor (VEGF), stromal cell-derived factor 1 monocyte chemotactic protein-1 and erythropoietin, in EPCs (13–15).