CXCL12 and neoplasm: Diverse cells within the tumor microenvironment are capable of secreting various cytokines and growth factors, including stromal cell-derived factor-1 (SDF-1), interleukin-6 (IL-6), vascular endothelial-derived growth factor (VEGF), insulin-like growth factor 1 (IGF-1) and hepatocyte growth factor (HGF) [79].