CXCL12 and neoplasm: Multiple and complex factors can affect NSC tumor tropism including tumor-derived growth factors hepatoxcyte growth factor (HGF), endothelial growth factor (EGF), vascular endothelial growth factor (VEGF), urokinase plasminogen activator (uPA), extracellular matrices (ECM), stromal cell-derived factor 1 (SDF-1), hypoxia inducible factor (HIF-1α, and inflammatory cytokines (e.g., IL-6 and IL-8) (15).