CXCL12 and neoplasm: The tumor-tropism effects of stem cells are mediated by multiple cell-surface and secreted proteins, and candidate cytokines/receptors including stromal cell-derived factor-1 (SDF-1)/CXCR4, stem cell factor (SCF)/c-Kit, hepatocyte growth factor (HGF)/Met, vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR), monocyte chemoattractant protein-1 (MCP-1)/CCP, and high-mobility group box1 (HMGB1)/RAGE (87,88,99,117,118).