S100A9 and neoplasm: Primary tumor cells release pro-inflammatory mediators such as VEGF-A, transforming growth factor-beta (TGF-β), and TNF-α, IL-1, IL-6, chemokines, MMPs, angiogenic factors, and anti-apoptotic proteins, which lead to selective expression of chemo-attractants S100A8 and S100A9 for tumor cells to home on the pre-metastatic sites [5,139,165,169,170,171,172,173,174].