They were mobilized to the bloodstream and then clustered in the pre-metastatic sites by a variety of tumor-secreted factors, such as CCL2, CSF-1, VEGF, PLGF, TNF-α, TGF-β, tissue inhibitor of metallopeptidase (TIMP)-1, and exosomes [116–118]. This evidence concerns the gene CCL2 and neoplasm.