For example, CCL2, CCL3 and CCL5 induce monocytes to secrete matrix metalloproteinase 9 (MMP9) to further promote tumor invasion and metastasis (19); CCL2 affects tumor vascularization and tumor metastasis by targeting colon cancer vascular endothelial cells (20); CCL8 promotes invasion and metastasis in breast, pancreatic, ovarian and prostate cancers (14) but high level of CCL8 expression in gastric cancer is a positive prognostic factor (21). Here, MMP9 is linked to gastric cancer.