MMP9 and gastric cancer: Matrix metalloproteinase (MMP) is considered to be an important component of tumor invasion in gastric cancer and a useful marker used to determine the biological aggressiveness of gastric cancer, and inhibition of the expression of these genes has a tumor-suppressive effect.[68,69] On the other hand, MMPs were found to be highly correlated with the microenvironment of tumors and immune cells, and Youqiong Ye et al[70] found that MMP2 and MMP9 regulate tumor immune checkpoints by modulating PD - L1.