MMP9 and breast cancer: The responsible mechanisms include the following: enhancing cell proliferation and migration by stabilizing DNMT1 or driving cancer progression by activating the TGF-β/CSF-1 signaling axis in breast cancer, promoting invasion and metastasis by activating Wnt/β-catenin signaling in HCC, activating the β-catenin/YB-1/EGFR pathway to promote progression in glioma, facilitating invasiveness via the NF-κB pathway to upregulate MMP-9 and MMP-2 expression in prostate cancer, and accelerating invasion in gastric cancer by phosphorylating integrin β1 and β3 [19–25].