Matrix metalloproteinase-9 (MMP-9), a significant matrix proteinase, is tightly regulated and expressed at low levels in normal physiological conditions, while it could be secreted by various cancer cells and plays vital roles in the progression of human malignant tumors, such as oral cancer, gastric cancer, lung cancer, liver cancer, breast cancer, and cervical cancer (25). The gene discussed is MMP9; the disease is breast cancer.