MMP9 and breast cancer: Besides, a meta-analysis conducted by Song et al.[16] testified that MMP-9 overexpression could act as a biomarker suggesting unfavorable results on both overall survival (OS) (hazard ratio (HR): 1.70, 95% confidence interval (95%CI): 1.41–2.04) and RFS (HR: 1.54, 95%CI: 1.17–2.01) in breast cancer patients.