Associated with poor prognosis; MTs promote the proliferation of breast cancer cells by promoting the cell cycle, regulating oxidative stress, inhibiting apoptosis, increasing DOX resistance of breast cancer, and enhancing the aggressiveness by upregulating MMP‐9 via AP‐1 and NF‐κB activation [37, 38, 39, 40, 41]. This evidence concerns the gene MMP9 and breast cancer.