Increased SOD2 expression in metastatic cancer cell lines as well as in tumor tissues is associated with increased SOD2 activity, thus, indicating a functional role for SOD2 during metastatic progression and poor outcome, particularly in BC [54,81,94,95,96], where elevated SOD2 expression is associated with metastatic progression in estrogen receptor (ER) negative BCs [93]. This evidence concerns the gene ESR1 and breast cancer.