Shen et al. [105] found that mtDNA copy number had a significant positive association with risk of breast cancer and a significant inverse association with several important endogenous antioxidants (glutathione, CuZnSOD activity, and catalase) and the prooxidant myeloperoxidase, suggesting that mtDNA copy number may be associated with breast cancer risk, possibly through an oxidative stress mechanism. Here, MPO is linked to breast cancer.