Unlike studies that have noted an association between the MPO A/A genotype and a reduced risk of breast cancer, we did not find an association between MPO G-463A polymorphism and breast cancer risk; however, consistent with the findings of Lin et al. [19], we did find a significantly reduced risk (OR = 0.56) in premenopausal women carrying the AA genotype. The gene discussed is MPO; the disease is breast cancer.