CYP2D6 and breast cancer: All the other polymorphisms with reported significant effects on breast cancer clinical outcomes, i.e., CYP2C8*3, CYP2D6*4, CYP2D6*6, CYP2D6*10, SULT1A1*2, and UGT2B15*2, were associated with an apparent worse response to tamoxifen, resulting in higher risk of tumor recurrence, shorter breast cancer-specific survival, or shorter overall survival.