To clarify the clinical significance of CYP2D6 allele in tamoxifen therapy, we recently carried out the prospective CYP2D6-tamoxifen study and reported the positive association between CYP2D6 genotype and response to tamoxifen using Ki-67 change in breast cancer tissues after short-term preoperative tamoxifen therapy [21], which has been known as a promising surrogate marker for clinical response to tamoxifen [21–24]. Here, CYP2D6 is linked to breast cancer.