The potential effect of CYP2D6 genetic variants on clinical response to tamoxifen treatment in breast cancer patients has gained much interest.53 The Food and Drug Administration recommended an update in the tamoxifen package insert in 2006 to reflect the increased risk of breast cancer recurrence in postmenopausal estrogen receptor-positive patients, who are CYP2D6 poor metabolizers. This evidence concerns the gene CYP2D6 and breast cancer.