CYP2D6 and breast cancer: However, these data are contradictory to the previously published data of Hertz (2017), who performed a retrospective analysis of two community-based studies to assess for the associations between the low-activity CYP2D6 genotype and disease outcome in 500 breast cancer patients treated with adjuvant tamoxifen monotherapy, and 500 patients who did not receive any systemic adjuvant therapy.