Despite the fact that women on adjuvant endocrine treatment for breast cancer are generally expected to be highly adherent as they are facing a serious life threatening disease, the treatment is effective, easy to use and generally well-tolerated, recent studies underline a clinically relevant decrease in adherence to tamoxifen and aromatase inhibitor therapy (including a reduction in cancer outcome) as soon as 12 months [13,14,25]. Here, CYP19A1 is linked to breast cancer.