CYP19A1 and breast cancer: Approximately 70 % of all breast cancers are hormone receptor positive (Harvey et al. 1999) and in this group of patients, upon completion of primary therapy (e.g., surgery, radiation, chemotherapy), adjuvant endocrine therapy (AET) with either tamoxifen or aromatase inhibitors (AIs) is an important component (Early Breast Cancer Trialists’ Collaborative Group 2005).