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). The gene discussed is NR4A1; the disease is breast cancer.