Breast cancer is the most common cancer worldwide, and the incidence is rising.1–3 In the United Kingdom, there are around 55 900 new cases of breast cancer annually and approximately 11 500 deaths per year.4 Around 80% of all breast cancers are hormone receptor-positive (HR+) tumors.5 Standard practice in high-income countries, including the United States and United Kingdom, is to offer women with early stage HR + tumors adjuvant endocrine therapy (AET) to reduce risk of recurrence.6,7. This evidence concerns the gene NR4A1 and breast cancer.