ESR1 and breast cancer: Breast cancers that are oestrogen receptor (ER) positive, progesterone receptor (PR) positive and human epidermal growth factor receptor 2 (HER2) negative account for up to 80% of all breast cancers, and yet ER+/PR+ cancers have significantly lower avidity and sensitivity on FDG PET compared to triple-negative breast cancers (ER-/PR-/HER2-) [2,3,4].