ESR1 and breast cancer: In early-stage breast cancer, treatment decisions are guided by the clinical subtypes of oestrogen receptor positive (ER+HER2−), human epidermal growth factor receptor 2 amplified (HER2+) and triple-negative breast cancer (TNBC), which are defined by the presence or absence of the oestrogen receptor and progesterone receptor and HER2 overexpression or gene amplification.