ESR1 and breast cancer: Breast cancer has been classified traditionally according to characteristics such as tumour size, lymph node involvement, histological grade and the receptors expressed: estrogen receptor (ER+) and/or progesterone receptor (PR+), human epidermal growth factor receptor 2 (HER2) amplified (referred to as HER2+), and triple negative breast cancer (TNBC), which lacks the expression of all three markers (ER and PR, and HER2) [2].