ESR1 and breast cancer: Based on the expression of the estrogen receptor (ER), progesterone receptor (PR), and the amplification of the HER2 (ERBB2) receptor, BC is categorised into four clinical subtypes: hormone receptor (HR)-positive (ER+/PR+/HER2−), triple positive (ER+/PR+/HER2+), HER2-positive (ER−/PR−/HER2+), and triple-negative breast cancer (TNBC) (ER−/PR−/HER2−, also HR−/HER2−) [3,4].