ESR1 and breast cancer: Based on the most recent European Society of Medical Oncology (ESMO) guidelines, BC can be subdivided into four molecular subtypes: 1) luminal A, in which HR are expressed while the HER2 is absent (HR+/HER2−); 2) luminal B, in which ER is positive while HER2 may or may not be expressed (ER+/HER2−/+); 3) HER2 enriched, in which (HR−/HER2+); and 4) basal-like subtype, which is also known as triple-negative BC (TNBC), tumor cells do not possess any of the 3 standard molecular markers (HR−/HER2−) [84,102,103].