ESR1 and breast cancer: Breast cancer is not a single disease and it is now accepted that there are at least four different biological subtypes which comprise endocrine responsive disease divided in Luminal A (estrogen receptor [ER] and/or progesterone receptor [PgR] positive (+), HER2 negative (-), low proliferating activity) and Luminal B (ER and/or PgR+, HER2-, high proliferating activity), HER2+ disease (HER2+ regardless of hormone receptor status) and triple-negative disease (ER, PgR and HER2 negative [1–3].