ESR1 and breast cancer: To assess survival prognosis and guide individual therapeutic decisions, breast cancer has been divided into distinct molecular subtypes based primarily on the expression status of hormonal receptors such as ER, PR, HER2 and Ki67 (tumor proliferation index) as follows: luminal A/B (ER and/or PR positive), HER2 enriched (HER2 positive) and triple-negative breast cancer (ER, PR and HER2 negative) [33].