Based on the receptors present, tumors of the breast can be broadly classified into (a) luminal A subtype (ER and/or PR-, HER-2-), (b) luminal B subtype (ER+ and/or PR+, sometimes HER-2+), (c) HER-2/ER- subtype (HER-2+ and ER-/PR-), and (c) basal subtype (ER-/PR-/HER-2-) breast cancer [8]. Here, ESR1 is linked to breast cancer.