Breast cancer is classified as luminal-like breast cancer, which includes luminal A and B subtypes, characterized by the expression of estrogen receptor (ER) and/or progesterone (PR), and basal-like breast cancer, characterized by the absence of all three hormonal receptors (R-PR-Human Epidermal Growth Factor Receptor 2 (HER2)-triple negative breast cancer). Here, ESR1 is linked to breast cancer.