The four main subtypes of breast cancer include Luminal A (ER or PR positive, or both, HER2 negative, low proliferation), Luminal B (ER or PR positive, or both, HER2 negative, high proliferation), HER-2/neu type (HER2 positive and ER and PR negative), Basal cell-like type (HER2 negative, ER and PR negative; triple-negative breast cancer) (51). Here, ESR1 is linked to triple-negative breast carcinoma.