Human epidermal growth factor receptor 2 (HER-2), progesterone receptor (PR), estrogen receptor (ER), and Ki-67 expression have made it possible to categorize breast cancers into four groups: TNBC (HER2-/ER-/PR-), Luminal A (HER2-/ER+/PR +, low proliferation), and Luminal B (HER2-/ER+/PR+, high proliferation). Here, ESR1 is linked to breast cancer.