ESR1 and breast carcinoma: The histological classification of breast cancer is mainly based on the expression pattern of human epidermal growth factor receptors 2 (HER2) and hormone receptors (HR) named as estrogen receptors (ER) and progesterone receptors (PR), as well as the tumor proliferation rate indicated by Ki-67, resulting in classified 5 major subtypes: HER2 positive, HR positive (HER2+ and ER+ or PR+ or both positive); HER2 positive, HR negative (HER2+/ER-/PR-); basal-like or triple negative (HER2-/ER-/PR-); luminal A (HER2-/ER+/PR+, low proliferation); and luminal B (HER2-/ER+/PR+, high proliferation)2.