ESR1 and breast cancer: Four surrogate molecular subtypes of breast cancer have been identified to date: luminal A (estrogen receptor [ER]-positive, progesterone receptor [PR]-positive, human epidermal growth factor receptor 2 [HER2]-negative, and a low Ki67 index [Ki67-low]), luminal B (ER-positive, PR-negative or Ki67-high, and either HER2-positive or HER2-negative), triple negative breast cancer (TNBC) (ER-negative, PR-negative, and HER2-negative), and HER2-overexpressing tumors (9).