ESR1 and breast cancer: Breast cancer is classified into four molecular subtypes based on estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (HER2/ERBB2) receptor status: luminal A (ER+/PR+/HER2−), luminal B (ER+/PR+/HER2+), HER2-overexpressing, and triple negative breast cancer (TNBC; ER-/PR-/HER2−) [2,3,4].