The subtypes of breast cancer include luminal A (ER/PR-positive, Ki-67-low, and HER2-negative), luminal B (ER/PR-positive, Ki-67-high, and HER2-negative/positive), HER2-positive, and triple-negative breast cancer (TNBC; ER/PR/HER2-negative), which differ in terms of progression, recurrence rate, and treatment options [1]. Here, ESR1 is linked to breast cancer.