ESR1 and breast cancer: Hormone receptor-positive breast cancer accounts for approximately 70% of breast cancer cases and has either estrogen receptor (ER) or progesterone receptor (PR) protein in the cancer cells; human epidermal growth factor receptor 2 (HER2, also known as ERBB2) breast cancer makes up 15% to 20% of breast cancer cases; TNBC is more heterogeneous and lacks ER, PR, and HER2 protein expression, accounting for approximately 15% of all breast cancer cases [10].