ESR1 and breast cancer: Triple-negative breast cancer (TNBC), which accounts for 10‒20% of all breast cancer cases, is an aggressive molecular subtype characterized by the absence of estrogen receptor (ER) and progesterone receptor (PR) coupled with low human epidermal growth factor receptor 2 (HER2) protein expression.1 This unique molecular profile has hindered the development of targeted therapies for TNBC.