ESR1 and breast cancer: Breast cancers are categorized into three main groups based on cellular markers: (i) positive for estrogen receptors (ERs) and/or progesterone receptors (PRs); (ii) positive for amplification of human epidermal growth factor receptor 2 (HER2) with or without ER and PR positivity; and (iii) triple-negative breast cancer (TNBC), defined by a lack of ER/PR expression and HER2 amplification [8,10,11].