ESR1 and breast carcinoma: Breast cancer (BC) is the most common malignancy reported in women and is classified into three main subtypes based on the molecular features of the tumor, including estrogen receptor positive (ER+), human epidermal receptor 2 positive (HER2+), and triple-negative BC (TNBC; ER−, progesterone receptor negative [PR−] and HER2−).1