ESR1 and breast cancer: Triple-negative breastcancer (TNBC), which accounts for approximately20% of breast cancer cases, is a particular subtype that lacks theexpression of human epidermal growth factor receptor 2 (HER2), estrogenreceptor (ER), or progesterone receptor (PR).1 In general, TNBC is biologically aggressive since it is highly proliferativeand is the subtype with the poorest prognosis due to its highly malignantnature.2 Although the development of alternativetherapeutics has continued over the last decades, chemotherapy remainsthe major approach for systemic treatment of TNBC nowadays.