ESR1 and breast cancer: BC can be divided into three groups, according to its molecular characteristics: estrogen receptor (ER) or progesterone receptor (PR) positive, human epidermal receptor 2 (HER2) positive, and triple-negative BC (TNBC; ER − , PR − , HER2 −), of which TNBC is the most common (Barzaman et al. 2020; Liedtke et al. 2023; Lehmann et al. 2011; Li et al. 2021).