ESR1 and breast cancer: Worldwide, breast cancer, accounting for approximately 30% of cancers in women (Siegel et al., 2020), can be divided into three subtypes based on estrogen receptor (ER), progesterone receptor (PR), and HER2 status: hormone receptor-positive, HER2-positive, and triple-negative breast cancer (TNBC) (Denkert et al., 2017).