ESR1 and breast cancer: Breast cancer is one of the main causes of cancer‐related death in women worldwide, accounting for an estimated 28% of new cancers.1, 2, 3 It is a highly heterogeneous disease, and multiple signaling pathways can mediate its initiation and progression.4 According to gene expression profile studies, different subtypes of breast cancer have been identified based on the expression of estrogen receptor (ER) and/or progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2).