Breast cancer can be classified into several subtypes based on the expression of hormone receptors [HR, including estrogen receptor (ER) and progesterone receptor], human epidermal growth factor receptor 2 (HER2), and Ki67 (Figure 1A): HR-positive, HER2-positive, and triple-negative breast cancer (TNBC) (Waks and Winer, 2019). Here, ESR1 is linked to breast cancer.