Typically, patients with luminal A breast cancer are treated with endocrine therapies, such as selective estrogen receptor modulators (SERMs; to interfere with the interaction between estrogen and ER) and aromatase inhibitors (AIs; to inhibit estrogen production), whereas anti-ERBB2 treatment can be applied to patients with ERBB2/HER2-positive breast cancer. Here, ESR1 is linked to breast cancer.