Among breast cancer subtypes, estrogen receptor (ER)+ luminal breast cancers are treated by endocrine therapy (e.g., aromatase inhibitor, tamoxifen) that targets the ER signaling pathway, whereas breast cancers with HER2 overexpression can be targeted by Trastuzumab (Herceptin), a monoclonal antibody that blocks HER2 signaling. Here, ESR1 is linked to breast cancer.