Breast cancer is one of the most common malignancies in industrialized countries and is characterized by distinct classes of tumors that respond differently to targeted therapies such as selective estrogen receptor modulator (SERM) treatments (for example, tamoxifen) in estrogen receptor (ER)-positive breast cancer or monoclonal antibodies (for example, trastuzumab) in HER2/Neu-positive breast cancer. This evidence concerns the gene ESR1 and breast cancer.