ESR1 and breast cancer: The selective estrogen receptor modulator (SERM) tamoxifen 1a with antiestrogen action in breast cells is the most commonly used drug in endocrine therapy for ER + BC [3]; the related metabolites 4-hydroxytamoxifen 1b, endoxifen 1c, and norendoxifen 1d also demonstrate potent antiestrogenic activity (Figure 1).