Tamoxifen (active metabolite 4-OH tamoxifen [4-OHT]), a selective estrogen receptor modulator, is the first clinically approved ER-targeted agent and the most widely used hormonal treatment for breast cancer in both pre- and post-menopausal women (Loi et al., 2010; Guan et al., 2019). The gene discussed is ESR1; the disease is breast carcinoma.