Tamoxifen, a selective estrogen receptor modulator (SERM), has shown clinical success by reducing the risk of estrogen receptor-positive (ER+) breast cancers by up to 69% as compared to placebo treatment, although approximately 20% of breast cancers are ER-negative (ER−) and do not respond to SERMs1, 2. The gene discussed is ESR1; the disease is breast cancer.