Hypoxic tumor microenvironment is associated with drug resistance to cancer therapies, such as chemotherapy and radiotherapy.7, 9, 24, 25 This is true for tamoxifen resistance during primary therapy of ER+ breast cancer.9 If we can monitor the dynamic development of tamoxifen resistance, we might be able to reverse/prevent the resistance and therapeutic failure. The gene discussed is ESR1; the disease is breast carcinoma.