The expression of ER-α was analysed by immunohistochemistry and Western blotting in a variety of benign (HUMA121) and malignant mammary cell lines, including ER-α(+) (MCF7, ZR-75, T47D) and ER-α(−) (MDA-MB 231) breast cancer cell lines. The gene discussed is ESR1; the disease is breast cancer.