The review of epidemiological data as well as in vitro experiments on ER-positive and ER-negative breast cell lines [66] allowed to postulate that phytoestrogens may exert two opposite actions, depending on their level in blood and concentrations used in experimental studies: At concentrations, not exceeding 10 μM, some phytoestrogens, like genistein, stimulate growth of ER-positive MCF-7 and T-47D cell lines, but not the ER-negative MDA-MB 231/435 breast cancer cell lines. The gene discussed is ESR1; the disease is breast carcinoma.