The present study demonstrated the non-competitive inhibition of the steroid metabolomics pathway in ERα+ MCF-7 but not in ERα- MDA-MB-231 breast cancer cells by low micromolar concentrations of RES, which led to a significant, twofold increase of free E2, capable of stimulating the proliferation of ERα+ breast cancer cells. The gene discussed is ESR1; the disease is breast carcinoma.