ESR1 and breast cancer: Physiological concentrations of this isoflafone (<10 μM) plus E2 induced apoptosis of MDA-MB-231 (ERβ-positive/ERα-negative) cells by dysregulating the Bax/Bcl-2 ratio and reducing the phosphorylation of ERK1/2, sustaining the antitumoral role of genistein against ERβ-positive/ERα-negative BC cells.