ESR1 and breast cancer: GPER-mediated gene transactivation was established through experiments that demonstrated c-fos gene transactivation by 17β-estradiol or phytoestrogens (quercetin or genistein) in ER-negative, GPER-positive human SKBR3 breast cancer cells or in human MDA-MB-231 cells following the reconstitution of GPER [5].