FOS and breast cancer: Although such xenoestrogens are more potent in activating ERα and ERβ (26), some research suggests that the two major phytoestrogens, genistein and quercetin, also stimulate c-fos expression via an ER-independent manner (i.e., GPER) in ERβ-positive MCF7 and ERα-negative SKBR3 breast cancer cells (27).