We discovered a novel compound, ethyl 3-[5-(2-ethoxycarbonyl-1-methylvinyloxy)-1-methyl-1H-indol-3-yl]but-2-enoate (referred to as MIBE) (Figure 1), which displays the unique property to bind to and inhibit GPER- and ERα-mediated signaling in breast cancer cells. Here, ESR1 is linked to breast cancer.