Although ERβ has been suggested as a cancer therapeutic target (27), some studies (28) show that when ERα/β positive MCF-7 cells are treated with an active metabolite of BPA [4-methyl-2,4-bis(4-hydroxyphenyl)pent-1-ene (MBP)] at levels (~1 nM) comparable to human environmental exposure, ERα protein expression is downregulated and proliferation is increased via ERβ. This evidence concerns the gene ESR1 and cancer.