As BTB could selectively inhibit the ER-mediated transactivation, we then investigated the effects of BTB on the inhibition of E2-induced cell proliferation in ER-positive MCF7, Ishikawa, and SKOV-3 cancer cells and compared this with the ER-negative MDA-MB-231, HEC-1-A, and OVCA429 cancer cells. Here, ESR1 is linked to cancer.