Notes: (E/Z)-4-hydroxy Tamoxifen is a mixture of (E)-4-hydroxy tamoxifen and (Z)-4-hydroxy tamoxifen.{28446} (Z)-4-hydroxy Tamoxifen is an active metabolite of the estrogen receptor antagonist tamoxifen (Item Nos. 13258 / 11629) and is formed from tamoxifen by the cytochrome P450 (CYP) isoform CYP2D6. (Z)-4-hydroxy Tamoxifen (100 nM) inhibits 17β-estradiol-induced increases in the mRNA encoding progesterone receptor in MCF-7 breast cancer cells.{67256} (Z)-4-hydroxy Tamoxifen inhibits the proliferation of MCF-7 and MDA-MB-231 breast cancer cells (IC50s = 27 and 18 µM, respectively).{28444}