Tamoxifen is converted by the liver cytochrome P450 into the 4-hydroxytamoxifen and is a more selective antagonist of the ERα subtype than ERβ. 4-hydroxytamoxifen binds to ERs within the same binding pocket that recognizes 17β-estradiol. The receptor recognition of 4-hydroxytamoxifen appears to be controlled by two structural features of 4-hydroxytamoxifen, the phenolic A ring, and the bulky side chain. The phenolic A ring forms hydrogen bonds to the side groups of ER's Arg-394, Glu-354 and to structurally conserved water. The bulky side chain, protruding from the binding cavity, displaces helix 12 from ligand-binding pocket to cover part of the coactivator binding pocket. The ER-4-hydroxytamoxifen complex formation recruits corepressors proteins. This leads to decreased DNA synthesis and inhibition of estrogen activity. Clomifene and torimefene produce binding affinities similar to that of tamoxifen. Thus, these two drugs are more selective antagonists of the ERα subtype than ERβ.