Superimposition of 3D complex models of ERα with 17β-estradiol (dark red; PDB ID: 1ERE), equilin (blue), Δ8,9-dehydroestrone (green) and estrone (purple). Estrogen receptors are represented as alpha carbon traces. Hydrogen bonds are labeled in dashed green lines. The comparison of these complex structures revealed that the four estrogens bind to ERα in a similar binding mode, with a hydrogen bond interaction occurring between the atom OE2 of the residue Glu 353 in the binding site of the receptor and the hydrogen atom of the phenol group in the A ring of estrogens. An additional hydrogen bond interaction occurs between the atom ND1 of the residue His 524 and the hydroxyl group in the D ring of 17β-estradiol. Image was generated using Accelrys InsightII 2000.