Alternatively, compound 4a docked against the crystal structure of ERα in complex with the ER antagonist 4-hydroxytamoxifen (PDB: 3ERT, Figures 3D,E) is able to fill the hydrophobic space and latch onto Arg-394/Glu-353. Tamoxifen prevents the helix 12 from closing properly upon the binding pocket of the ligand-binding domain, while compound 4a can extend the steroidal moiety through the opening left by the displaced helix 12. The D ring of steroid 4a modified at the 16-and 17-positions can go inside the pocket potentially stabilized by polar interactions with TRP-383, Phe-404, Leu-387, Met-388, and Leu-391. These docked structures may reflect the most probable mode of binding. Although the direct comparisons are speculative, our docking outputs are supported by our observation with compound 4a in ER luciferase reporter assays. The antagonist effect of compound 4a is interesting, because D-ring modifications of estradiol commonly result in weak ER agonists (Yang et al., 2000; Kreis et al., 2001).