As can be seen in Figure 2A, estranes 3a,b containing the D-ring-fused pyrimidine moiety did not inhibit ERα activity at 10 μM concentration. Moreover, these 3a and 3b stimulated ERα activity at low (10 nM) concentration acting as partial receptor agonists (Table S1). Androstene derivative 4b proved to be inactive as ERα inhibitor. Compound 4c showed weak inhibitory activity, while steroids 3c, 3d, 3e, and 4a highly inhibited E2-mediated ERα activity at 10 μM concentration. These compounds showed no ERα agonist activity in the luciferase reporter assay (Table S1).