ERA-45 is a synthetic estrogen and a selective agonist of the ERñ. It shows 286-fold selectivity for transactivation of the ERñ over the ERò, with EC<sub>50</sub> values of 0.37 nM for the ERñ (7-fold weaker than estradiol) and 13 nM for the ERò (20,000-fold weaker than estradiol). However, another found only about 35-fold potency for transactivation of the ERñ over the ERò. The drug has no antagonistic activity at either receptor. ERA-45 induced prostate cancer development in preclinical models when it was given in combination with testosterone, whereas testosterone alone did not do so. In contrast, the selective ERò agonist ERB-26 was protective against the development of prostate cancer produced by these two drugs. These findings suggest opposing roles of the ERñ and ERò in the prostate gland. The chemical structure of ERa-45 does not appear to have been disclosed.