An ligand binding competition assay using [3H] E2 was performed using baculovirus expressed ERα (A) or ERβ (B) with increasing concentrations of nonradiolabeled E2, DHEA or DHEA metabolites (DHEA-S, 7-oxo-DHEA, ADIONE and ADIOL). The values on the Y-axes are expressed as the percentage of [3H] E2 bound and each data point represents the mean of two independent binding assays. The competitor concentration causing 50% reduction in [3H] E2 binding (IC50) is found at the intersection of the binding curves with the 50% binding line (---------). E2, ●; DHEA, ▲; DHEA-S, ♦; ADIOL, ■; ADIONE, ♦; and 7-oxo-DHEA, □.