Effects of inhibitor 3a (A) and 4a (B) on rate of DNA synthesis. MCF-7aro cells were cultured in steroid-free medium without phenol red for 3 days before plating. Triplicate wells were treated with the indicated concentrations of the compounds in medium containing 1 nM T. Cells cultured with testosterone represented maximum cell proliferation and were considered as control. 3a and 4a induced a decrease in cell proliferation, evaluated by the thymidine incorporation assay, in a time- and dose-dependent manner. Results are the mean ± SE of three independent experiments whereas cultures were performed in triplicate. Significant inhibition relative to the control level is denoted by * (P < 0.001), ** (P < 0.01) and θ (P < 0.05).