To determine the transcriptional activity of the estrogen receptor α (ERα), MCF-7 cells were transfected with the plasmids containing luciferase reporter gene under the control of the promoter containing estrogen responsive elements. Assay was performed in steroid-free conditions (phenol red-free DMEM medium supplemented with 10% DCC serum). The reporter plasmid ERE-TK-LUC used in this work was kindly provided by Reid et al. (2003). The transfection was carried out for 24 h at 37°C using Metafectene (Biontex Laboratories). To control the efficiency and potential toxicity of the transfection, the cells were co-transfected with the β-galactosidase plasmid. The tested compounds were added to phenol red-free DMEM medium supplemented with 10% DCC serum. The luciferase activity was measured according to a standard protocol (Promega) using a Infinite M200 Pro luminometer (Tecan). The β-galactosidase activity was analyzed using a substrate, p-nitrophenyl β-D-galactopyranoside (ONPG). Briefly, cell lysates were mixed with 0.1 M phosphate buffer (pH 7.5) containing 1.0 mM MgCl2, 3.3 mM ONPG and 53 mM β-mercaptoethanol. After incubation for 1 h at 37°C, the absorbances at 405 nm were measured on the MultiScan FC reader (ThermoFisher). The luciferase activity was calculated in arbitrary units evaluated as the ratio of the luciferase activity to the galactosidase activity.