Taking into consideration the two-fold gain in cytotoxicity of compound 4a against ERα-positive breast cancer cells vs. ERα-negative cells and its high activity as ERα inhibitor we performed immunoblotting of ERα in MCF-7 cells. The data obtained by immunoblotting confirmed that compound 4a exerted ERα inhibitory activity. The incubation of MCF-7 cells with compound 4a resulted in the partial suppression of ERα expression, as can be seen in Figure 2B. Tamoxifen was used as the standard reference drug and its application resulted in an increase in ERα expression, which may be attributed to tamoxifen-induced stabilization of inactive ERα in the cell cytoplasm as discussed in Wijayaratne and McDonnell (2001). As compared with tamoxifen, compound 4a was found to be active as the partial ERα downregulator (Figure 2B).