To evaluate a potential functional link between E2 and CLDN6 in breast cancer cells, we treated MCF-7 (ERα+/ERβ+/GPR30+) and MDA-MB-231 (ERα−/ERβ+/GPR30-) cells with DMSO or E2 (from 5 nM to 100 nM) for 24 h. This evidence concerns the gene ESR1 and breast carcinoma.