To evaluate the biological role of NCOA2 in human breast cancer, we applied lentivirus-mediated RNA interference toward NCOA2 in three breast cancer cell lines (MDA-MB-231, ERα-, PR–, and HER2–; T47D, ERα+, PR+, and HER2–; MCF7, ERα+, PR+, and HER2+/–). This evidence concerns the gene ESR1 and breast cancer.