In in vitro experiments in breast cancer cell lines with different nuclear receptor status (MDA-MB-231, ERα-, PR–, and HER2–; T47D, ERα+, PR+, and HER2-; MCF7, ERα+, PR+, and HER2+/–), we demonstrated that NCOA2 knockdown strongly inhibited cancer cell growth. This evidence concerns the gene ESR1 and cancer.