To further verify the differentially expressed genes induced by 4-OH-TAM in MCF-7 cells and to understand their pathophysiological relevance in ER+- and ER--breast cancer cells, we examined their mRNA levels in three ER+-breast cancer cell lines (MCF-7, BT-474 and ZR-75-1) and two ER--breast cancer cell lines (MDA-MB-468 and MDA-MB-231) via qRT-PCR. The gene discussed is ESR1; the disease is breast cancer.