The aim of this study was to investigate the effects of ATR on the anti-proliferative effect and ferroptosis pathway of MDA-MB-231 (ER−, PR−, HER2−), BT-474 (ER+, PR+, HER2+), MCF-7 (ER+, PR+, HER2−), and SK-BR-3 (ER−, PR−, HER2+) breast cancer cells. This evidence concerns the gene ESR1 and breast cancer.