The cytotoxic effect of ATR, a lichen secondary metabolite, on breast cancer cell lines BT-474 (ER+, PR+, HER2+), MDA-MB-231 (ER−, PR−, HER2−), MCF-7 (ER+, PR+, HER2−), and SK-BR-3 (ER−, PR−, HER2+) of different histological subtypes and on the normal breast cell MCF-12A was determined for the first time using the MTT assay and the xCELLigence RTCA system. Here, ESR1 is linked to breast cancer.