We evaluated the toxicity effect of ROS-induced drug release 3-(4-morpholinopyrido[3′,2′:4,5]furo[3,2-d]pyrimidin-2-yl)phenol (RIDR-PI-103) (described in [20]) on normal fibroblasts, normal mammary epithelial breast cells MCF10A and estrogen receptor positive (ER+) MDA-MB-361, triple negative breast cancer (TNBC) MDA-MB-231 and HER2+ MDA-MB-453 breast cancer cell lines [25] using a 72 h 4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell proliferation assay. The gene discussed is ESR1; the disease is breast cancer.