PGR and breast cancer: Additionally, the cytotoxicity of the ZFO NPs was assessed against triple negative human breast cancer cell line MDA-MB-231 (defined as tumors that lack expression of estrogen, progesterone receptor, and human epidermal growth factor [7]), human breast cancer cell line (MCF-7), and human embryonic kidney 293 (HEK-293) cell lines with different cytotoxicity assays such as 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT), crystal violet, and lactate dehydrogenase (LDH).