In this study, we first examined the antitumor effects of quercetin and its methylated (4Me-Q) and acetylated (4Ac-Q) derivatives on two types of human breast cancer cells, MCF-7 (wt-p53, caspase-3-ve) and MDA-MB-231 (mt-p53, caspase-3+ve). Here, CASP3 is linked to breast cancer.