In this study, two breast cancer cell lines (MCF-7 and MDA-MB 231) were treated with daidzein for 48 h in order to observe the effects of this bioactive molecule on H3 and H4 histones on H3K27me3, H3K9me3, H3K4me3, H4K8ac, and H3K4ac marks, on six selected genes (EZH2, BRCA1, ERa, ERb, SRC3, and P300). This evidence concerns the gene NCOA3 and breast carcinoma.