Breast cancer is the most common cancer in women and a significant cause of death. Mutations of the oncosuppressor genes BRCA1 and BRCA2 are associated with a hereditary risk of breast cancer, and dysregulation of their expression has been observed in sporadic cases. Soya isoflavones have been shown to inhibit breast cancer in studies in vitro, but associations between the consumption of isoflavone-containing foods and breast cancer risk have varied in epidemiological studies. Soya is a unique source of the phytoestrogens daidzein (4′,7-dihydroxyisoflavone) and genistein (4′,5,7-trihydroxyisoflavone), two molecules that are able to inhibit the proliferation of human breast cancer cells in vitro. The aim of the present study was to determine the effects of genistein (5μg/ml) and daidzein (20μg/ml) on transcription in three human breast cell lines (one dystrophic, MCF10a, and two malignant, MCF-7 and MDA-MB-231) after 72h treatment. The different genes involved in the BRCA1 and BRCA2 pathways (GADD45A, BARD1, JUN, BAX, RB1, ERα, ERβ, BAP1, TNFα, p53, p21Waf1/Cip1, p300, RAD51, pS2, Ki-67) were quantified by real-time quantitative RT-PCR, using the TaqMan method and an ABI Prism 7700 Sequence Detector (Applied Biosystems). We observed that, in response to treatment, many of these genes were overexpressed in the breast cancer cell lines (MCF-7 and MDA-MB-231) but not in the dystrophic cell line (MCF10a).