SMAD3 and breast cancer: Deficiencies in BRCA1 gene in the presence of H2O2 induce the exportation of phosphorylated protein Smad3 from the nucleus to cell cytoplasm (cytoplasmic factor that binds with other proteins to activate or inhibit the transcription of specific genes); this reduces the Smad3-Smad4 interaction mediated by TGF-b (growth factor responsible for activating Smads by phosphorylation) and slightly decreases the transcriptional activity of both proteins, as evidenced in studies on the response to oxidative stress in breast cancer cell lines (e.g., MCF-7) (Li et al. 2009).