We treated the breast cancer cell line MDA-MB-231 with mitomycin C (MMC) and Taxol and performed qPCR and Western blotting to analyze the gene expression of YY1 and FEN1. We found that in response to treatments with MMC and Taxol, the mRNA level of YY1 was down-regulated by more than 2 folds, while the mRNA level of FEN1 was up-regulated by 3 to 6 folds (Figure 4A and B). This evidence concerns the gene YY1 and breast cancer.