S100A9 and breast cancer: To support the upregulated S100a9 signal caused by the loss of Brca1 could increase S100a9 protein levels, we knocked down BRCA1 in WT mouse mammary epithelial cells B477, and human breast cancer MDA-MB-231 cells and found that S100A9 protein levels were increased in both cell types via immunofluorescence (IF) staining (Fig. 3f) and Western blot analysis (Fig. 3g).