S100A9 and neoplasm: We found that (1) S100a9-positive MDSCs invaded tumor-adjacent mammary gland tissues (Fig. 4e); (2) S100a9 protein was secreted by MDSCs into the tumor-adjacent mammary gland tissues of Brca1-MT mice and the supernatant of culture cells in vitro (Fig. 4f); and (3) the protein levels TGF-β and Il-10, which can trigger the translocation of S100A9 to the membrane and cell surface to promote S100A9 protein secretion29,37,38, were increased in mammary tissues of both 4-month-old and 6-month-old MT mice (Fig. 4g, h).