MDSCs elevated microRNA-101 expression in ovarian cancer cells and promoted the CSC phenotype.22 MDSCs were also reported to endow multiple myeloma cell stemness by inducing piRNA-823 expression and DNA methyltransferase 3 beta (DNMT3B) activation.23 Peng et al found that MDSCs promoted breast cancer cell stemness through activating IL-6/STAT3 and NO/NOTCH cross-talk signaling.13 PMN-MDSC-derived exosomal S100 calcium-binding protein A9 (S100A9) was reported to promote the stemness in colorectal cancer.24 However, the regulation and mechanism of PMN-MDSCs on BCSCs were unclear. Here, S100A9 is linked to breast cancer.