S100A9 and central nervous system cancer: There have been a few studies that explored CSC-macrophage interactions such as PLGF-VEGFR1 and IL1RAPL1-IL-8 in glioma [32], AGER-S100A9 in hepatocellular carcinoma [33], CD90-CD11b in mouse breast cancer [34], and hCAP-18/LL-37-FPR2 and P2X7R in pancreatic ductal adenocarcinoma [35].