MPO and cancer: We annotated the staining according to cells expressing CD15+MPO− (CD15+; immature or non-activated neutrophils (brown only), CD15−MPO+ (MPO+; any cell expressing MPO other than neutrophil (pink only)), CD15+MPO+ (mature and activated neutrophils expressing MPO (brown and pink co-staining)) expression in immune cells, or CD15+ expression in malignant tumor cells (CD15+ TC) (Fig. 1A).