MPO and lung cancer: To understand whether immune cells, specifically myeloid cells, in the tumor microenvironment of lung cancer are affected by oncogenic KRAS, we performed immunofluorescent staining on lung sections from each experimental group of the L-iKRAS mouse using macrophage marker F4/80 and neutrophil marker myeloperoxidase (MPO) (Figure 4A and Supplemental Figure 4A).