It is also highly and permanently expressed on macrophages in a variety of chronic infections or tumors (hepatitis C virus infection, pneumocystis pneumonia, gastric cancer, colorectal cancer, etc.)(11, 13–15, 36), which have received growing attention recently for the essential role in dampening immunity, further emphasizing the need for the mechanistic elucidation of PD-1 expression in macrophages. Here, PDCD1 is linked to colorectal cancer.