Compared to the inhibitory CTLA-4/(CD80/CD86) system, which especially provides an immune checkpoint during immune response initiation in lymphoid tissues, the PD-1/PD-L1 system is considered more important in the periphery where PD-1 can engage with PD-L1 on non-immune cells such as epithelial cells, endothelial cells, or tumor cells (16, 17). The gene discussed is PDCD1; the disease is neoplasm.