Depending on the specific disease, including cancer and chronic viral infections, the circulating CD8+PD-1+ T cell population comprises 20–70% of total CD8+ T cells in hepatocellular, gastric, and lung cancer patients (4, 7, 8), while in solid tumors the CD8+PD-1+ T cell population can constitute as much as 80–90% of the resident CD8+ T cell population (8). Here, PDCD1 is linked to cancer.