PDCD1 and cancer: In recent years, immune checkpoint inhibitors such as anti-programmed cell death protein 1 (PD-1) and anti-cytotoxic T lymphocyte–associated protein 4 (CTLA-4) have achieved great results in cancer treatment, however, because to low response rates and immune-related adverse events (irAEs), most patients are unable to benefit (Fritz and Lenardo, 2019).