Immune checkpoint inhibitors such as anti-programmed cell death protein 1 (PD-1; pembrolizumab and nivolumab) (50), anti-cytotoxic T lymphocyte-associated protein 4 (CTLA-4; ipilimumab and tremelimumab) (51), anti-PD-1 ligand 1 (PD-L1: atezolizumab, avelumab and durvalumab) (52) have developed, and demonstrated to significantly improve outcome in patients suffered from devastating cancers (Figure 2A). Here, PDCD1 is linked to cancer.