PDCD1 and cancer: Immune checkpoint inhibitors have a potential property to improve clinical outcomes in cancer immunotherapy [106, 107], such as programmed cell death protein 1 (PD-1)/programmed cell death-ligand 1 (PD-L1) axis, cytotoxic T lymphocyte antigen-4 (CTLA-4), indoleamine 2,3-dioxygenase (IDO), cluster of differentiation 40 (CD40), as well as 4-1BB (CD137).