PDCD1 and cancer: Immune checkpoint inhibitors targeting programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), T cell immunoglobulin and ITIM domain (TIGIT), T cell immunoglobulin mucin-3 (TIM-3), and cytotoxic T lymphocyte antigen 4 (CTLA4) have been effective in treatment of various cancer types [12].