PDCD1 and cancer: Immune checkpoint inhibitors, such as anti-programmed cell death protein 1 (PD-1), anti-programmed cell death ligand 1 (PD-L1), and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA4), have been widely used in anti-cancer immunotherapies and resulted in significant improvements in disease outcome for a variety of cancers [31].