PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) weaken the cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and the programmed cell death 1 (PD-1)/ligand 1 (PD-L1) pathways, and modulate the ligand–receptor interactions between cancer cells and the patient’s immune cells within the tumor microenvironment, depriving cancer cells of a key strategy of evasion from immunosurveillance.