The advent of immunotherapy has revolutionized cancer therapy. Immune checkpoint inhibitors (ICIs) are a group of monoclonal antibodies that commonly target the receptors cytotoxic T-lymphocyte antigen 4 (CTLA-4), and programmed cell death protein 1 (PD-1) or its associated ligand (PD-L-1). By attenuating these pathways, the ICIs deprive cancer cells of a key strategy of evasion from immunosurveillance. Antibodies that bind to CTLA-4, PD-1, and PD-L1 lead to T-cell activation and destruction of both tumor and normal host cells. This evidence concerns the gene PDCD1 and cancer.