Within cancer immunotherapy, cytotoxic T lymphocyte antigen-4 (CTLA-4) and programmed cell death protein 1 (PD-1) are the canonical immune-checkpoint receptors that illustrate ligand–receptor interactions between T cells and APCs that modulate the T cell response to antigen (MHC- mediated molecule complexes that are recognized by the TCR). This evidence concerns the gene CTLA4 and cancer.