Immune checkpoints [including Programmed Cell Death Protein 1 (PD-1) and Cytotoxic T-Lymphocyte-Associated Protein 4 (CTLA-4)] work when proteins on the surface of immune cells such as T cells recognize and bind to chaperone proteins (such as PD-L1) on other cells, including certain tumor cells. This evidence concerns the gene PDCD1 and neoplasm.