The discovery of the immune checkpoint proteins CTLA-4 (Leach et al., 1996) and PD-1 (also known as PDCD1) (Ishida et al., 1992) has revolutionised our understanding of immunity in cancer and has resulted in the introduction of therapeutic vaccines, immune checkpoint inhibitors, T-cell receptor-engineered T cells and chimeric antigen receptor T cells (CAR-Ts). Here, PDCD1 is linked to cancer.