In the late 19th century, inactivated bacteria was shown to reduce sarcomas in patients.49 Further groundbreaking findings in cancer immunology led to the development of several immunotherapies, including immune checkpoint blockade.50 Programmed cell death protein 1 (PD-1) on the T cell membrane interacts with its ligand programmed death-ligand 1 (PD-L1) to prevent over-activation of T cells.51 Cancer cells induce PD-L1 expression to suppress tumor-specific T cells, but this immune evasion can be overcome by blocking the interaction between PD-1 and PD-L1. This evidence concerns the gene PDCD1 and cancer.