Tasuku Honjo is a Japanese immunologist and Distinguished Professor at the Kyoto University Institute for Advanced Study, where his Department of Immunology and Genomic Medicine identified programmed cell death protein 1 (PD-1) as an inducible gene on activated T lymphocytes in 1992. By showing that blocking PD-1 releases the brakes the immune system places on itself, Honjo established the molecular basis of checkpoint-inhibitor cancer immunotherapy, work that earned him the 2018 Nobel Prize in Physiology or Medicine shared with James P. Allison. Earlier in his career he discovered activation-induced cytidine deaminase (AID), the enzyme responsible for class-switch recombination and somatic hypermutation that diversifies antibodies. His PD-1 discovery underpins the anti-PD-1 antibody nivolumab (Opdivo), developed with Ono Pharmaceutical and Bristol-Myers Squibb, now a multibillion-dollar therapy class spanning melanoma, lung, kidney and other cancers; the resulting patents and licensing royalties make his laboratory an unusually direct academic-to-industry node for partners selling reagents, antibodies and immune-profiling platforms into checkpoint-biology labs. He also won the 2016 Kyoto Prize in Basic Sciences.