Born on September 5, 1939, in Nagoya, Aichi. A graduate of Kyoto University's Faculty of Science (Chemistry), he completed a master's degree at Osaka University's Graduate School of Science and earned his doctorate at the University of California, San Diego. After a period at the Salk Institute, he moved in 1971 to the Basel Institute for Immunology in Switzerland, where he took up immunology research in earnest. In 1976 he discovered V(D)J recombination — the mechanism by which vertebrates produce a seemingly infinite diversity of antibodies as a small number of gene segments rearrange to generate immense variety. For this achievement he became the first Japanese to receive the Nobel Prize in Physiology or Medicine solo, in 1987. He has been a professor at MIT since 1981 and was concurrently director of the RIKEN Brain Science Institute from 1994. In recent years he has shifted the focus of his research from immunology to neuroscience, producing results on the mechanisms of memory and learning and on Alzheimer's disease.

A thoroughgoing rationalist and internationalist. An independent-minded researcher who won his standing in America and Europe on ability alone, without relying on Japanese academic cliques or connections. With sharp logic and tenacious experimentation, he has always cut through to essential questions. A spirit of inquiry that has kept him shifting his research object from immunology to neuroscience is also characteristic of him.

Tonegawa's discovery of V(D)J recombination is one of the largest in the latter half of the twentieth century, rewriting the textbooks of immunology and molecular biology. Wide-ranging fields of modern medicine — vaccine development, immunotherapy, treatment of autoimmune disease — rest upon his achievement. His becoming the first Japanese Nobel laureate in Physiology or Medicine was a great encouragement to younger researchers and opened the way for subsequent Japanese laureates including Honjo Tasuku (2018) and Yamanaka Shinya (2012). He remains active at the MIT Picower Institute for Learning and Memory.