Susumu Tonegawa (1939-2026): A molecular biologist who transformed immunology and neuroscience.

Susumu Tonegawa transformed two fields of biology. His discovery of the genetic mechanism generating antibody diversity established the foundations of modern immunology and earned him the 1987 Nobel Prize in Physiology or Medicine. Remarkably, he then began a second scientific career in neuroscience, pioneering genetic approaches to dissect the molecular and circuit mechanisms of learning and memory and, ultimately, the neuronal ensembles that store specific memories. He was also an ambitious institution builder whose leadership shaped neuroscience at Massachusetts Institute of Technology and RIKEN. As former trainees from his immunology and neuroscience labs, we remember a scientist of extraordinary curiosity, intensity, and intellectual courage, who was never afraid to leave familiar territory in pursuit of a fundamental question.