In a plexiglass box it had never been shocked in, a mouse froze in fear. The memory was false: Susumu Tonegawa's laboratory had tagged the hippocampal cells encoding that box's memory a day earlier, during an uneventful visit, then switched those same cells on with pulses of light in a different chamber at the instant the mouse felt a mild shock, engineering a fear where none had occurred. Tonegawa, the molecular biologist and neuroscientist who had already reshaped immunology by discovering how the body builds a vast supply of antibodies from a limited set of genes, becoming the first Japanese-born scientist to win the Nobel Prize in Physiology or Medicine, died on July 11, 2026, at 86, according to MIT News.

The 2013 experiment, carried out with graduate student Steve Ramirez and research scientist Xu Liu and published in the journal Science, gave neuroscience its first physical demonstration of a false memory. Researchers had engineered mouse brain cells to light up whenever they fired during memory formation, then used that same light to reactivate a memory of one safe room while the animal was being mildly shocked in a different room. The mouse later froze in the first room as though it had been hurt there. "Whether it's a false or genuine memory, the brain's neural mechanism underlying the recall of the memory is the same," Tonegawa said, according to MIT News. He put the finding in more human terms too: "Humans are highly imaginative animals. Just like our mice, an aversive or appetitive event could be associated with a past experience one may happen to have in mind at that moment, hence a false memory is formed," he said in a statement from RIKEN, the Japanese institute that co-funded the research.

Tonegawa was born in Nagoya, Japan, in September 1939. He studied chemistry at Kyoto University before crossing to the United States for a doctorate, earning his Ph.D. in biology from the University of California, San Diego, in 1968 for research on bacteriophages. Postdoctoral work under Renato Dulbecco at the Salk Institute led him, in 1971, to the Basel Institute for Immunology in Switzerland, where he made the discovery that defined the first half of his career.

At Basel, working with colleague Nobumasa Hozumi, Tonegawa found that immune cells do something no one had shown before: they physically cut and splice their own antibody genes as they mature, joining separate DNA segments to generate millions of distinct antibodies from a genome that could not otherwise hold instructions for so many. The two published their evidence in the Proceedings of the National Academy of Sciences in 1976. Eleven years later, the discovery brought Tonegawa the 1987 Nobel Prize in Physiology or Medicine as its sole recipient, the first time a Japanese-born scientist had won that category. The same year brought the Albert and Mary Lasker Award for Basic Medical Research; three years earlier, the Emperor of Japan had conferred on him the Order of Culture.

Tonegawa joined MIT's Center for Cancer Research as a professor in 1981 and spent the rest of his career on its faculty, more than four decades in Cambridge. In 1994, already a Nobel laureate in his mid-fifties, he redirected his research entirely, founding and becoming the first director of MIT's Center for Learning and Memory to study how the brain stores experience. A $50 million gift from the Jeffry M. and Barbara Picower Foundation, announced in 2002, expanded the center into a new building, and it was renamed the Picower Institute for Learning and Memory in December 2005. Tonegawa led it until 2006, then continued as its Picower Professor of Biology and Neuroscience while also directing the RIKEN Brain Science Institute in Japan from 2009 to 2017.

He was, by his own description, a fan of one team only. "No, I am not a baseball fan, I am a Red Sox fan!" Tonegawa told MIT News in 2004, the year the team invited him to throw a ceremonial first pitch at Fenway Park in honor of Boston's scientific community. About twenty five members of his lab watched from the stands holding signs that spelled his name letter by letter, according to MIT News. Standing on the mound, his first reaction was to the distance ahead of him: "Wow, it's pretty far." He called the evening, in which the Red Sox came back to win in the ninth inning, "a once in a lifetime occasion."

The 2013 memory study drew attention well beyond neuroscience journals, prompting comparisons to science fiction premises about engineered recollection. Howard Eichenbaum, a memory researcher at Boston University who had no part in the work, called it "a fantastic feat" and "a real breakthrough," as reported by Smithsonian magazine. Tonegawa's team went on to show that engrams, the physical traces of individual memories, are distributed across multiple brain regions rather than confined to one, opening an experimental approach to a question philosophers and psychologists had debated for a century: where, physically, a memory lives.

Tonegawa is survived by his wife, Mayumi Tonegawa, their children Hidde and Hanna Tonegawa, and two grandchildren. He was predeceased by his son, Satto Tonegawa. Colleagues at the institute he founded described a scientist who never stopped redirecting his own curiosity. "His intellectual fearlessness, extraordinary creativity, and relentless pursuit of fundamental questions opened entirely new frontiers in both immunology and neuroscience," said Myriam Heiman, director of the Picower Institute, in MIT's announcement of his death. Li-Huei Tsai, who had led the institute before her, added: "His passion, boundless energy, and unwavering pursuit of the fundamental mechanisms underlying memory were contagious, inspiring generations of neuroscientists to join and advance the field."

Tonegawa built one career proving that the instructions for the body's defenses are not fixed at birth but reassembled inside each developing immune cell, and a second proving that a single memory can be traced to specific neurons, tagged, and even overwritten with an artificial one. The Picower Institute he founded continues that second line of work under the name of the gift that expanded it. The 1976 paper he wrote with Hozumi and the 2013 paper he oversaw with Ramirez and Liu remain foundational citations in two different fields of biology that most scientists spend an entire career in only one of. Per MIT News, his ashes are to be buried in Kyoto, Japan, following a private funeral. Two Nobel-caliber discoveries, decades apart, trace back to the same insight: that biology is not written once and fixed, but rewritten, cell by cell and synapse by synapse, for as long as an organism lives.