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In 2025, a team of scientists revealed some unexpected findings about how the inherited DNA repeat associated with Huntington's disease causes the illness—by slowly expanding throughout a person's life, then becoming toxic after enlarging to several times its original length. The findings were in a specific brain region, the striatum, but it wasn't clear if the same process was directly causing the harm in other parts of the brain affected by the disease.

Now, the research team, led by scientists at the Broad Institute, Harvard Medical School and McLean Hospital, has found that the same mechanism is indeed at play in another brain region damaged in Huntington's—the neocortex. Appearing online in Cell, the findings suggest that recently initiated efforts to develop medicines targeting this mechanism could protect multiple brain areas.

"These new results suggest that such medicines, if and when they're successfully developed, might be able to address not just one of the pathologies in the disease, but the two big ones," said Steve McCarroll, a geneticist, neuroscientist and co-senior author of the study.

Scientists have known for decades that people who inherit a version of the huntingtin (HTT) gene in which a three-letter stretch of DNA ("CAG") is repeated 40 or more times are at high risk of developing Huntington's disease, a fatal neurodegenerative disorder. The 2025 study upended the field. It found that in the striatum, the brain region most damaged in the disease, the inherited HTT mutation becomes toxic only after accumulating 150 or more of the three-letter repeats.

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