One of the cruellest and most devastating diseases – Huntington's – has been successfully treated for the first time, say doctors.
The disease runs through families, relentlessly kills brain cells and resembles a combination of dementia, Parkinson's and motor neurone disease.
An emotional research team became tearful as they described how data shows the disease was slowed by 75% in patients.
It means the decline you would normally expect in one year would take four years after treatment, giving patients decades of "good quality life", Prof Sarah Tabrizi told BBC News.
The new treatment is a type of gene therapy given during 12 to 18 hours of delicate brain surgery.
The first symptoms of Huntington's disease tend to appear in your 30s or 40s and is normally fatal within two decades – opening the possibility that earlier treatment could prevent symptoms from ever emerging.
Prof Tabrizi, director of the University College London Huntington's Disease Centre, described the results as "spectacular".
