Scientists have found a drug which appears to slow the progress of the debilitating condition Huntington's Disease, which currently has no cure.
Huntington's disease is caused by brain cell loss
Animal tests by Cambridge University researchers showed that rapamycin also delays the onset of the disease.
The drug is already used in humans to prevent organ rejection after transplants.
Huntington's Disease groups hailed the research, published in Nature Genetics, as a significant advance.
The disease, caused by a mutation in the huntingtin protein which makes it become toxic, is an inherited condition.
It affects the central nervous system and can lead to loss of muscle control, dementia and depression.
Huntington's normally affects people in middle age, but it can strike at any time.
It is estimated that around 50,000 people in the UK either suffer from the disease or are at risk of developing it.
'Balance risks with benefits'
The research by the Department of Medical Genetics at Cambridge University, funded by the Medical Research Council and the Wellcome Trust, found that rapamycin can reduce the levels of a toxic protein causing Huntington's.
It does this by speeding up the break-down of the protein in cells.
Dr David Rubinsztein, who led the research, said: "This is an exciting development which could be tremendously important for people suffering from Huntington's Disease.
"Rapamycin is designed for long-term use, which is obviously crucial for someone who has this disorder.
"It is not without side effects, but you could argue that you'd be balancing side effect risk with the potential benefits."
He said more research would be need to be carried out before the drug could be used to treat Huntington's in humans, but studies would be done "as fast as possible".
Dr Rubinsztein said the fact it was possible to test someone to assess their risk of developing Huntington's meant it would be easy to target the drug treatment at those who would benefit from it.
Susan Young, a regional care advisor for the Huntington's Disease Association, said: "At the moment, the drugs that we use are to control each of the individual symptoms, not the disease.
"This research means it might be possible to delay the onset of symptoms. It could give people a lot more life."
Ms Young said once it became available, people may be more willing to be tested to see if they are likely to develop Huntington's.
Many currently prefer not to be tested because there is nothing that can be done to prevent the disease's progression.