A severe form of multiple sclerosis (MS) has been linked to a single mutant gene.
The gene defect was found in two Canadian families, several members of which had a rapidly progressing version of MS for which no effective treatment exists.
It is evidence that at least some forms of MS can be inherited, said scientists writing in the journal Neuron.
"This mutation puts these people at the edge of a cliff, but something still has to give them the push to set the disease process in motion," said lead researcher Dr Carles Vilarino-Guell, from the University of British Columbia in Canada.
Although only one in 1,000 MS patients is thought to have the mutation, its discovery helps uncover the biological pathway involved in rapidly progressing MS - which accounts for about 15% of cases of the disease.
It could also shed light on the more common "relapsing-remitting" form of MS, scientists believe.
The gene could provide a target for new therapies, or a way of screening high-risk individuals.
Co-author Dr Anthony Traboulsee, from the MS Society of Canada, said: "If you have this gene, chances are you will develop MS and rapidly deteriorate.
"This could give us a critical early window of opportunity to throw everything at the disease, to try to stop it or slow it. Until now, we didn't have much basis for doing that."
MS is a progressive disease in which the immune system attacks the fatty myelin insulation surrounding nerve fibres.
As a result, nerve signals are disrupted - leading to a wide range of mild and severe symptoms, which may include paralysis.
Dr Sorrel Bickley, head of biomedical research at the MS Society, said: "Whilst the gene variant identified was associated with rapidly progressing forms of MS in the two families studied, the variant itself is rare and most people with MS won't have it.
"This research does however give us an insight into how progressive forms of MS develop, which could help to inform the development of new treatments in the future."