The first new genes for three decades linked to multiple sclerosis have been identified by UK and US researchers.
Many genes are thought to play a role in MS
Approximately 60,000 people in the UK suffer from the incurable disease of the nervous system.
The finding, published in two journals, will not lead directly to new tests or treatments, as experts say as many as 100 more genes may play a role in MS.
However, a Cambridge University researcher said he now expected swifter progress to reveal them all.
The joint project used the latest genome-scanning technology to look at the genetic make-up of thousands of MS patients, looking for signs of tiny genetic differences which might mean a greater risk of developing the illness.
It concluded that people carrying either of two genetic variants, called IL7R-alpha and IL2R-alpha, had an increased risk of between 20% and 30%.
Dr Stephen Sawcer, from Cambridge University, said that even though this only represented a tiny increase in risk, it was a "landmark" discovery.
"This ends 30 years of complete frustration," he said. "But now we finally have the technology we need to help us find these genes."
A test for this gene alone would reveal nothing about a person's chances of developing MS later in life, he said, as only one in 10 people in the UK did not carry it.
"We estimate that there are between 50 and 100 other genes which carry an additional risk.
"These finds are the first of many, and after three decades finding nothing, we would expect to find many more of these genes over the next few years."
It was only once many more "MS genes" were revealed, he said, that scientists would then be able to identify people carrying large numbers of them and examine what other factors might be triggering their illness.
Both of the genes identified by the research are known to have a role in the body's immune responses.
MS is caused when the immune system wrongly attacks the sheaths surrounding the nerves, destroying them and the body's ability to send signals along them.
This leads to muscle weakness, difficulties with balance and speech and vision problems.
The Harvard Center for Neurodegeneration and Repair was one of several US universities and research institutes which worked together on the project.
Adrian Ivinson, its director, said: "This study illustrates the power of collaboration - individually, none of us could have completed a study of this scale and complexity."
Dr Lee Dunster, head of research and information at the MS Society, welcomed the publication of the studies in the journals Nature Genetics and the New England Journal of Medicine
He said: "One of the great unknowns about MS is what causes it and this looks like a welcome breakthrough in getting to grips with the genetics behind the disease.
"People with MS often worry about what caused it, and particularly whether it will affect their children, so a better understanding of the role of certain genes is good news.
"These latest findings will be of great interest to researchers trying to develop future treatments."
The research is published in the New England Journal of Medicine and Nature Genetics.