A gene that helps to stave off the effects of multiple sclerosis (MS) has been discovered by scientists.
MS is a disease of the central nervous system
A Danish-UK team found that a known risk gene for MS, called DR2b, is always partnered by a twin gene - DR2a.
The researchers, writing in the journal Nature, said DR2a tempers the effects of the risk gene and reduces the severity of MS symptoms.
They believe in the future the gene's symptom-fighting features could be exploited for potential treatments.
There are about 85,000 people with MS in the UK.
The precise cause of the disease, in which the body's immune system attacks the central nervous system, is unknown, but a range of genetic and environmental factors are being explored.
Two-thirds of MS sufferers carry the pair of DR2 genes, but carrying the genes does not necessarily mean a person will go on to develop MS.
The researchers looked at mice that carried different combinations of the twin genes.
They discovered the mice with just the risk gene, DR2b, had a form of multiple sclerosis with extremely aggressive symptoms.
Those carrying both genes were less likely to get MS, and if they did, they had a milder form of the disease.
The scientists said they believed the two genes were interacting.
They said the risk gene, DR2b was "influencing" the immune system to attack the body, while the DR2a gene was counteracting this attack and dampening the effects.
Professor Lars Fugger, one of the researchers in the study and a clinical immunologist at the Medical Research Council Human Immunology Unit, Oxford University, said: "The DR2b gene clearly tells the immune system to go hard into battle against the body's own tissue, so it starts to work in a way that actually damages the person.
He added that when the team looked through different populations, the genes were always found together. He said this was most likely down to evolutionary pressures.
"For this reason, natural selection has eliminated the gene on its own, but allowed it to be inherited only when it is accompanied by another gene [DR2a] which tempers its effect.
"This is a new way to assess how genes contribute to autoimmune diseases overall."
The researchers said they hope their findings may be useful in helping to find possible treatments for MS.
Professor Fugger said that the mechanisms that reduce MS symptoms could potentially be exploited.
Simon Gillespie, chief executive of the Multiple Sclerosis Society, said: "This is a very interesting finding which adds another piece to our understanding of the MS puzzle.
"Genes are known to be one of a combination of factors which can lead to the development of MS. We share the researchers' hope that it could eventually play a part in helping to treat this very variable and unpredictable condition."