Scientist Stephen Hawking has motor neurone disease
A gene linked to a type of motor neurone disease that runs in families has been found after a 10-year search.
Along with a related gene reported last year, it opens up an unexplored area of research into the condition, investigators said.
The finding will also help doctors screen and counsel families at risk of the disease, the US and UK team wrote in Science.
Up to 10% of cases are inherited within families because of genetic mutations.
Motor neurone disease (MND) involves the progressive wasting of the muscles, while usually leaving the mind unaffected.
It affects some 5,000 people in the UK.
The first MND gene - SOD1 - was found in 1993 and it has been a major focus of research.
But then researchers found a protein called TDP-43 is deposited in the neurons of 90% of people with the condition.
However, it was not apparent in animal models with the SOD1 mutation, suggesting that the first gene found is not linked with the major underlying biology of the disease.
For the past decade the UK and US team have been looking for a gene believed to be located on chromosome 16.
They eventually found a mutation in the FUS gene in one family with inherited MND - also known as amyotrophic lateral sclerosis.
Further studies showed that 4% of all families had FUS mutations.
The FUS gene is related to TDP-43, the gene for which was found by the same researchers last year.
Professor Christopher Shaw, from the Institute of Psychiatry at Kings College London, said the FUS gene was a very important clue as to what causes motor neurons to degenerate.
"It's very interesting, we really have wrung SOD1 out. We have looked at cells and mice endlessly, but the major pathways are not SOD pathways.
"The genetic pieces of the jigsaw puzzle are beginning to fit together, leading us in new and exciting directions of research."
He said the research would have immediate implications for genetic testing of families affected and also for reassuring those with sporadic cases that their relatives were not at risk.
Dr Belinda Cupid, research manager at the MND Association, said the fact that two genes had been discovered in 12 months reflected the "accelerating pace" of such research.
"Not only will it open up an entirely new avenue of scientific investigation, it will also allow researchers to compare the different known causes of MND and start to home in on the main biochemical events that cause motor neurones to die."