Researchers say they have made the most significant breakthrough for 15 years in the quest to understand the fatal condition Motor Neurone Disease (MND).
Stephen Hawking, who has lived for 35 years with MND, is exceptional
A team says a mutated gene is behind one form of the disease - and can be used to understand it better.
Campaigners say the study, published in Science, is the most important since a first gene was identified in 1993.
MND involves the progressive wasting of the muscles - while usually leaving the mind unaffected.
It affects some 5,000 people in the UK. At least five people each day die of the condition.
The international team, led by King's College London, found that in one family affected by a rare, inherited form of the condition, there were mutations in the gene coding for the protein TDP-43.
The protein has long been known to accumulate abnormally in MND patients, but it had been thought this was an innocent by-product of the disease.
Now, says Professor Chris Shaw, who led the research, it is clear that this protein is "directly toxic" to motor neurones.
While the inherited form of the disease this mutation causes is extremely rare - accounting for just 1% of MND cases - researchers can use their findings to give animals the disease and investigate how it develops.
"It is a new biological tool to understand the disease and develop treatments," says Professor Shaw. "It is another part of the jigsaw, but there are still, admittedly, a lot of pieces missing."
The first part of that jigsaw was uncovered in 1993, when researchers found the gene mutation SOD1 was responsible for one form of MND. Experts say this finding improved understanding, although it has yet to impact significantly on treatment.
But the MND Association says this latest finding should now speed that up.
"This new information will be a springboard to greater understanding of the processes that cause motor neurones to die," said Dr Brian Dickie, director of research.
"It is through such understanding that we will develop the treatment strategies to defeat this devastating disease."