Scientists say they have discovered a new gene linked with late-onset Alzheimer's disease.
The scientists analysed the DNA of over 1,000 people
People with a damaged copy of the gene, GAB2, may be at four times increased risk of developing dementia, Neuron journal reports.
Experts said the latest findings were some of the most significant to emerge since the discovery of the ApoE4 Alzheimer's gene.
Late-onset Alzheimer's affects one in 10 people over 65 and half of over 85s.
The researchers, from 15 institutions including the Institute of Neurology in London, analysed the DNA of 1,411 people and found GAB2 influenced the risk of dementia among those with APOE4.
GAB2 appears to modify the effects of this better known Alzheimer's gene.
In turn, this leads to the formation of the characteristic protein "tangles" found in the brains of people with Alzheimer's, the researchers told Neuron.
To explore the gene's role, Dr Eric Reiman and colleagues experimentally "silenced" GAB2 in neurons and observed an increase in a key protein, tau, that contributes to these tangles.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "This impressive research suggests a common gene could be responsible for a four-fold increased risk of developing Alzheimer's disease.
"It is the most important risk factor gene to be identified in relation to tangles, which develop in the brain in Alzheimer's disease."
He added: "The results are some of the most significant finds for genetic risk factors since the discovery of ApoE4, which revolutionised treatment research."