A single-letter swap in the genetic code can increase the risk of some brain tumours six times, researchers have found.
In people with the danger gene the letter A, short for the biological molecule adenine, is replaced by G (guanine).
When the substitution occurs it dramatically raises the risk of developing certain types of glioma brain tumour. The tumours are slower growing than some others but can still be lethal. They account for around 20% of newly diagnosed brain cancers.
The US discovery, reported in the journal Nature Genetics, could lead to new and better diagnostic tests and treatments.
Professor Margaret Wrensch, one of the scientists from the University of California at San Francisco, said: "Understanding how this variant causes people to get these less aggressive, but still lethal, tumours will be extremely important. It may eventually lead to methods to reverse the course of these tumours or possibly to prevent their formation."
Co-author Dr Robert Jenkins, from the Mayo Clinic in Rochester, Minnesota, said: "Based on our findings, we are already starting to think about clinical tests that can tell patients with abnormal brain scans what kind of tumour they have, just by testing their blood."
The DNA region studied has been conserved through evolution as far back as the duck-billed platypus, a primitive egg-laying mammal, said the scientists. The letter-swap change was found in a particular region of chromosome 8, one of the 23 pairs of protein-wrapped DNA packages in every normal cell that contain the genes.
Scientists believe this DNA region may provide the instructions for making a microRNA, a tiny molecule that controls gene activity.
"The altered microRNA might target tumour suppressor genes, it might activate a cancer gene, it might be involved in regulating the stability of the genome (genetic code), or there might be something else going on altogether," said Dr Jenkins.
"One of the big challenges of the current genomic era is to assign functions to all these new gene variants."