Glioma is the most common form of brain tumour
Genetic warning signs of an increased risk of the commonest kind of brain cancer have been discovered.
UK and US scientists identified genetic indicators that someone is at greater risk of developing a glioma - which accounts for 50% of all brain tumours.
However, the teams from London, Texas and California write in Nature Genetics that environmental factors also affect if someone will develop the cancer.
A UK charity said the work opened up new avenues for research and treatment.
About 4,550 people are diagnosed with brain tumours each year. Only 14% are alive after five years.
Gliomas begin in glial cells, which play a key role supporting and insulating nerve cells in the brain.
There are various kinds, with glioblastoma being the most aggressive, deadly and common.
People who are diagnosed with it rarely survive more than five years.
The two research teams looked at single nucleotide polymorphisms (SNPs). These are points in the genetic code which vary from person to person.
In the first study, a team of scientists from London's Institute of Cancer and the University of Texas looked at over 521,000 SNPs in almost 1,900 glioma patients and 3,670 healthy people and found 34 SNPs linked with glioma.
These 34 were then independently analysed by scientists in Germany, France and Sweden who looked at the DNA of 2,500 people with glioma and almost 3,000 healthy people.
This allowed researchers to pinpoint 14 key SNPs on five genes.
As people have two copies of each gene, one from each parent, they can have up to 10 of the variants.
The researchers said those who had eight or more were three times more likely to develop glioma than the general population.
Professor Richard Houlston of the Institute of Cancer Research, who led the study, said: "This is a major discovery.
"We've found the first real evidence that variations in the genes which many people carry can increase their risk of this deadly disease."
The team hope their findings can eventually be used to help identify those most at risk for the disease and also to provide potential targets for treatment or prevention.
In the second study, a team from the Mayo Clinic and the University of California San Francisco found a connection between DNA alterations on chromosome 9 and glioblastoma risk.
They looked at the DNA of over 900 glioma patients, and 4,000 healthy people.
It was found that people with three particular SNPs had a 50% increased risk of developing glioblastoma.
The same section of chromosome 9 was one of the five points identified by the Institute of Cancer Research study.
But the Mayo Clinic's Dr Robert Jenkins, who led the research, said having the SNPs did not mean someone was guaranteed to get the cancer.
"Increased relative risk is just that - relative."
A normal person's risk of developing a glioblastoma is about one in 10,000, but the researchers say the risk is about one in 7,000 for a person carrying one of these SNPs.
However, scientists in both studies say further research is needed to confirm their findings, and that it is too early to screen people for the genetic variations.
Professor Melissa Bondy, of the University of Texas, said: "We have only just begun to understand the causes of brain tumours.
"Our findings give reasons for hope for those who might be affected and an incentive for a more comprehensive investigation of what has been a mysterious disorder."
And Dr Lesley Walker, director of information at Cancer Research UK, which part-funded the UK study, said: "Compared with many other cancers, little is known about the lifestyle or genetic factors that influence the risk of developing brain tumours."
She said it was important to "unlock" some of the "genetic secrets" behind glioma.
"Identifying these genetic variants will open up new avenues for scientists to explore, helping them to better understand how gliomas develop, identify who might be most at risk and ultimately find improved ways to diagnose and treat the disease," she added.