Scientists have identified a further gene which increases a woman's risk of developing breast cancer.
Identification of the gene may help assess a woman's breast cancer risk
An international study of 20,000 women found having a faulty version of the CHEK2 gene doubles their cancer risk.
UK women have a one in nine chance of developing breast cancer. Carrying CHEK2 increases that to one in four.
The American Journal of Human Genetics study said the findings brought a comprehensive genetic test of breast cancer risk a step closer.
Two other faulty genes, BRCA1 and BRCA 2, which increase a woman's breast cancer risk by between 50 and 80%, were identified in the mid-1990s.
Women can already be tested to see if they have inherited these genes.
Previous research had suggested there could be a link between having a faulty CHEK2 gene and developing breast cancer.
The normal version of the gene acts like a car mechanic, shutting down faulty cells so they can be repaired and do not pass on affected DNA.
Mutated versions of CHEK2 are unable to initiate this shutdown so faults in other genes are more likely to evade the body's repair process and continue replicating, potentially leading to a tumour.
'No family history'
In this study, an international team of scientists, led by experts from Cancer Research UK's Genetic Epidemiology Unit at the University of Cambridge, looked at the CHEK2 genes of 10,860 breast cancer patients and 9,065 healthy women in the UK, Australia, Finland, Germany and the Netherlands.
Their analysis confirmed that the faulty version of the gene, was more common in women diagnosed with breast cancer than in healthy women.
The variant was found in 201 women with breast cancer and 64 healthy women.
From these figures, the researchers suggest having the faulty variant more than doubles a woman's risk of developing breast cancer, whether or not there is a
history of the disease in the family.
The risk also appeared to be greater for women diagnosed at a younger age.
The gene is said to carry a 'low risk' in comparison with the BRCA genes, but unlike them, CHEK2 increases risk of disease in people who do not have a family history.
Scientists believe other 'low risk' genes also exist, each increasing breast cancer by a small amount.
Professor Doug Easton, who led the research, said: "Women with a strong family history of breast cancer can already receive genetic tests for the BRCA genes. "
But he said it was less clear when specific CHEK2 tests would be appropriate, because it was not just women with a family history who could be affected.
Professor Eason: "As we identify more genes that impact on hereditary breast cancer, we move closer to a comprehensive genetic test to accurately assess the risk of inheriting the disease."
Professor Robert Souhami, Director of Clinical and
External Affairs at Cancer Research UK, said: "Identifying the first of a new set of breast cancer genes
puts us in a much better position to tackle breast
cancer, both through testing high-risk groups and
eventually through new clinical strategies. "