Scientists have identified another damaged gene which they believe can increase women's risk of developing breast cancer.
Inheriting a faulty version of a gene called CHK2 vastly increases the risk of breast cancer, according to a team funded by the charity Cancer Research UK.
The scientists believe that the healthy form of the gene helps to repair damage to genes within breast cells that, uncorrected, can lead to cancer. But when damaged itself, this gene, like two other faulty genes already identified, will not do the job.
The researchers, based at the Institute of Cancer Research in London, Cambridge University and the Erasmus Medical Centre in Rotterdam, hope such discoveries will pave the way for improved genetic testing for breast cancer and lead to better ways of preventing and treating the disease, which is diagnosed in nearly 40,000 women each year.
About half these cases are thought to include an inherited factor, although previously only two genes had been specifically identified.
It was already known that the faults in genes BRCA1 and BRCA2 greatly increased the chances of a woman developing the cancer, but these accounted for only about 2% of breast cancer cases.
CHK2 is more prevalent in its abnormal form, but does not seem quite so vicious when things go wrong and probably accounts for 1% of all breast cancer cases.
But scientists believe that inherited risk more often depends on a combination of faulty genes, each with a modest effect, which is why it is regarded as so important to identify conclusively more of them.
The scientists, who report their discovery in the journal Nature Genetics today, studied how often women with breast cancer had inherited a CHK2 gene with a piece missing. They compared 1,071 breast cancer patients, who had a family history of the disease but did not have faulty BRCA genes, with a group of 1,620 healthy women.
Among the patients, 5.1% had inherited the faulty version of CHK2, compared with only 1.1% of healthy women. The presence of the faulty gene does not automatically lead to breast cancer, but in families where some members had inherited the faulty gene it was linked to breast cancer more often than would be expected by chance.
Michael Stratton, of the Institute of Cancer Research, said: "Predisposal to breast cancer often comes with the additive effect of a number of genes, each of which on its own has just a small effect.
"Our research provides the first well substantiated example of one of these additive genes, although we need to learn much more about it before we can consider using this information in clinical practice."
