Scientists have identified a further gene which increases a woman's risk of developing breast cancer. 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.
Identifying the first of a new set of breast cancer genes puts us in a much better position to tackle the cancer Professor Robert Souhami, Cancer Research UK 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.