In a healthy BRCA2 gene, molecules from the gene attach themselves to Rad51 like Velcro strips.

BBC: How gene causes breast cancer

"When the BRCA2 gene is faulty, a cell's genetic information becomes increasingly unstable, making it more likely that cancer will develop, " he said.

BBC: How gene causes breast cancer

Scientists from the Centre for Research on Women's Health in Toronto led the study which looked at women with the BRCA1 and BRCA2 gene mutation.

BBC: Tamoxifen 'cuts genetic cancer risk'

Professor Ashok Venkitaraman and colleagues at the Hutchison Medical Research Council Centre in Cambridge, working with Dr. Luca Pellegrini and Professor Tom Blundell at the Cambridge University Department of Biochemistry, discovered that the BRCA2 gene controls a key molecule called Rad51.

BBC: How gene causes breast cancer

But for the average breast cancer patient who has neither the BRCA1 or BRCA2 gene mutation nor a close relative with breast cancer (Jolie's mother, for example, died from ovarian cancer), the risk of a new breast cancer in the nonaffected breast is less than 1%.

CNN: When removing breast is not the answer

The test could also help the one in 300 woman who carry a faulty gene known as BRCA1 or BRCA2.

BBC: DNA test reveals 80 markers for inherited cancer risk

By combining the gene test for BRCA1 and BRCA2 with this extra genetic information, women who have a high number of the newly identified markers could find they have a nearly 100% risk of getting breast cancer.

BBC: DNA test reveals 80 markers for inherited cancer risk

Ms Harris said that a family history of breast cancer was more likely to be a case of "bad luck" than a faulty gene, as hereditary breast cancer caused by genetic mutations such as BRCA1 or BRCA2 account for only 5-10% of cases of the disease.

BBC: 'A faulty gene caused my cancer'

For example, 23andMe tests for some gene variants involved in mildly raising breast cancer risk, but not genes called BRCA1 and BRCA2 that vastly raise breast cancer risk.

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