Researchers have found another breast cancer gene that can greatly raise the risk of the disease in women of European heritage, according to a report published on Monday.
They said the gene worked in tandem with the well-known BRCA1 and BRCA2 genes to raise the risk of breast cancer by as much as 80 percent.
The team, at Iceland's Decode Genetics, said their findings suggest women with certain mutations in two of the genes have an almost certain risk of breast cancer.
The new gene is called BARD1, Decode's team reports in the online journal Public Library of Science Medicine.
"The BARD1 variant works together with the BRCA2 mutation in Iceland and increases the likelihood of breast cancer from 45 percent in those who have only the BRCA2 mutation up towards 100 percent in those who also have the BARD1 variant," Decode chief executive Kari Stefansson said in a statement.
And women with the BARD1 variant who develop breast cancer are more likely to have tumors in both breasts, Stefansson said.
Breast cancer is known to run in families and has a genetic component, although most cases occur in people with no family history of the disease. Several genes are known to be involved, including the BRCA1 and BRCA2 genes, p53 and others.
The known BRCA1 and BRCA2 mutations only account for about 3 percent of all cases of the disease.
The Decode team is trying to find other genes that work with these genes.
They found 1,090 women in Iceland who had breast cancer, and compared them to 703 Icelandic women who did not. The company has a database carrying the genetic profiles of virtually everyone in Iceland.
A certain BARD1 mutation was found in 5.4 percent of breast cancer patients and 3.1 percent of women who did not have breast cancer - representing an 80 percent increase in risk, the researchers said.
Simply having the BARD1 mutation was not especially dangerous to a woman, but women who had it and a specific mutation on BRCA2 had a "dramatic" risk of breast cancer, the researchers found.
It may be worth developing a test for that particular combination, the researchers and the company said.
This BARD1 mutation has been found in Italian and Finnish families with a history of breast and ovarian cancer.
It has not been found in people of Chinese, Japanese, African-American and Yoruban descent, the researchers added.
"Therefore, the variant may be restricted to individuals with European ancestry and could contribute to the higher load of breast cancer seen in this group. However, other BARD1 variants have been discovered in African-American and Japanese individuals. The contribution of these variants to the risk of disease is still uncertain," they wrote.
Breast cancer is the second leading cause of cancer deaths among women after lung cancer, affecting 1.2 million women globally and killing more than 400,000 every year, according to the International Agency for Research on Cancer .