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WASHINGTON - A genetic mutation that raises the risk of breast cancer is found in up to 60 percent of U.S. women, making it the first truly common breast cancer susceptibility gene, researchers reported on Sunday.

Reports from several teams around the world identified changes in four other genes that raise the risk of breast cancer significantly. Several are found in many men and women.

More than 60 percent of the women in the United States probably carry at least one of the mutations in one of the genes, called FGFR2, the researchers said.

"This is a truly landmark breakthrough for breast cancer research, because these genes are the first confirmed common genetic risk factors for breast cancer," said Jianjun Liu of the Genome Institute of Singapore, who took part in one of the studies.

The researchers, reporting in the journals Nature and Nature Genetics, said the discoveries are the most important genes associated with breast cancer since BRCA1 and BRCA2 were identified.

Women with faulty copies of BRCA1 or BRCA2 have a 50 percent to 85 percent chance of getting breast cancer in their lifetimes. But they are rare genes, and only account for 5 percent to possibly 10 percent of breast cancer cases.

Researchers have been testing women for other genes associated with breast cancer, to find its causes, to understand how and why it develops, and to make more effective treatments.

Better techniques to analyze DNA, and the publication of the human genome, the map of all DNA in the body, have made this a much faster and easier process.

David Hunter of Harvard University and a team at the U.S. National Cancer Institute looked at more than 2,200 women of European ancestry.

COMMON GENE, BIG RISK

They found four common mutations in FGFR2 associated with the breast cancer in women after menopause who do not have known relatives with breast cancer.

The mutations raise the risk of breast cancer risk by 20 percent if they carry one copy of the gene and by 60 percent if they carry two copies. And close to 60 percent of the women they studied carried at least one copy.

The findings do not yet have any real relevance for women, Hunter stressed.

"It is premature to recommend screening women for these gene variants, at least until the scientific community has further combed through the genome-wide findings and found all the variants that are associated with increased risk," Hunter said in a statement.

Douglas Easton of Britain's University of Cambridge led a team of researchers around the world to look at tiny changes in the DNA code called single nucleotide polymorphisms or SNPs -- pronounced "snips" -- in the DNA of 21,860 people with breast cancer and 22,578 people without it.

They found mutations in four genes that were more common in the people with breast cancer -- FGFR2, TNRC9, MAP3K1 and LSP1.

FGFR2 may be a logical candidate for a breast cancer gene -- it is a receptor, a kind of molecular doorway, for compound called tyrosine kinase which is involved in several cancers.

In a third study, a team at deCODE genetics, the University of Nijmegen in the Netherlands and elsewhere studied 22,000 people to find two other gene mutations associated with breast cancer. One is also near TNRC9.

"DeCODE estimates that these two variants are contributing factors in one quarter of breast cancer cases in women of European origin," the company wrote in a statement.

Breast cancer kills 500,000 people a year globally according to the World Health Organization, and 1.2 million men and women are diagnosed with it every year.