Two new breast cancer genes, of which one reduces the chance of developing the disease while the other increases it, have been identified by an international team of scientists led by Cambridge University researchers.

The new study, published in Nature Genetics, found that one of the DNA segments increases breast cancer risk by about 12 per cent in those who carry one faulty copy of the gene, and by 23 per cent if they carry two mutated copies.

The other region reduces the risk by about 4 per cent in women with one faulty copy of the gene and by 11 per cent with those with two mutated copies. Professor Doug Easton, lead author and director of Cancer Research U.K.'s Genetic Epidemiology Unit at Cambridge University, said, "These two new genes bring us closer to developing a better test to identify women who are at a high risk of developing breast cancer, but there are still many more pieces of the puzzle to find."

Researchers have identified 13 genetic regions with common changes that alter breast cancer risk. The increased risks acquired from these faults are small, but as more are found it will be possible to create tests for combination of genes that significantly increase the risk.

Such tests could help doctors make better decisions about prevention, diagnosis and treatment of women who are more likely to develop breast cancer. The research team scanned the entire genetic code in more than 400 women with breast cancer to identify faulty genes that appeared more regularly in the patients with the disease than in healthy women.

They then tested the most promising regions in more than 40,000 women with breast cancer, and 40,000 without the disease, in an international collaboration involving more than 100 scientists from 16 countries.  One in five women.

In the general population are thought to carry two faulty copies of the marker found to increase risk — a gene known as rs4973768. One in 14 women are thought to carry two faulty copies of the marker that decreases risk, known as rs6504950. Breast cancer is the most common cancer in the U.K. with more than 45,500 new cancers diagnosed each year.