A gene found in almost one in eight people may greatly increase the risk of breast or ovarian cancer, say experts.
Breast cancer kills thousands of women each year
If confirmed, this would make the TGFBR1*6A gene the most common "cancer susceptibility" gene yet found.
Researchers at Northwestern Memorial Hospital in Chicago found that carriers also had an increased risk of colon cancer.
The finding, published in the Journal of Clinical Oncology, may lead to better tests for those at risk.
A number of genes which appear to carry an increased risk of cancer have been identified by researchers - the best known are the BRCA1 and 2 genes, which are thought to account for between 5% and 10% of breast cancer cases.
The Northwestern researchers suggest that their gene may play a role in approximately 7% of all breast cancers, 11% of all ovarian cancers and more than 5% of colon cancer cases.
Carrying it may boost an individual's risk of developing breast cancer by almost half.
Dr Boris Pasche, who leads the team working on the gene, said that he felt that testing for the new gene was on the horizon.
However, more research was needed to see if there was a relationship between the new gene and the BRCA genes.
Dr Pasche said: "In the near future, it will be commonplace for people to know what genes make them more susceptible to cancer, and we'll have many more options for preventing those cancers.
"Most cases of cancer are caused by damage to the genes that builds up over a lifetime - but some people are born with a high risk of the disease.
"When inherited, the TGFRB1*6A gene makes people susceptible to having certain cells grow and divide uncontrollably, which may contribute to cancer development."