Research has identified a molecular "switch" in blood samples that makes a woman vulnerable to breast cancer.
The marker is linked to the BRCA1 breast cancer gene but was also present in those who did not have the mutation and later developed the disease.
BRCA1 and BRCA2 variants are responsible for 10 percent of breast cancers and are inherited from parents.
A woman with the BRCA1 gene has an 85 percent risk of developing breast cancer, resulting sometimes in the drastic step of breast removal.
This leaves around 90 percent of breast cancer cases unexplained with no credible way of predicting the likelihood of non-inherited breast cancer.
Scientists claim that the "switch" is part of the process by which certain molecules acting on DNA cause genes to be activated or deactivated.
"We identified an epigenetic signature in women with a mutated BRCA1 gene that was linked to increased cancer risk and lower survival rates," lead researcher Professor Martin Widschwendter, from University College London, said.
"Surprisingly, we found the same signature in large cohorts of women without the BRCA1 mutation and it was able to predict breast cancer risk several years before diagnosis.
"The data is encouraging since it shows the potential of a blood-based epigenetic test to identify breast cancer risk in women without known predisposing genetic mutations."
For their research the scientists analysed blood sample DNA from 119 postmenopausal women who later developed the disease over a period of up to 12 years, and 122 who remained cancer-free.