Scientists have identified a molecular "switch" in blood samples that increases a woman's chances of developing breast cancer.
The marker is associated with the BRCA1 breast cancer gene, but was also found in women without the mutation who went on to develop the disease.
Around 10% of breast cancers are caused by BRCA1 and BRCA2 gene variants inherited from parents, leaving 90% of cases unexplained.
But until now there has been no reliable way of predicting the likelihood of non-inherited breast cancer.
Lead researcher Professor Martin Widschwendter, from University College London, said: "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."