Testing for the activity of two genes could help doctors identify women at an increased risk of dying from breast cancer, research suggests.
Women whose tumours displayed a specific activity pattern were three times more likely to die within 10 years than those with a different pattern, a study found.
The two genes, known as F12 and STC2, are thought to play a key role in freeing cancer cells to spread around the body.
The study, conducted by scientists at London's Institute of Cancer Research, focused on almost 2,000 women with HER2 positive breast cancer, who make up about a fifth of patients.
Their tumours produce high levels of the protein HER2, which fuels cancer growth.
Women whose tumours had highly active F12 genes and low activity in the gene STC2 were found to have a 32% chance of dying within 10 years.
Those with low F12 activity and high STC2 activity had only a 10% chance of dying.
Lead researcher Dr Paul Huang, head of the Protein Networks Team at the Institute of Cancer Research, said: "Survival rates for breast cancer are now much higher than they were a few decades ago, but the disease remains deadly once it has spread round the body.
"Our study sheds light on how cancer cells unstick themselves from healthy tissue, and it could help pick out women at high risk of their cancer spreading and becoming fatal.
"We found that the activity of two genes which may help control how tightly cells are glued together is linked to breast cancer survival.
"If the results are confirmed in larger studies, it could give us a new way of assessing women's survival chances in the clinic, and adjusting treatment accordingly."
Professor Paul Workman, chief executive of the Institute of Cancer Research, London, said: "We have seen major strides in the treatment of breast cancer, but once it begins to spread round the body it is still often fatal.
"This new study helps us understand some of the processes that control how breast cancers spread, and identifies a pattern of genetic activity that could be used to pick out women particularly at risk."
The findings are published in the journal Oncotarget.
Catherine Priestley, clinical nurse specialist at the charity Breast Cancer Care, said: "This is a promising study which helps shed light on how specific genes in breast tumours can affect the cancer's ability to spread around the body.
"Anything that could assist in identifying those who are at an increased risk of their breast cancer spreading is a great thing.
"It is encouraging that this sort of research may help turn the tide in the treatment of breast cancer, and contribute to a future where more and more patients are treated in a way that is tailored to them."