Scientists have pinpointed a telltale set of genes which appear to predict if breast cancer is going to spread to the lungs.
The researchers looked at how breast cancer spreads
The team from the Memorial Sloan-Kettering Cancer Center say the genetic "signature" could help predict how serious the disease might be.
Writing in Nature, they say their finding could help doctors offer more accurate prognoses to cancer patients.
They add it might also provide a new target for breast cancer treatments.
Researchers traced the genes by injecting mice with cancer cells from a patient with an aggressive and metastatic breast tumour - one which had spread.
They then watched to see which cells migrated to the animals' lungs and analysed the genetic make-up of those cells.
Research was carried out on mice so that scientists could follow the biological process which caused a breast tumour to metastasise to the lung.
The scientists identified a "thumbprint" of genetic activity involving 54 genes that appeared to be particularly associated with lung metastasis.
Some genes functioned more vigorously than normal, while the activity of others was suppressed.
The same genetic pattern was found in a group of 82 early stage breast tumours removed from patients.
More than half the patients with this "thumbprint" went on to develop lung metastases, compared with only 10% whose primary tumours did not carry the gene set.
The same research team previously identified different genes that help breast cancer cells spread to bone.
The lead researcher, Dr John Massague, who also carries out research at the Howard Hughes Medical Institute, said: "Our work shows that the ability of a tumour to form metastases depends on the combined action of multiple genes - and a different set of genes is required for each organ the tumour spreads to."
He added: "If you can successfully target these genes with a drug, you are helping slow the growth of any primary tumour and also blocking the growth of any tumour cells that have spread to the lungs."
He said the way doctors managed the cases of cancer patients was different for those likely to relapse, so knowing who was at risk could improve care.
Knowing which organ the cancer could return in would also allow doctors to keep watch for early signs of the cancer returning.
The researchers say they will now look at more patients with breast cancer, to see if a larger study will confirm their initial findings.
They will also search for gene patterns in other forms of cancer which often spread to the lungs.
Henry Scowcroft, of Cancer Research UK, said: "This research is very interesting.
"Understanding exactly how cancer cells spread around the body is extremely important, as this is often why cancer becomes a life-threatening disease."
He said if the team were successful in identifying exactly how the genes, and the proteins they produce, were involved in aiding cancer spread, it would open up " very exciting prospects" for improving breast cancer care.