A rogue gene that drives the spread of aggressive breast cancer could help scientists develop better forms of diagnosis and treatment.
The mutant gene, known as KLF6-SV1, may provide a test marker for more dangerous breast cancers.
Targeting it could also help to treat some women with a poor prognosis.
"Breast cancer is a genetically complex disease and it remains a challenge to predict disease outcomes and which patients may benefit from more aggressive treatment," said study leader Dr Goutham Narla, from Case Western Reserve University School of Medicine in the US.
"Our research has uncovered a promising gene marker that will not only help us better identify tumours that behave badly, but provide a basis for developing and personalising therapies to better treat our patients."
The gene is linked to disease recurrence and metastasis, the spread of cancer around the body.
Scientists looked at tissue samples from 671 breast cancer patients kept at a tumour bank in the Netherlands.
Women with high levels of the gene variant were 50% more likely to die from their disease.
The findings are published today in the journal Science Translational Medicine.
Dr Narla said: "More studies need to be done, but this could provide an important prognostic marker to determine which patients need to be treated more aggressively or watched more closely."