A gene has been linked to 70% of hard-to-treat breast cancers which are resistant to hormone therapies, in US research.
The study published in Nature used a new technique which tested hundreds of genes at once, rather than one at a time.
Scientists said there was "a lot of potential for significant impact" if drugs could be developed.
Cancer Research UK said it would be interesting to see where the study led.
Hormones can force tumour growth, so drugs which interfere with that process, such tamoxifen and aromatase inhibitors, are used as treatments.
Up to a third of breast cancers, however, are not hormone driven, so these drugs do not work and there are fewer treatments available for these patients.
The researchers at the Whitehead Institute for Biomedical Research, Massachusetts, used small, disruptive, snippets of genetic material which can turn off genes.
They injected cancerous cells with the snippets to investigate which genes were necessary for tumour formation and growth.
They found that the gene - PHGDH -was highly active, far more than usual, in 70% of tumours which did not respond to hormone therapies.
Over expression of the gene results in the chemistry of a cancerous cell changing and is involved in the production of an amino acid - serine.
The hope is that by identifying the gene which leads to some breast cancers, a drug can be developed which interferes with its activity.
Dr Richard Possemato told the BBC: "There is a lot of potential for a significant impact if a therapy targeting the serine pathway were found to be effective.
"However, as we do not treat any patients in our study, or develop any chemical inhibitors of the pathway, it would be very premature to predict the response in the general population."
The technique used allowed researchers to analyse large numbers of genes, they said "the technological advance is one of scale".
Cancer Research UK's Henry Scowcroft said: "The more scientists delve into cancer's inner secrets, the more clues to future treatments they discover.
"This early work has identified a possible new avenue for future research into a hard-to-treat form of breast cancer, and it will be interesting to see where it leads."