Gene responsible for 70 percent of hard-to-treat breast cancers has been found by researchers.
The study used a new technique, which tested hundreds of genes at once, rather than one at a time.
Hormones can force tumour growth, so drugs that interfere with that process, such as tamoxifen and aromatase inhibitors, are used as treatments.
However, up to a third of breast cancers 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 that 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 percent 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.
"There is a lot of potential for a significant impact if a therapy targeting the serine pathway were found to be effective," lead researcher Dr Richard Possemato told BBC.
A detail of the study has been published in Nature.