The findings could have implications for how each woman is treated
Scientists say they have linked a faulty copy of a gene with the development of breast cancer.
Researchers in London and Italy studying biopsies found that Tip60 did not work as actively in breast cancer tissue as it did in normal tissue.
Low Tip60 activity was particularly associated with aggressive tumours.
The scientists said their findings, which are published in Nature, had implications for the treatment of women with the disease.
If low levels of Tip60 activity suggest a woman has a particularly aggressive form of the disease, she could be treated accordingly.
"More aggressive types of breast cancers tend to recur after treatment, spread to other parts of the body, and respond less well to chemotherapy," said Dr Tim Crook, the team leader at The Breakthrough Breast Cancer Research Centre.
"The identification of Tip60's role in breast cancer is a step forward towards predicting the aggressiveness of the disease and then individualising chemotherapy for women."
One faulty gene
Tip60 is a so-called tumour suppressor gene: it helps to hold the growth of cells in check.
Scientists found that reduced activity of the gene contributed to uncontrolled cell growth, which in turn allowed breast cancer to develop.
They also found that reduced levels of Tip60 were caused by only one faulty copy of the gene.
When we are born we inherit a copy of the gene from our mother and our father. Typically with tumour suppressor genes, both copies must be faulty for the function to be lost.
But the scientists said that the Tip60 gene stops working if only one copy is faulty.
Antonia Dean of Breast Cancer Care said scientists were still a long way from fully understanding the role genes play in the development of breast cancer, "and how this interacts with or is mediated by other factors".
"This is initial research. Further studies are needed to fully determine how these findings can be translated into practical benefits for people with breast cancer and the exact role Tip60 may have in the development of the disease."