The genetic code of the most common form of hereditary breast cancer has been mapped for the first time, offering hope for diagnosis and treatment of the disease in the future.
Researchers say they have "fully sequenced" the DNA of two breast cancers caused by a faulty BRCA1 gene, which is responsible for aggressive and highly drug-resistant tumours.
The team from the Breakthrough Breast Cancer Research Centre at the Institute of Cancer Research (ICR) say they hope that their work will lead to more tailored treatment for patients.
Dr Rachael Natrajan, one of the scientists involved in the study, said: "It is exciting to find new genes which could be involved in causing and driving breast cancer. Now these have been identified we have to do more work to find out the role that they play.
"Ultimately, this knowledge could help us develop new treatments that target the specific defects of each patient's disease."
Breast cancers genetically passed down through families account for up to 10% of all cases, affecting around 4,500 people in the UK each year.
The scientists said cases caused by the BRCA1 gene are "usually aggressive" and "do not benefit" from targeted drugs such as tamoxifen and herceptin.
The research, published today in the Journal of Pathology, found that despite both tumours being caused by the same source they mutated in almost completely different ways.
Professor Jorge Reis-Filho, who co-authored the study, said: "This research has big implications for how we treat hereditary breast cancer in the future.
"We often consider patients with a faulty BRCA gene as one group but our work shows that each tumour can look very different from each other genetically. Now we understand this, we can start to identify the best treatment strategies to save more lives of hereditary breast cancer patients."
Using the information gathered they then scanned the genetic code of several other breast cancers and discovered three genes, not previously linked to the disease, which they believe are worth investigating further.
The study also included teams from the Institut Curie in France, the University Medical Centre Utrecht in the Netherlands, The Cancer Research UK London Research Institute in London and the University of Nottingham.
Last week the ICR, writing in the British Journal of Cancer, said all women under 50 who are diagnosed with triple-negative (TN) breast cancer should be screened for the BRCA1 gene fault, which also carries with it an additional high risk of developing ovarian cancer.
It said the screening could identify hundreds of extra women every year who may benefit from tailored therapy.
Baroness Delyth Morgan, Chief Executive, Breast Cancer Campaign said: "These results reinforce our understanding that each breast cancer develops differently, even from the same inherited faulty gene, and so treatment options need to be tailored to each patient to ensure the best possible outcome. The work targeting the causes of breast cancer will be continued by Dr Rachael Natrajan with her new five-year scientific fellowship, funded by Breast Cancer Campaign."