Scientists are developing a test to predict how breast cancer patients will respond to chemotherapy.
Gene controls tumour response
The key appears to be how well a gene called BRAC1 is working within tumour cells.
If the gene is defective, the tumour is likely to be more resistant to some drugs, but less so to others.
A gene test should help doctors tailor care accordingly, say researchers from the Breast Cancer Campaign and Cancer Research UK.
Not only could this make treatment more effective, it could avoid people being exposed to unpleasant side effects unnecessarily.
The researchers found that the fully-functioning version of BRCA1 made breast cancer cells ten to 1,000 times more resistant to one type of drug that works by damaging DNA within cancer cells.
However, it made the cells over 1,000 times more sensitive to a second type of drug that work by blocking cell division.
Lead researcher Dr Paul Harkin, of Queen's University, Belfast, said: "Our results were quite dramatic.
"Essentially, cancer cells with functional BRCA1 are highly resistant to one type of chemotherapy but extremely sensitive to another.
"For scientists like myself it's very exciting, because knowing a tumour's BRCA1 status may be invaluable in deciding which type of chemotherapy to use."
Fellow researcher Dr Jennifer Quinn, of Breast Cancer Campaign, said: "Breast cancers can differ greatly from one woman to the next, so finding ways of matching a treatment to a patient is one of the key areas of current research.
"Our study is part of a movement towards a new approach to breast cancer, in which women will be treated on the basis of information gathered about their individual tumours."
BRAC1 plays an important role in stopping cancer developing, and it is known that women who inherit a damaged version have a high risk of developing breast cancer.
Up to 80% of women who inherit a damaged version of BRCA1 will develop breast cancer at some point in their lives.
Approximately one in 850 women in the UK inherit a faulty BRCA1 gene.
However, inheriting the fully functioning version of the gene is not guarantee of avoiding problems, as the gene can be switched off.