A gene test could reduce unnecessary treatment for women with breast cancer, research has found.
Different genes can be active in tumours
The test can show whether or not a woman with breast cancer is likely to respond to treatment with the drug docetaxel.
It would enable doctors to provide alternative treatment to those women who are likely to be resistant to the drug.
Chemotherapy or hormonal treatment after surgery for breast cancer is crucial for improving a woman's survival chances.
However, patients react differently to chemotherapy drugs and some women are resistant to treatment.
Until now there has been no way to distinguish between women who might respond and those who probably will not.
Docetaxel - known commercially as Taxotere - is one of the newest and most widely used chemotherapy drugs.
Although the drug can be an effective treatment, it is also associated with a number of potential side effects.
It can reduce the number of blood cells in the bone marrow, and cause liver damage.
There is also a risk that it can trigger an allergic reaction, which may lead to chest pain, dizziness or shortness of breath.
The researchers, from Baylor College of Medicine in Texas, took breast tumour biopsy samples from 24 patients before treatment, and then measured the way the tumour responded to docetaxel.
They found some genes were more active in responsive tumours, and different genes were more active in those that were more resistant to the drug.
Lead researcher Dr Jenny Chang said: "When validated, this type of molecular profiling could have important implications in defining the optimum treatment for individual patients, and reduce unproductive treatment, unnecessary toxicity, and overall cost."
Clara MacKay, of the charity Breast Cancer Care, welcomed the research.
She said one of the major problems with the treatment of breast cancer was that it was not often tailored to the specific needs of the individual.
"This means that some patients are having unnecessary treatments that will have little or no benefit.
"They may also have to experience the side effects of these treatments, which may have a long-term impact on their health."
The research is published in the Lancet medical journal.