Scientists have found a way to identify which breast cancer patients are likely to respond to a type of chemotherapy.
The new test will also predict who is unlikely to see any benefit from the treatment and help to save them from unnecessary side effects.
The findings - presented by researchers at the European Breast Cancer Conference in Spain - mean doctors should be able to tailor treatment to the patients who will benefit and avoid giving toxic drugs to those that will not be helped.
By conducting a review of four large breast cancer trials, the researchers found that an abnormality on chromosome 17, called CEP17, is a "highly significant indicator" that the tumour will respond to chemotherapy drugs called anthracyclines.
Anthracyclines are anti-tumour antibiotics that interfere with enzymes involved in DNA replication. They are widely used against a variety of cancers.
The researchers found that if patients with CEP17 were treated with anthracyclines, they were around two-thirds more likely to survive, and to survive without a recurrence of cancer, than those not treated with anthracyclines.
The results provide more tools for doctors to make personalized, or tailored, medicine a reality in cancer care.
Our aim was to identify patients for whom anthracyclines provided benefit and to seek to ensure that future treatment was targeted to this group. The results suggest that only those patients with CEP17 tumours should receive anthracyclines.
CEP17 is on the same chromosome as other genes known to be involved in breast cancer, such as HER-2, and can be detected with a simple test.
The researchers say that the existence of a readily available test for CEP17 means doctors could immediately start to better tailor chemotherapy to patient needs.
They say extra work on CEP17 is needed to see if it could reveal more about breast cancers.
"CEP17 works as a biomarker for predicting response to anthracyclines, but we don't know why it works," says Prof Bartlett.
"Our next step is to discover this and to try to make the cancers that don't have the marker behave like the ones that do."