Testing for a gene called ESR1 can show which women with breast cancer would benefit most from oestrogen-blocking drugs, Nature Genetics work suggests.
Hormones can fuel cancer growth
Women with extra copies of the gene, which carries DNA code for an oestrogen receptor, had tumours that were more likely to respond to tamoxifen.
An estimated 31,000 post-menopausal women are diagnosed with breast cancer in the UK each year.
In 80% to 85% of these cases, the disease is fuelled by oestrogen.
In these women, anti-oestrogen drugs such as tamoxifen, can slow tumour growth and cut the risk of the cancer returning.
Doctors can already check whether breast tumours are "hormone sensitive".
Now experts from Switzerland and Germany say a test for one specific type of oestrogen receptor gene - ESR1 - could help doctors identify cancers that would be particularly likely to respond to anti-oestrogen therapy.
They analysed 2,000 breast cancer samples and found a fifth of the tumours had extra copies of ESR1 gene.
In a small follow-up study of 175 women with breast cancer who were being treated with tamoxifen, they found that women with extra copies of ESR1 survived longer than those who did not, even though the tumours in both groups of women had at least one copy of ESR1.
Dr Emma Pennery of Breast Cancer Care said: "This research adds to our growing knowledge about the complexity of breast cancer, including the many variations of how breast cancer cells originate and what stimulates them to grow.
"We talk to patients on a daily basis who are encouraged by studies that add to this.
"Our increasing understanding means treatments can be increasingly tailored to the individual, helping to ensure people with oestrogen-receptor positive breast cancer benefit from improved outcomes."
Cancer Research UK's Dr Simak Ali, who is based in the department of oncology at Hammersmith Hospital, said: "If confirmed, this result should help us to identify people with breast cancer who are most likely to benefit from drugs like tamoxifen.
"Testing women for extra copies of ESR1 could also provide a way of identifying people with a higher risk of developing breast cancer."