Scientists have revealed that an error with a gene can result in some women failing to respond to breast cancer treatment.
Thousands of women diagnosed with breast cancer are given tamoxifen for five years after an initial round of treatment or surgery.
However, it is estimated that just two-thirds of patients actually benefit, while some women do not respond to the drug at all.
Researchers suggested the lack of response was caused by women having too much of the gene FGFR1 - which builds a resistance to tamoxifen among women with hormone sensitive breast cancer. This means their cancer has a greater chance of returning.
However, the research, led by a team from the Breakthrough Breast Cancer Research Centre at the Institute of Cancer Research (ICR), showed these women could be given new treatments under which the action of FGFR1 is switched off.
Around 4,500 breast cancers diagnosed each year are thought to have too much of the FGFR1 gene. The study was published in the journal Cancer Research.