Testing for an abnormal protein in the blood could identify men with prostate cancer who cannot be helped by two new drugs.
Patients whose tumours contain a shortened protein called AR-V7 do not respond to enzalutamide and abiraterone, research has shown.
The protein is detectable in the blood, raising the possibility of a test that could help doctors plan the best form of treatment for men with advanced prostate cancer.
Enzalutamide and abiraterone work in different ways to tackle prostate cancer that has become resistant to standard hormone treatments.
In a new study, no patients who tested positive for AR-V7 responded to either drug. A large proportion of those who lacked the protein did respond.
Enzalutamide and abiraterone have proved highly successful in extending the lives of 80% of men with advanced prostate cancer, but do nothing for the other 20%.