Scientists have found genetic hotspots for bowel cancer they say will help doctors better treat the disease.
They say a third of the 37,500 people diagnosed with the cancer in the UK each year harbours the DNA code that renders common drugs ineffective.
Being able to identify these patients early will mean they can be started sooner on treatments that will work.
The next step, say the authors in the British Journal of Cancer, is to devise screening tests for the gene faults.
The hotspots all sit in a gene called K-ras which carries the DNA code needed to switch off and on cell growth.
Experts already knew that some bowel cancers were caused by faults in this gene that left the growth switch permanently "on".
And these patients tend not to respond to cancer drugs like cetuximab and panitumumab.
The University of Dundee researchers analysed 106 bowel cancer tumour samples to search for K-ras errors.
They found faults at three already recognised gene locations, and several new locations of the gene.
The scientists now believe as many as 33% of bowel cancer patients have a K-Ras defect - around 12,375 people in total, and 3,000 more than had previously been assumed.
Lead researcher Dr Gillian Smith said: "These results highlight additional gene faults which potentially could be tested for in bowel cancer patients to determine which people will respond best to which drugs."
She said the findings could also help experts develop new drugs to fight cancer.
Dr Lesley Walker of Cancer Research UK, who funded the work, said: "This important study shows how the most fundamental science can have a direct impact on the treatment of patients in the clinic.
"There is increasing interest in the identification of molecular markers to flag up the forms of cancer which would respond to a particular treatment and these findings could help health professionals plan and deliver more personalised and effective treatment for people with bowel cancer."