Gene-screening is being used to identify women most likely to benefit from one type of breast cancer chemotherapy.
The technique could lead to a simple test enabling doctors to administer personalised treatment, say researchers.
In future the same method may offer a way of predicting which patients will respond to other cancer drugs.
An international team of scientists scanned 829 genes in breast cancer tumour cells.
They selected out six which, if missing or faulty, would prevent the chemotherapy agent paclitaxel working.
Dr Charles Swanton, from Cancer Research, said: "Health professionals may in the future be able to use this information to direct treatment to patients most likely to benefit -- and avoid giving treatment that is less likely to be effective to patients with drug resistant cancers.
"Ultimately similar approaches could reduce the cost of delivering cancer care whilst enabling improved patient access to beneficial treatments."
The new findings, reported in The Lancet Oncology medical journal today, suggest that half the women currently prescribed the drug could do without it.