Doctors have found a genetic trait which means they can identify which lung cancer patients will benefit from a new chemotherapy treatment.
The drug is used to treat tumours in the lung
Boston Dana-Farber Cancer Institute looked at the drug gefitinib, used to treat non small cell lung cancer.
This form of the disease is very hard to treat, but accounts for most UK lung cancers.
Writing in Science, the researchers say they are now working to develop a test which could screen for the gene fault.
Researchers had been trying to discover why some patients with the disease saw their tumours shrink rapidly after the treatment with the drug, whose brand name is Iressa, and others did not.
The team from the Massachusetts General Hospital Cancer Center found those who benefited from the treatment are more likely to have a mutated form of a gene in their tumours that assists cell growth.
The gene produces a protein called epidermial growth factor receptor (EGFR).
Gefitinib works by disabling the epidermal growth factor receptor (EGFR) on the surface of lung cancer cells, halting a sequence of signals that can lead to the uncontrolled growth characterising a malignant tumour.
The team from the Massachusetts General Hospital Cancer Center looked at lung tumour tissue from 58 Japanese and 61 American patients.
They found that the gene mutation was present in 15 of the samples from Japanese patients, but only one American sample.
Previous research has shown gefitinib is more effective in Japanese than American people.
Other tests showed tumour tissue from a woman with cancer that had
spread to the lining around her lungs - a condition called adenocarcinoma - was
very responsive to the drug.
When the tumour DNA was analysed, it was found to have the same EGFR gene
mutation as the wider research identified.
The scientists then analysed tumour samples from people who had been successfully treated with the drug and four who had not.
All those who benefited, and none of those who did not, had the gene mutation.
Dr Daniel Haber, who led the research, said: "This discovery will help us significantly improve the treatment of many lung cancer patients and is also an important next step in the molecular targeting of cancer drugs.
"We're hopeful that what we have learned will eventually lead to more new drugs that will target tumour-specific mutations without affecting normal tissues."
Dr Thomas Lynch, who also worked on the research, said: "These findings will help determine which patients will benefit from the drug and which should not receive it.
"In addition, if we know that a patient is likely to respond, we might be able to start treatment earlier with this drug that is more effective and has fewer side effects than standard chemotherapy.
Dr Stephen Spiro, a spokesman for the British Lung Foundation told BBC News Online the study suggested the Japanese, particularly women, were most likely to benefit from the drug.
But he added: "There is no doubt that treatment for cancer is going to have to be much more targeted."