For years, cancer doctors have known that many patients aren't helped by the drugs they're given. For instance, only 20 per cent of those with advanced lung cancer will benefit from certain drugs.

Even among those receiving a very effective drug such as tamoxifen for breast cancer, 30 per cent don't respond.

The trouble is that, until now, there hasn't been any reliable way of working out who will do best on which treatments. But that is about to change. An important study has found that people with a certain type of gene won't benefit from selected cancer drugs.

This is hugely significant because it means gene tests can be used to work out which patients will do best on a drug, and as a result cancer treatments will become much more effective.

Meanwhile, thousands of patients will be saved from the debilitating side-effects of chemotherapy they are given without gaining any benefit.

And because doctors will be able to better target expensive treatments, drugs currently not recommended by NICE because they are not cost-effective could soon be re-evaluated.

'It's not so much a new dawn, more that someone has opened the blinds and let the sun in.' That's how Dr David Sidransky, a leading cancer specialist, has described the significance of the research, which was presented at a major cancer conference in America earlier this month.

The gene involved is called K-ras - it affects the rate at which tumours divide and spread. Researchers have suspected for some time that a common, faulty form of the gene stops certain drugs working.

'The faulty gene could be carried by as many as 40per cent of people with lung and colon cancers,' explains Professor Ian Cree, a pathologist at the Translational Oncology Research Centre at Queen Alexandra Hospital in Portsmouth.

'What's new is that now we've got the evidence showing if you target treatment at people who have a "neutral" version of the gene, you get much better results.'

Recent studies have linked many other genes with poorer or better responses to chemotherapy. Leading experts now hope that the success with K-ras will stimulate proper gene testing, and the effect on treatment could be dramatic.

'We are on the edge of a whole new era of molecular diagnosis,' says Professor Karol Sikora of Imperial College School of Medicine and one of the UK's leading cancer experts.

'Eventually, cells from your cancer will be tested for a whole range of genes and biomarkers, and a computer analysis will tell you the balance of benefits and side effects of various drugs.'

In fact, doctors already do one routine gene test on the NHS. Women with breast cancer are only offered the drug Herceptin - which can cut the chances of the cancer coming back by 50 per cent - if they have the HER2 gene. If you don't have the gene, the drug won't do you any good.