In a small number of cancer sufferers, certain healthy cells can undergo a process whereby they turn off the cancer-fighting mechanism in neighbouring cells, robbing them of their ability to fight the disease.

The HKU team was able to identify the switch-like control that turns off the anti-cancer mechanism in 3 per cent of patients.

The researchers say the discovery improves the chances of spotting the cancer while it is still in the early stages, and allows them to test neighbouring cells to identify the mechanism.

Previously, doctors would have to rely on blood tests to detect HNPCC, but in a small percentage of people, the tests would only find healthy cells.

'This is a good gene in a bad neighbourhood,' said Professor Leung.

'Three per cent of cases had this mechanism,' she said, 'but if you use the same principle and apply it to all genetic diseases, such as breast cancer, 2 per cent of the total population are affected.'

Colonic cancer is the second most common cancer in Hong Kong, with 3,500 cases diagnosed each year. HNPCC accounts for up to 4 per cent of these.

Sufferers have few or no symptoms until late in the development stage, at which point it can be too late for treatment. In half of the cases of hereditary types of cancer, the disease can be passed on to sufferers' offspring.

'Genetic tests should not be limited to examining the disease-causing gene, but also extended to study the termination signals of the neighbouring genes,' said Professor Leung.

It is thought the study, published in Nature Genetics last month, will help many families suffering from various hereditary diseases and improve the success rate of prevention and treatment.

Queen Mary Hospital and the charitable Hereditary Gastrointestinal Cancer Registry at St Paul's Hospital now perform all of their tests with reference to the HKU study's findings.

Quiet killer

Colonic cancer is the second most common cancer in Hong Kong

The number of people diagnosed every year is: 3,500