"We suggest for those who have family members with the disease or with cause-unknown mental retardation to check for whether they are carriers," the university's obstetrics and gynaecology professor, Leung Tak-yeung, said.

As the gene defect was transmitted mainly from a carrier mother to her son, women in doubt could consider undergoing the test before coming to a decision on whether to have babies, he said.

The blood test can also be conducted on pregnant women. If they are found to be carriers, an amniotic fluid test can be conducted for the fetus.

Screening for Down's syndrome has become common and is included in the public health service, but the technology for Fragile X is new. It uses a polymerase chain reaction test that can swiftly make multiple copies of a specific DNA segment.

The conventional diagnosis for Fragile X syndrome is a more labour-intensive and time-consuming test, which is used mainly for children with cause-unknown mental retardation and their mothers who may want to get pregnant again.

Now, with the more efficient test, anyone in doubt can choose whether to be screened.

The university, which offers the tests as a private service at the Prince of Wales Hospital in Sha Tin, charges HK$800 for each test, which can give results in six hours. It can do 100 tests a day and will be receiving referrals from private doctors.

The disease-causing gene mutation is located on the X chromosome in different levels of severity, leading to different chances of transmission from mother to son. The test can also identify the different levels of severity in a carrier.

During trials in the past few months, the university found 20 carriers, 14 full mutations and 75 normal cases with a 100 per cent accuracy.

It is estimated that about one in up to every 1,000 women carry the gene defect without showing any symptoms of it.