FOR construction worker Cheng Yu-tong, last month should have been a proud and happy one. It had shaped up as a good summer, one which would at last reunite his small family. For 10 years, he had snatched long weekends such as this to rush to Guangzhou - queueing at Lowu in blistering sunshine or thunderous rain squalls and bearing gifts during cold Lunar New Years - to be with his mainland wife. Since their Hong Kong-born son started school, moving in with him two years ago, Mr Cheng had known the strain of being a single father and looked forward to having his burden eased. His wife finally received her single-entry visa last month and the family started a new era in their Shamshuipo flat. Yet, within weeks, Mr Cheng had begun to wonder about his seven-year-old son. The boy had his mother's eyes and jawline, but seemed to bear little resemblance to his father. As in any cross-border relationship, the couple had spent more nights apart than they had together. On the work site, he had heard tales of infidelities on both sides of Lowu, and his thoughts had taken a troubling turn. 'Is this my son?' he wondered. Mr Cheng, his wife and their son are a composite picture, drawn from many similar cases. The difference today is that men like Mr Cheng are acting on their suspicions. Cross-border families form the largest proportion of customers paying for Hong Kong's first commercial DNA paternity tests. As Mr Cheng ponders his wife's loyalty and his son's appearance, he gambles the happiness of his family on three slim vials of blood in a Sha Tin laboratory. The futures of scores of families across Hong Kong rest on the results of tests on the DNA in the blood. Some will be reassured, others pained. 'DNA FINGERPRINTING', proclaims the advertisement. 'Most advanced technology for paternity testing. Can be applied to immigration disputes and civil litigation.' For the first time, anyone with a few thousand dollars can walk into the Mid-Levels office of a commercial firm and ask for DNA fingerprinting of themselves, their partner and their child - no questions asked. But while many welcome the power of DNA fingerprinting to cut through bureaucracy, others question whether an overdose of technology could prove fatal to families. For years, the Government Laboratory has used DNA tests in criminal forensic work and to establish identities and relationships, often at the request of the courts or other government departments. But Molecular Technology Innomed (MTI), with laboratories at Sha Tin and an office just an uphill stroll from Lan Kwai Fong, is now offering the technology to all comers. 'Most of our work is paternity testing,' said business development manager Terence Lau Lok-ting, a molecular biology graduate from the University of Science and Technology. 'It's a kind of molecular diagnosis. It's just starting in Hong Kong and we think it's the most important industry in the coming century. 'We tried to see if there was a need in the Asian market before we offered this service. In recent years, a lot of people have gone to China to get together with mainland women and this has generated a lot of cases.' Demand is quite high and growing from men whose pregnant mainland girlfriends press them for marriage or financial support. Husbands, reunited with their mainland wives and families after long separations, also want to check whether their children are really their own. 'Some don't know themselves,' Mr Lau said. 'Women want to figure out who is the father. Sometimes a mother, a child and two men come in together. We don't ask the reason, but we can guess.' Blood samples are taken from each participant - or amniotic fluid if the child is still in the womb - and results are available in two to three weeks. For the usual three tests - mother, father and child - the charge is $7,000. There are other reasons for buying a DNA analysis from MTI. Many cases are referred by consulates or law firms to establish identity when a person has lost a birth certificate or documents are unclear. 'For example, in immigration, if documents can't prove the relationship between two people, or they have lost their birth certificate, the only way to prove it to the officials is DNA testing. Genetics don't change,' Mr Lau said. The Legal Aid Department last year commissioned the company to do DNA tests on illegal child migrants. 'They used several sample cases, but after that everyone who lost documents went through law firms,' Mr Lau said. Some walked in directly and asked for the test. 'We also accept cases from law firms for property succession. If someone dies and a person turns up and says he's their son, we can test the siblings and prove whether he is a child, a full sibling.' It has taken more than a decade for DNA fingerprinting technology to be offered commercially in Hong Kong. Many wonder whether it should be in the marketplace at all. 'It's most unfortunate that it seems we've reached a stage of doubting the closest relationships you've ever had - those with your family,' said Priscilla Lui Tsang Sun-kai, spokeswoman for Against Child Abuse. 'It's a challenge to the existing family system and it deserves serious attention. 'I think the mainland aspect may have contributed to one aspect of this but, even locally, people may face the same concerns. The doubts and uncertainties cast a huge shadow on families, so that we cannot rely on family relationships, we have to rely on technology.' To a child who has grown up trusting in the love of a 'father', the trial of undergoing a blood test to determine whether he is his father's son must seem baffling, Mrs Lui said. How could an antiseptic-smelling clinic, a painful needle and a vial of blood cancel the relationships which are the bedrock of his life? 'In the past, it was in extraordinary circumstances that you would go for DNA testing. Now, it can be simply if you doubt your partner's loyalty,' Mrs Lui said. 'To commercialise the whole system is quite scary. I don't think this industry should be encouraged at all because of grave concerns for families.' Chinese University Professor of Paediatrics Patrick Yuen Man-pan said he saw troubles ahead: 'It can create a lot of hard feelings. I can see the problems coming, with everyone suspecting their wife of having an affair. You cannot force your wife and child to have their blood tested. They have rights, too.' Yet there are occasions where DNA fingerprinting can shore up a family, helping to bind it together against a common foe. Nineteen-year-old Xu Heng-lun - who became a test case in the fight of child immigrants against the Government last year - used blood to try to prove his identity. When the Government refused to accept the relationship between Mr Xu and his Hong Kong-born father - because his father's name at birth was Xu Gu-lun, but he later went by the name of Xu Gu-sheng - it asked the teenager to take a DNA test. The Immigration Department has not yet accepted the DNA evidence, and cases are scheduled for a further hearing in the Court of Final Appeal in January. ANOTHER case, in the High Court on Friday, involved a 12-year-old Hong Kong boy whose father's name had been omitted from his birth certificate. 'He's seeking to get an ID card now, and the Immigration Department is saying: 'We don't believe you're the son of this man',' said barrister Bill Clarke. The boy's mother sneaked across the border in 1985 to give birth. At the time, the family feared his father could be prosecuted if he was named on the birth document, so his name was left off. Armed with their DNA fingerprints, father and son entered the court on Friday morning, where Deputy Judge Saunders made an order declaring the father-son relationship. 'As far as I know, this is the first time Section Six of the Parent and Child Ordinance has been used to declare parentage according to DNA evidence,' Mr Clarke said. 'With this new technology, it's so much more certain. There can be no corruption influencing the production of paper documents. 'It's a very useful tool in Hong Kong immigration cases. It's shown to a probability of well over 99 per cent that they are the child of the parent,' the barrister said. But the Immigration Department is loathe to open the doors to science, instructing parents to collect copies of their children's birth certificates from mainland hospitals. 'I don't think it will be a problem for them to get a birth certificate from the relevant hospital in China,' said Immigration Department spokesman Alvin Tam Ho-man. 'The parents, or some relatives, can go back to get a copy. 'Immigration officers are very cautious about the genuineness of relationships between the applicant and their parents. Documentary evidence is always required. A blood test is not a substitution.' Senior government chemist Dr Betty Law Man-yee offers figures which show DNA fingerprinting - to the layman at least - as a highly accurate technique. 'We would never say that someone is the father, but if we get over 99.8 per cent [correlation], we would say it is extremely likely he is the father,' said Dr Law, who runs one of the Government's two biochemical sciences sections. Her department handles some paternity cases, when asked by the courts, law firms, the Social Welfare Department (SWD) or, in some cases, private individuals. 'Basically, it's to establish 'is this my child?' for reasons of custody, to assist the SWD on the future of the child,' Dr Law said. 'In those cases, they don't have the necessary documents.' Dr Law has no reservations about the commercial acceptability of paternity testing, as long as quality control of the laboratory systems and staff is upheld and clinics have international accreditation. 'It's a free market. There's supply and demand,' she said. 'A lot of reasons can break up a family. This, I'd admit, is one. But there should be a lot of other things that bind a family together. I wouldn't think a DNA test would split up a family. 'I'm aware that, these days, human relationships have deteriorated and there's not a lot of trust between people. It's sad, but it's a fact of life. That's my personal view.' The answer, Dr Law says, lies in teaching people the value of trust in loving relationships, rather than shooting the white-coated messenger. 'Technology has advanced to a stage where, for not a lot of money, we can get useful information . . . It can be quite frightening,' she said. 'It's a very thin line between good and evil as to how you use that technology. I would hate to be in a position to judge whether I should carry out a test for this person or not.' Family Law Association executive member Stephen Peaker said he can see both sides One scenario would be a man who is being pursued for financial support on behalf of a child he has never treated as his own. The other is a family man who uses DNA testing to see whether he can absolve himself of responsibility for the children who have grown up as his family. 'You have to look first and foremost for the interests of the child,' Mr Peaker said. 'If the child has regarded this man as his father for seven years, after seven years of treating someone as your father - then finding out they're not your father - it's going to be very traumatic. 'As far as children are concerned, testing should not be done unless absolutely necessary. 'DNA testing is highly undesirable unless there's a real uncertainty whether the child is the child of the father in question.' As for Mr Cheng, his wife and his son - and many like them, waiting to discover their future - what can they expect from this scientific judgment? Back in the Central offices of MTI, Mr Lau draws a breath and shrugs. 'Most of them match because they have confidence before they do the test,' he said. 'Most, but not all.'