Body of evidence
When Constable Leung Shing-yan was shot dead at a Tsuen Wan housing estate in 2001, DNA left on a surgical mask at the scene helped police link the killing to the 2006 Tsim Sha Tsui underpass shooting.
When serial rapist Cheung Shing attacked five young women after luring them into a stolen taxi outside Kowloon karaoke bars, again it was DNA that helped put him behind bars in 1996 for 24 years.
From a flake of dandruff to the saliva left on a postage stamp, the multiple ways people unwittingly leave a DNA trail has proven a curse for criminals and an invaluable tool for police forces worldwide. It has also solved immigration and paternity cases and identified victims of the 2004 tsunami.
Last week the man who pioneered the science that revolutionised crime investigation revealed the latest developments and predicted how DNA may be used in the future.
Delivering the Shirley Boyde Memorial Lecture at the University of Hong Kong, British geneticist Sir Alec Jeffreys spoke of how his unexpected discovery has swept the world, but also raised concerns that innocent people's DNA was being stored by police.
Sir Alec recalled the moment when the DNA identity concept dawned on him - 9.05am on September 10, 1984.
'That was a eureka moment. That was the moment when, entirely by accident, we came up with the idea that DNA fingerprinting was possible,' he said.
DNA may now be synonymous with forensics but Sir Alec, a professor of genetics at the University of Leicester, said there was 'never a forensic thought' in his mind in the years leading up to the discovery. Even when he made the breakthrough, he failed to comprehend its full potential.
'My feeling was that it would take years, if not decades, before this would move into real casework and it would always be a technology of last resort, not first resort,' he said. 'I could not have been more wrong on both counts.'
DNA profiling quickly became the most common form of forensic analysis, and collecting DNA is now one of the first things police do at a crime scene.
A year after Sir Alec's eureka moment, DNA fingerprinting was put to the test in an immigration dispute. It proved that a young boy who was facing deportation to Ghana was the son of a woman who had British citizenship.
'We managed to bring a family back together,' Sir Alec said.
Shortly after, DNA was used to resolve a paternity case and then, in 1988, it was called on to help solve a chilling rape and murder of two girls in Britain. Police had a confession for one of the murders but DNA testing was able to prove their suspect was innocent. They then conducted the first mass voluntary collection of DNA samples, which unmasked the real culprit. Colin Pitchfork became the first criminal to be convicted of murder based on DNA evidence and was sentenced to life in prison.
'Without DNA I'm fairly sure that he would not have been caught and would have gone on to kill again,' Sir Alec said. 'That was quite sobering stuff.'
The technology is in the headlines in Hong Kong this week as the Hospital Authority and the Social Welfare Department have issued appeals for people to come forward and give DNA samples to help solve the riddle over a baby mix-up in a Sai Ying Pun hospital in 1976. Kelvin Li Kwok-yin, now 31, is searching for his biological parents and the authority wants women who had babies at Tsan Yuk Hospital between November 28 and December 14, 1976, to come forward for testing.
The success of using DNA to identify criminals led to the establishment of databases containing criminals' DNA profiles, with Britain starting the first in 1995. According to Sir Alec, if DNA is retrieved from a scene, police have a 54 per cent chance of finding the prime suspect on the British DNA database.
DNA testing has been a boon for police, but Sir Alec is concerned about the darker side of his discovery. Not only does the British database contain the DNA of those convicted of crimes, it also stores the details of anyone arrested even if they are proven innocent. 'We have a situation in the UK where the DNA database is now populated not only by convicted criminals but by hundreds of thousands of innocent people,' he said.
Sir Alec, who was knighted in 1994, cites the case of a 14-year-old boy who stole a few pence worth of sweets. Police took a mouth swab and his DNA profile was uploaded onto the database. 'His DNA will stay there until he is 100 years of age and then it will be destroyed.'
Sir Alec said he was 'wholly opposed' to the idea of police holding DNA profiles of all citizens.
Hong Kong police have been legally allowed to collect DNA samples from suspects and convicted criminals since 2001. The city's DNA database now includes the profiles of more than 21,000 people, including more than 18,000 with convictions, 800 suspects and 2,400 people who gave their DNA voluntarily. More than 800 people have been linked to various crimes by DNA technology since the database was established in 2001.
The DNA of those convicted of an offence punishable by seven years' jail or more will be kept indefinitely. The DNA profile of anyone found innocent is deleted immediately, but people who voluntarily donate their DNA must request that their profile be removed from the database.
Chief Superintendent of the Crime Support Unit Albert Ng Kam-wing said DNA profiling was one of the most compelling methods of forensic analysis.
He said a number of serious cases had been solved as a result of DNA profiling, and cited the Tsim Sha Tsui underpass shooting as a case where DNA evidence was 'of corroborative assistance'.
'DNA technology can be utilised to enhance the crime-fighting capability of police as it can unearth new clues from biological evidence that cannot otherwise be found by conventional means,' he said.
Deputy director of public prosecutions Kevin Zervos SC said DNA profiling had been a great development for the criminal justice system.
'The monumental advance that was made with fingerprinting has now been eclipsed by DNA profiling,' he said. 'It's reliable evidence that in certain cases is conclusive of guilt.'
He recalled a case in which a 14-year-old girl was sexually assaulted by a taxi driver. The driver had touched her breast and days later managed to phone her. Police traced the call and found the man, who initially denied touching the girl. But DNA found on skin tissue left in the girl's bra cup belonged to the man. The man later pleaded guilty.
Although DNA may be best known for solving rape and murder cases, Mr Zervos said it could also be used to prosecute white-collar crimes - in a fraud case, for instance, to prove that a person had handled certain documents.
He said growing awareness of DNA was putting people on notice that if they engaged in criminal activity there was a chance their DNA could be recovered.
'It acts as a general deterrent and a disincentive to commit crime,' Mr Zervos said.
Forensic pathologist Philip Beh Swan-lip said Sir Alec's work had revolutionised crime investigation. Without his discovery, Dr Beh said, police would still be relying on fingerprints, police line-ups and blood stains to identify criminals.
'He was the one who made it possible to use DNA in a simple way, to break down DNA into manageable data to help identify a person,' said Dr Beh, an associate professor at the University of Hong Kong's pathology department.
Although the use of DNA has raised complex ethical issues, one of its greatest triumphs has been in helping to overturn wrongful convictions. This is especially so in the US, where the Innocence Project has used DNA to exonerate more than 200 people. Sir Alec described meeting one of the beneficiaries, a man who had served nine years in jail, including two on death row.
'It's an extremely emotional moment when you meet someone whose life has been saved, literally, by DNA,' he said.
Despite such successes, Sir Alec sounded a note of caution regarding how DNA is used by the courts. He said people should not assume DNA was infallible.
'It's always important for any court to think about the nature of DNA and whether it's relevant to the criminal act,' he said.
Sir Alec said the 'holy grail' in DNA identification would be an instant read-out. It still takes hours to get a DNA reading, although scientists have made impressive gains. Sir Alec cites the speedy identification of former Iraqi leader Saddam Hussein as an example.
'It was a mere 16 hours from him being taken out of that hole in the ground before it was confirmed that it was Hussein,' he said. 'If you could get it much faster, not in hours but in minutes or seconds, that would open up a massive new area.'
Real-time identification could pave the way for immigration officials to identify people by their DNA as they cross borders. Scientists are also exploring how DNA can be used to reveal a person's physical appearance.
Whatever the future holds, DNA technology has already taken on a life of its own, and taken Sir Alec along for the ride. It has shaped the life of a modest scientist who received his first chemistry set at the age of eight and soon after got what he calls his 'badge of honour' - a scar from a splash of sulphuric acid.
The scar is now covered by a beard, but Sir Alec retains the same fascination with science that propelled him from childhood to his eureka moment and all the subsequent successes of his breakthrough.
'If you had told me all that 20 years ago I would have been frankly astonished,' he said. 'It's been a bit of a fairy tale for me, but a good fairy tale.'