Two British men who have not been convicted of any crime are challenging the police's legal right keep samples of their DNA and fingerprints at the European Court of Human Rights.
The men, both from Sheffield, are just two of the many thousands of people arrested since 2004 who have not been charged. The genetic signatures of those people have become part of Britain's growing and permanent police database.
The government says a large database will help police solve crimes - DNA evidence was crucial in securing the conviction and life prison terms of killers Steve Wright and Mark Dixie last week.
But critics say retaining the details of innocent people is both unfair and prone to abuse.
One of those is 14-year-old Kathryn Lay from Maldon in Essex.
She used to have huge respect for the police, especially because her late father served as an Essex police officer for more than 20 years.
But that respect evaporated after she was arrested and released without charge, because the police refused to destroy her DNA sample.
Her mother Angela explained how she was arrested. "Last year Kathryn was on a school bus with her classmates," she said.
"There were some typical high jinks - but things went a bit too far and one of the boys complained to the police and named Katherine and a couple of other girls - even though they hadn't been involved.
"Kathryn was asked to come to the police station to help with inquiries - but when she arrived she was arrested; they took DNA and put her in a police cell.
"After about 40 minutes, everyone realised a mistake had been made and she was released without charge.
"When I learned that Essex police intended to hang on to her DNA samples I complained - but I was told it was 'policy' to retain samples."
Mrs Lay described the police move as unfair and suggested the government was using underhand tactics to expand the database.
"I don't have a problem with a database that has everyone's DNA on it," she said.
"But to collect samples via the back door, and to lump in my daughter with real criminals is not right."
A spokesman for Essex Police said he was not able to comment on Kathryn's case.
But he said the force, like others in England and Wales, followed "strict criteria" in collecting and retaining the DNA of anyone arrested for a "recordable offence" - one that is deemed serious enough to be held on the Police National Computer.
The government changed the law in 2003 to allow police to keep the DNA of anyone arrested - but not charged - for a recordable offence, and it came into effect in January 2004.
The change was brought in as part of a wider reform of the criminal justice system in England and Wales.
It was justified by ministers as a means of helping police solve more crimes.
But the Liberal Democrats have criticised the current system as "fundamentally blurring the line between guilt and innocence" - and other critics suggest it was never properly debated.
Gareth Crossman, of civil liberties group Liberty, said he hoped that if the men won their case in Strasbourg it would "mark the beginning of a rational debate on whether permanently retaining samples from innocent people can be justified".
Dr Helen Wallace, of the pressure group GeneWatch, is firmly convinced the law should be changed again, so only criminals' samples are on file.
"The danger with keeping the samples of innocent people is that it might one day be misused by government, or fall into the wrong hands," she said.
One scenario now being painted by critics is that of clever criminals throwing police off the scent - or even framing innocent people - by deliberately contaminating crime scenes with bogus DNA.
"Vulnerable people could be put at risk, and there is also a real concern that once you start putting the good guys on a database, then the bad guys can misuse that that to their advantage," said Dr Wallace.
Innocent people who try to get their DNA profiles removed from the database often face a fruitless struggle.
Advice from the Association of Chief Police Officers (ACPO) states that "exception cases will by definition be rare".
One man who did succeed was Jonathan Cann of Twickenham in Middlesex, whose son - also called John - was arrested after he stopped a drunk assaulting a women driver.
Mr Cann said: "My son intervened after he saw this drunken man trying to smash a woman's car windscreen and drag her out.
"The man turned on John, who laid him out with one punch.
"When the police came, they arrested John - despite there being dozens of witnesses, who said he had behaved like a hero."
For three months Mr Cann battled to compel police to take his son's DNA sample off the database.
But it was only after his local MP and a national newspaper supported his case that the police agreed to back down and erase the details.
However, a Home Office spokeswoman said innocent people "had nothing to fear" from their DNA details being retained by police.
"DNA techniques are revolutionising the way police detect crime," she said.
The Home Office says it is watching the result of the Strasbourg case, and will "reconsider" the policy of DNA retention in the light of the eventual verdict.