The trial of David Butler, which ended last month, provided a near-perfect stage for airing these new doubts. When the police first investigated Foys murder in 2005 they failed to produce a suspect. But then, as part of a cold case review last year, officers rechecked whether the DNA discovered under Anne Marie Foys nails had any matches on the national DNA database.

For some reason, first time around, no matches were found. This time, however, one turned up: a sample recovered from a cigarette butt found in 1998 after a burglary at the defendants mothers house. The police originally believed the butt had been left by the burglar. Instead it led them to Mr Butler who had apparently dropped it during a visit to comfort his mother. After the taking of a full DNA profile, which, again, matched the DNA under the finger nails, the cabbie was charged with murder. This was at the heart of the prosecutions case.

But Michael Wolkind, Butlers QC, took the science apart. The testing procedures were unreliable, he told the jury. The analysis of the DNA under Foys nails had been done at a time before higher-quality standards for handling samples were established. And, he said, even if the DNA was the defendants, there could be a perfectly innocent explanation for how it got there.

Butler suffers from a dry skin condition so severe that his nickname in the local cab trade is flaky. He could have taken a passenger to the Red Light district, handed over some notes in change and passed on his DNA to the passenger who then met with Foy and later handed the notes, complete with Butlers DNA, to her.

The idea that Mr Butler violently attacked her is beyond belief, Wolkind told the jury. Mr Butler never met the deceased, and unsafe science cannot change that fact. Waiting to face-off in court on the day I visited were two of the current luminaries of the DNA-forensics field: Prof David Balding of Imperial College, London and Prof Allan Jamieson, head of the Glasgow-based Forensic Institute, who has become a familiar thorn in the side of prosecutors seeking to rely on DNA evidence.

Prof Balding, a dapper Australian, is one of the worlds leading DNA statisticians. That is to say his speciality is calculating the likelihood of a sample coming from a particular source. He is a firm believer in the power of DNA as a forensic tool.

Prof Jamiesons approach is more combative. He has appeared as an expert witness for the defence in several important DNA-centred trials, most notably that of Sean Hoey, who was cleared of carrying out the 1998 Omagh bombing which killed 29 people. Jamiesons main concern about the growing use of DNA in court cases is that a number of important factors human error, contamination, simple accident can suggest guilt where there is none. Police and prosecutors, he alleges, have come to see DNA evidence as a shortcut to convictions, and juries are ill-equipped to understand the complex scientific data.

Wherever you have humans involved, youll have the potential for mistakes, he tells me. Theres a growing realisation that the system is not foolproof. In particular, he worries about the tiny amounts of DNA (known as Low Count DNA) that can now be used as the basis of a trial. Modern technology allows forensic teams to capture DNA from two or three cells, as opposed to hundreds or thousands of cells, as used to be the case, and Jamieson believes these sort of minuscule samples are unreliable.

Does anyone realise how easy it is to leave a couple of cells of your DNA somewhere? he asks rhetorically. You could shake my hand and I could put that hand down hundreds of miles away and leave your cells behind. In many cases, the question is not Is it my DNA?, but How did it get there? On February 10, after 11 hours of jury deliberation, Butler was cleared.

Afterwards, at his home in Wavertree, he accused the police of being fixated on the DNA and failing to provide any other evidence. If theyd been a bit more robust with their investigations over five-and-a-half years it would not have got to this stage, he said. The DNA stopped good policemen doing a good policemans job. It was like that was all [the evidence] they needed. They could never say it was my DNA. They talked in probabilities but you cannot put a probability on a mans life.

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The case against DNA