DNA in the dock: how flawed techniques send innocent people to prison

Many juries believe crime-scene DNA evidence is watertight - but this is far from the case. As forensic technology gets ever more sophisticated, experts are only just realising how difficult interpreting the evidence can be

For David Butler, it began with a knock on the door early one November morning, seven years ago. When he opened it, officers from the Merseyside police were standing on his doorstep. The retired taxi driver was being arrested for murder.

The police said they had evidence connecting Butler to the death of Anne Marie Foy, a 46-year-old sex worker who had been battered and strangled in Liverpool in 2005.