Hardly a week goes by without a mention of DNA’s contribution to criminal justice. Last week Sandip Patel was convicted of killing a prostitute near Baker Street 30 years ago: DNA belatedly proved that his hairs were caught in her ring. A few days before, a double murderer, Colin Pitchfork, was controversially granted a parole hearing 36 years after being the defendant in ‘the first case of its kind in Britain to use DNA profiling’, as the Times put it.