Advocates of rapidly growing databases of criminals' DNA say they're a potent crime-fighting tool with little downside for law-abiding citizens. Typical of this school is Howard Safir, New York City's Police Commissioner, who told the New York Times last summer: "We should be collecting [DNA] from everybody. The only ones who have anything to worry about from DNA are criminals."

Well, not quite. British officials admitted in January that robbery scene DNA matched to England's databank of 660,000 genetic profiles of convicts and arrestees had led to the arrest of an innocent man. The odds against two people having DNA fingerprints similar enough to cause such a mismatch are huge-about 37 million to one. Nevertheless, further testing and a string of alibis later proved that police had detained the wrong person.