Terrorists and kidnappers inadvertently convict themselves by licking the envelope of a ransom note. Criminals of every ilk deposit saliva on glasses, telephones, and cigarette butts and leave spittle and sweat stains. Under a new profiling system know as Short Tandem Repeats, DNA is measured only at 13 specific sites, where chemicals are repeated in a kind of stutter. The count of these repeats creates a unique DNA fingerprint. Except for in identical twins, these 13 numbers are highly variable from one person to another, pinpointing an individual with a probability of one in several billion.
Criminals often move from state to state to avoid detection.
With more and more states contributing crime evidence and DNA fingerprints, the FBI has a growing web of information to link crime scenes across state boundaries. Every new sample added to the FBI's national database is immediately checked against all DNA fingerprints from unsolved crimes throughout the nation. This provides early recognition and linkage of serial crimes even before a suspect is identified, providing a broad net to quickly ensnare some of the most elusive and violent offenders. At this time, most states require some sort of DNA testing but differ widely on who gets sampled. DNA fingerprinting is generally done on sex offenders, but if we are to realize the full value of this technological breakthrough, sampling must be expanded to a much wider audience.