More than 200 prisoners have been exonerated since 1989 by DNA evidence — almost all of whom had been incarcerated for murder or rape.

DNA probe analysis grew out of basic genetic research, with far different aims. A kind of serendipitous gift to police science, it takes advantage of a peculiarity within the human genetic code. Along the three feet of the double helix in each complete DNA molecule there exists, in addition to the tens of thousands of proteincoding genes, a so-far indecipherable wilderness called the intron. The intron, although it seems mostly chaotic, nevertheless contains certain repetitive sequences of the genetic alphabet, which geneticists sometimes call "stutters" or "burps."

To exploit these stutters for identification purposes, scientists use a technique that forms this genetic material into a distinctive pattern, similar to the universal bar codes on retail merchandise.

The degree of certainty that can be attained depends on a number of factors, one of which is the number of probes applied. A single probe might produce a pattern unique to one person in a hundred. The application of a second probe with the same discriminating power then produces a combined pattern unique to one person in ten thousand. After several more probes, the final pattern might be unique to one person in a trillion, two hundred times as many people as exist on earth today. If this pattern matches the bar code for the suspect's DNA, the test has produced an identification every bit as reliable as a human fingerprint.