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AT THIS POINT IN THE O.J. SIMPSON trial, one thing—and maybe only one thing—is certain: 12 citizens of Los Angeles County, tutored by the finest instructors money can buy, are about to become experts in the arcane science of DNA analysis. The courtroom seminar will fill volumes, but it all boils down to a few essential facts—the ABCs of DNA.

DNA—short for deoxyribonucleic acid—is a complex, two-stranded molecule wound into a double helix that makes up the chromosomes of every living cell. Each person's DNA is a unique sequence that serves as a blueprint for all inherited traits. By chemically extracting DNA from blood, semen, skin, saliva or hair follicles found at a crime scene, forensic scientists can say within a certain range of probability whether it matches DNA taken from the accused.

Developed in England during the 1980s, DNA analysis is done in two ways. RFLP—short for restriction fragment length polymorphism—is the more discriminating analysis, but it requires more genetic material. Using this method, pieces of DNA, say from a dime-size blood stain, are illuminated with radioactivity and captured on X-ray film as dark bands. If two DNA samples share a pattern of bands, they are considered a virtual match. With RFLP the chances of a second person having an identical pattern can be one in tens of millions.

Another test, called PCR (for polymerase chain reaction), can be conducted on blood and tissue samples as small as the head of a pin, but it is less conclusive. PCR has been described by forensic scientists as molecular Xeroxing because individual genes, too small to be detected by themselves, are replicated millions of times in the laboratory and then compared for certain common chemical sequences. At best, PCR—shown as a series of blue dots—can only single out the one person in 800 to 4,000 with a particular pattern.

Although OJ.'s defense team is expected to attack the accuracy of DNA evidence, it has been used effectively in more than 25,000 criminal cases in North America—exonerating the accused in more than a quarter of them. Indeed, DNA analysis, by showing a mismatch between the suspect's DNA and that collected from the crime scene, can only definitively establish that the sample did not belong to the suspect. A match can only establish high probability of the suspect's guilt. In the Simpson trial expect days, or more likely weeks, of wrangling over just what that probability is. For O.J. Simpson it may be the argument that seals his fate.

This special section of articles on the O.J. Simpson trial was written by Richard Jerome, Mark Lasswell and David Ellis. It was reported by Lyndon Stambler, Lorenzo Benet, Kurt Pitzer, Carolyn Ramsay, Danelle Morton, Vicki Sheff-Cahan and Karen Brailsford in Los Angeles and Sherry Thomas in Chicago.