Order in The Lab!

By LEON JAROFF

Rising to his feet during a pretrial hearing in the O.J. Simpson case in Los Angeles last week, Deputy District Attorney William Hodgman waxed indignant. "If the defense wants to go fishing," he complained, "they're going to have to use their own pole and own tackle box." Hodgman was upset over the defense team's demands that the prosecutors turn over a laundry list of records including reports of prowler and burglary calls in the neighborhood of the murder site, records of local unsolved murders, and -- on the chance that Nicole Simpson's dog may have attacked the killer -- records of emergency-room visits for dog bites during the 24 hours after the slayings. Most intriguing of all was the defense's demand that prosecutors investigate an unidentified witness who had reportedly seen two white men leaving the crime scene. The prosecution insisted, in turn, that it was not ignoring leads or holding back evidence from the defense.

If the thrust and parry had a tone of urgency, it was because both sides were aware that the time to prepare their case was growing short. As expected, the judge set an early date for the opening of the trial: Sept. 19. No one could know for sure whether any more witnesses would turn up before then (Simpson was offering $500,000 for information that would prove his innocence), but the case may not turn on what people saw or didn't see. By the time of the trial, the court will have what is likely to be the most important evidence: results of DNA tests done on blood found at the homes of O.J. and Nicole Simpson.

A hearing earlier in the week centered on that subject and was notable for some pretrial confessions -- confessions of ignorance about testing, that is. The only kind of science he studied in college, Judge Lance Ito admitted, was political science, and both Deputy District Attorney Marcia Clark and defense attorney Robert Shapiro in effect conceded that they lacked the expertise to come to his aid. Ito ordered both attorneys to round up expert witnesses to testify about how to handle and interpret the DNA tests.

DNA fingerprinting, as the process is called, is a complex, high-tech forensic test that can link a suspect to the commission of a crime -- or establish his innocence. While still controversial, use of the tests is gaining widespread acceptance in American courtrooms, including California's. That fact was hardly lost on either the Simpson defense or the prosecution. Attorney Shapiro insisted that his own experts as well as those hired by the * prosecution had the right to conduct the DNA tests. He requested that the prosecution turn over half the samples of blood that were collected by investigators after the slayings.

After much wrangling, a compromise was reached. The defense's experts could visit Cellmark Diagnostics in Germantown, Maryland, where the prosecution had sent the samples for testing, and could pare off 10% of each sample. But the material removed would in effect be held in escrow. After hearing expert testimony, Judge Ito would rule on the disposal of the sliced-off portions.

The tests being performed in Maryland involve two different techniques: RFLP (for restriction fragment length polymorphisms) and a newer process called PCR (polymerase chain reaction). Both are based on the fact that no two people except for identical twins have identical DNA.

The DNA strand found in the nucleus of a human cell contains a sequence of some 3 billion chemical units, each of them representing one of the four "letters" in the genetic code. Most of the sequence is the same in all humans, but there are variations, especially in the "junk" stretches of between the genes, where seemingly meaningless triplets of code letters are repeated over and over again. In some of these stretches, no two people have the same number of repeats.

In RFLP tests, which take several weeks, the DNA is extracted from blood or hair or other tissue samples and exposed to enzymes that "recognize" certain sequences of code letters and snip the strand at those sequences, cutting the DNA into fragments of varying lengths. These segments are placed in a gel and subjected to an electrical charge that pulls them down into the gel. Because short pieces move faster than longer ones, the fragments are separated into a pattern of bands that is recorded on X-ray film. That pattern differs from person to person, depending on the number of repeats in each segment, and by comparing the images on overlying X-ray films, lab technicians can determine if there is a match between samples.

Several thousand cells and long stretches of DNA in pristine condition are required for RFLP tests, and lawyer Shapiro's concern is that RFLP testing at Cellmark will completely consume some of the smaller samples. But the prosecution insists that RFLP tests are essential because they can be so accurate. Scientists estimate that the chance that a properly conducted RFLP test will incorrectly identify a person's blood are less than 1 in a million.

PCR tests are less precise but require just several days, need only the DNA from a few dozen cells and, because only short segments of are needed, can be effective even if the DNA is somewhat degraded. Using PCR and working with tiny samples, technicians use enzymes to copy each of these segments time and again until they have enough DNA for testing. One PCR method involves comparing the DNA in one of several genes having code-letter sequences that can vary slightly from person to person. The chance of this test incorrectly linking a suspect with a forensic sample is only about 1 in several thousand.

A newer PCR test developed by Dr. Thomas Caskey, of the Baylor College of Medicine in Houston, is even more accurate; if there is a match, the odds that it could have occurred by chance are about 1 in 100,000. Caskey's technique involves comparing short triplet repeat sequences in as many as 13 locations in human DNA, sites in which the number of repeats varies widely among individuals.

Like most U.S. prosecutors, Rockne Harmon, a deputy district attorney in California's Alameda County, has an abiding faith in the answers that DNA provides. "There has never been a case in which a person has been convicted using DNA evidence and later been proven to be innocent," he says. "In fact, people have actually been freed from prison thanks to DNA evidence that turned up after they were convicted."

O.J. Simpson hasn't been convicted, but he sits day after day in a Los Angeles jail cell. Whether he wins freedom or stays behind bars may depend on the tale of DNA.

With reporting by Dan Cray/Los Angeles