The O.J. Simpson murder case could boil down to a titanic clash between silver-tongued lawyers and modern forensic science represented by a DNA fingerprinting technique that won the Nobel Prize in 1993.
At issue will be the nature and quality of the genetic evidence Los Angeles prosecutors obtained for testing and-if the case goes to trial-whether a jury trusts it.
The test, although controversial in legal circles, is beyond dispute in science. If performed correctly under ideal circumstances, DNA evidence can either place Simpson at the murder scene or clear him, giving lawyers for both the defense and the prosecution vital reasons to have the testing done.
“When DNA exonerates a defendant there usually is no reason for the prosecution to move forward,” said Mark Stolorow, director of operations at Cellmark Diagnostics of Germantown, Md., the nation’s largest independent DNA testing service.
“Most of those cases ultimately result in the release of the defendant,” Stolorow said. “Police then go and investigate someone else.”
DNA analysis, which was developed by an English geneticist in 1985, is considered the most important advance in forensics since fingerprinting. Its use in U.S. courts has skyrocketed, from a mere 14 cases by 1987 to 12,000 cases by mid-1993.
So far, the exact nature of the Simpson evidence has not been released by authorities. However, DNA testing usually involves blood and tissue samples taken from the victims, suspects and crime scene.
When the tests are completed, prosecutors will be handed one of two objects: either a piece of X-ray film, 8 inches square, bearing various charcoal-colored bands like a computer bar code; or else a strip of paper, chemically treated and bearing a pattern of blue or clear dots.
Such X-ray smudges and dot patterns represent the genetic code of an individual.
Every living thing-plant, animal, person-is unique genetically (except for identical twins, which share the same DNA). Since 1980, scientists have been able to identify people by the variations in their genetic codes.
Each human being has between 50,000 and 100,000 genes scattered among the 3 billion chemical subunits (nucleotides) found in the nucleus of every cell. White blood cells, skin, sperm cells, cells surrounding hair roots and cells in saliva-all those cells are of interest to forensic scientists.
The DNA molecule contains four types of nucleotides, which repeat many times in coded sequence. Because the nucleotides are complementary, an adenine always bonds with a thymine wherever it occurs. And guanine always pairs up with cytosine.
This property permits scientists to locate specific DNA sequences: They use a known sequence of nucleotides as a probe to seek out its complement like a magnet.
Here’s how it works:
After the DNA is extracted from cells, it must be cut by molecular scissors called restriction enzymes into manageable lengths for study. The fragments are placed at one end of a sheet of gel and then exposed to an electrical field, which causes them to move through the gel.
The smaller, lighter fragments move more rapidly, so the molecules become separated into bands by size.
Next, the DNA fragments are transferred to a nylon membrane where they are introduced to the genetic probes that have been made radioactive (to make them visible). The probes seek out their complementary cousins and bind to them, setting off tiny flares on X-ray film. The result is a clear sheet of film covered with dark bands.
That is a DNA fingerprint.
The technique for producing this type of fingerprint uses restriction fragment length polymorphisms (RFLPs, or “riflips”). Riflips are minute variations in the genetic code that are unique to each person.
But riflip testing usually is limited to blood and semen stains, fluids rich in DNA. Other sources of DNA, like hair or saliva from a cigarette butt or the flap of an envelope, cannot be readily detected with the riflip technique.
That is where the Nobel Prize-winning discovery known as the polymerase chain reaction (PCR) comes in.
“Many samples come in the door in quantities either too small or too degraded to use in DNA fingerprinting,” said Stolorow. “That’s why PCR is so powerful.”
A kind of molecular Xeroxing, PCR copies even the most minute amount of DNA and amplifies it billions of times for study in a test tube. Visible patterns of blue and white dots result on paper strips. The information can be plugged into a computer database to determine the likelihood of a match between a suspect and samples at a crime scene.
PCR, like blood tests, cannot prove identity but can establish with very great probability that the match is accurate. If that’s not enough, when coupled with a new test called STR (short tandem repeats), the odds against the DNA belonging to someone else become even greater, Stolorow said.
The time required to perform DNA analysis varies from lab to lab. At the FBI Laboratory in Quantico, Va., the tests take about six weeks, but a backlog of cases has resulted in a current turnaround time of 120 days, officials said.
In Los Angeles on Monday, legal experts predicted a major courtroom showdown in the Simpson murder trial over the DNA testing.
The FBI, which began DNA profiling in 1988, has testifed in more than 360 criminal cases in 47 states and has provided expert witness testimony in more than 200 admissibility hearings about the new technology.
Even so, the admissibility of DNA evidence is still controversial in many states, including California.
In ruling whether such evidence can be presented to a jury, a judge must decide whether it is valid and reliable, scientifically acceptable, and sufficiently valuable when weighed against its potential to confuse or prejudice a jury.
The U.S. Supreme Court has turned down several requests to address the scientific evidence question of DNA testing. But a 1992 report by the prestigious National Research Council confirmed the “general reliability” of DNA evidence in criminal cases.
However, the report emphasized that reliability depended on quality control in the process of collecting, analyzing and interpreting the data.
“DNA typing is a powerful tool for criminal investigation and justice. But there are certain standards that need to be met consistently from a technical point of view,” said Dr. Victor McKusick, a professor of medical genetics at Johns Hopkins University who chaired the NRC committee.
The technique has been widely accepted in California cases in federal courts, but its reliability is hotly contested at the state level. The California Supreme Court has yet to rule on the validity of DNA tests, and state appeals courts have issued conflicting judgments on the admissibility of DNA testing.
“What this means is that DNA is going to be a giant issue” in Simpson’s trial, said John Shepard Wiley Jr., a former federal prosecutor and professor at UCLA’s law school.
The controversy in California state courts revolves around the question of assigning statistical probability that any particular tissue or blood sample matches a suspect.
In a 1993 California case, People vs. Bravo, the three-judge appeals court ruled that a convicted rapist’s lawyer was not incompetent for failing to ask for DNA testing. Appeals Judge Earl Johnson, in a dissenting opinion, strongly disagreed, suggesting that California lagged behind other states and federal courts in accepting DNA testing.
“There currently is a split of authority within California about the admissibility of DNA test evidence to prove an affirmative match, that is, to establish the defendant on trial was the one and only person whose bodily tissues or fluids could match those found at the scene of the crime,” Johnson wrote. “However, in the rest of the country, there is no such split.”
