Simpson trial puts DNA on the lips of every American

After all the legal manoeuv- ering, grandstanding and courtroom bickering, the OJ Simpson trial has reached the heart of the case: what are the odds that blood drops found near the bodies of the former American footballer's alleged victims are not his?

More than three months after the start of trial, the long-awaited DNA battle is under way - a confrontation that could not only determine Mr Simpson's fate, but may also be a watershed in the use of such testing in cases across the US.

The opening salvo came from a prosecution DNA expert, Dr Robin Cotton, and was the most damaging testimony against Mr Simpson so far. She told the court fewer than one person in 170 million has the same genetic characteristics as those found in a blood drop at the murder scene which matched that of the sporting legend.

The chances of someone sharing the same DNA profile as his murdered former wife, Nicole Brown, whose blood matched that found on a sock in his bedroom, were even more remote, she said: one in 6.8 billion.

Trial observers have long predicted America's obsession with the trial would begin to fade once the science began. But, although public attention shifted in the aftermath of the Oklahoma bombing, it evidently takes more than that to extinguish the extraordinary interest that the case still commands.

This week genetics has become a regular talking point on breakfast television shows, a subject that would usually stand about as much chance of competing for air-time as a feature on accountancy.

Newspapers have devoted acres of space to "everything-you-need-to-know- about-DNA". The mention of ''deoxyribonucleic acid'' over drinks in a bar is no longer the act of a party-pooper.

In the courtroom, the jury has received what amounted to a crash course in biology from Dr Cotton, director of the Cellmark Diagnostics, one of three laboratories that examined blood samples in the case. With all the patience of a fifth-form school mistress, she spent two days explaining that DNA is a genetic blueprint, "a thread ... a zipper ... an alphabet", which contains the assembly instructions for each human being.

There is no doubt the DNA debate is the most important issue in the trial to date.

Prosecutors do not have a murder weapon or any witness. The only tangible evidence connecting the former sports idol to the murder of his former wife and her waiter friend, Ronald Goldman, is the "trail of blood" - traces from all three - which prosecutors say leads from the murder scene to Mr Simpson's estate.

There is also a larger issue at stake. DNA evidence is used in about 10,000 criminal cases a year in the United States, three-quarters of which are alleged sex assaults. It is also a means of settling a host of other disputes, including paternity suits and the identity of meat in a poacher's freezers. Prosecutors have come to regard it as more convincing than the often erratic memories of eye-witnesses.

Yet there are deep divisions among American experts over its reliability, implications for civil liberties, and the calculations used to establish the likelihood of one person having the same DNA pattern as someone else.

Mr Simpson's defence team includes two of the country's top DNA lawyers, Barry Scheck and Peter Neufeld, who have long called for reforms - not least, they argue, because the figures are sometimes based on samples as small as 100 people.

Cross-examining Dr Cotton, Mr Neufeld flourished an unpublished letter to Nature magazine written by 25 scientists, raising the same point.

Mr Simpson's lawyers also have other lines of assault. They argue that police officers may have framed Mr Simpson by stealing his blood from a sample bottle and sprinkling it at the scene.

Under that theory, the discovery of his blood at the scene is not incriminating, although this has not stopped the defence from attacking the use of DNA tests, or from claiming blood samples may have been inadvertently contaminated by shoddy investigation practices. Theirs is a scatter-gun approach, driven by the hope that something will sow a seed of reasonable doubt among the jury - or cause confusion.

Watching with increasing impatience is Judge Lance Ito. On Thursday, he threw another tantrum, this time during a squabble between attorneys during the cross-examination of Dr Cotton.

The judge slammed both hands on his desk and fined both sides $250 on the spot. As the lawyers dug out chequebooks, with some sniggering, he told them the fine was a "personal sanction" and not to be billed to their clients.

"Thank you," said Mr Simpson, who has already spent millions trying to clear his name.