Defense attorneys in the Scott Peterson (search) trial have called mitochondrial DNA evidence (search) questionable science, frustrating experts and putting under a microscope what has become a mainstream tool of American justice.

Mitochondrial DNA, the genetic identification method cited last week in Peterson's preliminary hearing, has been used hundreds of times in the nation's courtrooms, helping convict the guilty and free the innocent, experts say.

It first appeared in a sensational 1996 Tennessee murder trial, but it has been used less frequently in California, which has higher barriers for new evidentiary techniques.

Prosecutors in the Peterson case are using mitochondrial DNA to make a case that a human hair found in pliers in Peterson's boat came from his wife, Laci, whom he is accused of killing last year.

The evidence is key to a possible prosecution argument that Peterson used the boat to ferry his pregnant wife's body to a watery grave on the day she disappeared from their Modesto home. Peterson, 31, is now charged with murder in the deaths of his 27-year-old wife and their unborn son.

Mark Geragos (search), Peterson's attorney, has attacked the mitochondrial DNA evidence, calling it the unreliable subject of "raging debate" among scientists.

Not so, said Dr. Terry Melton, chief executive officer of Mitotyping Technologies in State College, Pa., one of a handful of laboratories in the United States that extract cellular blueprints from evidence.

"It's been around for about 20 years," Melton said. "The armed forces used it to ID remains of Vietnam veterans for 10 years. Now it's being introduced quite a bit in court."

Experts say mitochondrial DNA — a tiny ring-shaped molecule that's much smaller than the more familiar nuclear DNA that reveals genetic makeup — helped identify victims of the Sept. 11, 2001, terror attack in New York. It can be extracted from hair and bones when little else remains of a body. The process takes a few days and typically costs about $2,500, Melton said.

Geragos grilled the prosecution's FBI witness about the science's weak points, prompting admissions of computer glitches and breakdowns in lab equipment. He plans to call his own witnesses to discredit forensic science techniques used to link the hair to Laci Peterson.

That argument is a long shot, analysts say, because mitochondrial DNA evidence is now typically one of many pieces of evidence used to build cases and most states have allowed it as courtroom evidence.

"It's seen as a legitimate type of science," said Fred Galves, professor at University of the Pacific's McGeorge School of Law in Sacramento.

"The more it's used and introduced into evidence, the more difficulty the defense is going to have in fighting it," said Randy Grossman, a San Diego County deputy district attorney who used it in a murder conviction last year.

Melton, who says she has testified 50 times in U.S. courtrooms since 1998 on mitochondrial DNA evidence, said challenges like Geragos' are bound to become a thing of the past "because there's simply nothing new or novel about any of the lab work."

That's not to say it's foolproof.

Galves said mitochondrial DNA from the human body cannot specifically identify an individual. Nor is it as reliable as the more familiar nuclear DNA samples, which can prove an identity based on a person's genetic fingerprint. But mitochondrial DNA, if matched with similar samples from a person's mother or sibling, can show a statistical likelihood of identification and rule out others.

"It's a piece of the puzzle, another bit of information you add to what you know about your case," Melton said.

Chattanooga, Tenn., prosecutors first used it in September 1996 to help convict Paul Ware, 27, for the rape and murder of a 4-year-old girl. Mitochondrial DNA in a hair found in the girl's throat and other hairs on her bed were successfully matched to a saliva sample from Ware.

Mitochondrial DNA also has been used to clear suspected criminals. In a 2001 Oklahoma case, it freed a man convicted of a 1981 murder by showing that a hair found in the gag stuffed in the victim's mouth did not belong to the person found guilty. Investigators had testified at the trial that the hairs were consistent with the defendant's hair, but the newer form of testing revealed otherwise.

Melton said one-third of the requests for DNA work at her Pennsylvania lab are from defense attorneys. Likewise, Galves said defense challenges like Geragos' in the Peterson case aren't entirely representative of the legal industry.

"I don't think the criminal defense bar has a real interest in poking holes in DNA," Galves said. "DNA can be their friend in a way that no other evidence can."