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Peterson defense witness calls DNA technique 'unreliable'

Scott Peterson, right, with attorney Mark Geragos are shown last week at the preliminary hearing.
Scott Peterson, right, with attorney Mark Geragos are shown last week at the preliminary hearing.

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A defense expert testified that a DNA testing technique used to identify a hair found in Scott Peterson's boat is unreliable. CNN's David Mattingly reports (November 4)
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MODESTO, California (CNN) -- A DNA testing technique used to identify a hair found in Scott Peterson's boat is unreliable, a defense expert testified in a preliminary hearing Monday.

William Shields, of the State University of New York, said tests of mitochondrial DNA, or MTDNA, on hair samples are "not very probative" of where the hair came from.

"They are most likely to result in false exclusions [of people]," he said. "They're unreliable."

Prosecutors objected to Shields' testimony, characterizing him as a professional witness who makes up to half his income testifying for the defense in trials, rather than a research scientist who works in a laboratory. But Stanislaus County Superior Court Judge Al Girolami said he would allow Shields' testimony.

Defense attorney Mark Geragos has been trying to attack the validity of MTDNA analysis, which an FBI lab performed on a hair found on a pair of pliers in Scott Peterson's boat.

Prosecutors contend the hair came from his wife, Laci. He is accused of murdering her, along with their unborn child.

MTDNA, which is passed from mother to child, is not as definitive as conventional DNA because it is not unique to each person. However, people who are closely related can share the same MTDNA sequences, and FBI analysts compared the hair found on the pliers with a sample from Laci Peterson's mother. (More on MTDNA)

Geragos has been trying to get the hair evidence excluded, saying MTDNA is not reliable enough to make an identification.

The often painstaking scientific testimony about MTDNA has slowed down the preliminary hearing for Peterson, 31, who is accused of killing his pregnant wife last December and dumping her body in San Francisco Bay.

The hearing, which began last Wednesday, had been expected to wrap up this week, but court officials now say that it could last until next Thursday.

The bodies of Laci Peterson and her unborn son washed up on the shore of the bay in April, and Scott Peterson was arrested and charged with murder. Prosecutors are seeking the death penalty.

Among the witnesses expected to take the stand this week is Amber Frey, the woman with whom Scott Peterson was having an affair at the time his wife disappeared. Frey, who says she did not know Scott Peterson was married, contacted police after she found out about Laci Peterson's disappearance and cooperated in the investigation.

At the conclusion of the hearing, Girolami will decide whether prosecutors have presented enough evidence to bind Peterson over for trial.

CNN Correspondent Rusty Dornin and Producer Chuck Afflerbach contributed to this report.

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