In any criminal trial involving DNA evidence, the defense has a difficult task: explaining away the scientific smoking gun of the modern-day trial. Tiny amounts of this genetic blueprint often become the final word on who did, or did not, commit a crime, and overcoming such trusted evidence is a difficult task.

But Martin Murphy, the attorney defending Dirk Greineder, the 60-year-old Boston allergist who is charged with the murder of his wife of 32 years, has a theory. Although his client's blood was found on a knife and glove used in the slaying, Murphy has tried to convince the jury that the once-respected doctor's DNA could have been transferred to Mabel Greineder's face when she used her husband's towel to stop a nosebleed, and then transferred again to the knife and glove when she was attacked.

Murphy called a series of DNA experts Thursday to advance this "transfer" theory and to chip away at the jury's confidence in DNA experts brought earlier by the prosecution.

Greineder, who is charged with first-degree murder, says he briefly walked ahead of his wife, 58, during a stroll the morning of Oct. 31, 1999, when she was bludgeoned and stabbed to death. His lawyers claim the police failed to consider the possibility of a serial killer, noting that two other people had been killed in nearby Massachusetts parks around the time Greineder's wife was slain.

The prosecution, however, claims Greineder needed his wife out of the way to continue his secret life of porn and prostitutes.

Dan Krane, a professor at Wright State University, took the stand to discuss "calling" an allele, or the process by which data from a DNA sample is matched to the profile of an individual or individuals. He criticized the work of Cellmark, the company that analyzed the glove samples, saying that the company ignored certain markers and did not use the standards maintained by agencies such as the FBI.

"This is not a simple interpretation," said the professor, referring to the many different "peaks," or markers, that correspond to different individuals' DNA. "We're discussing samples ... in which mixtures were involved."

Krane said that Cellmark changed its threshold DNA levels midway through its examination. Krane applied the lower of their two standards and found evidence of DNA that might have come from a third party.

"It's not just a single individual's DNA profile, but rather a number of individuals' that are present in these DNA samples," he told the jury.

On cross-examination, prosecutor Richard Grundy attempted to discredit Krane, saying he was not an accredited forensic scientist and implying that his professorship was in part paid for by the fees he gave the school from his frequent testimony in criminal cases.

Grundy also sparred with the professor over his assertion that the DNA could possibly have been transferred to the knife and gloves from an alternative source other than the killer, despite never having done any transfer experiments.

"So your testimony to the jury is that you've read some articles, you believe them, and you think they're pretty good?" Grundy asked.

After Wednesday's testimony from a glove expert backfired, Murphy tried again Thursday, this time calling a rubber glove expert, Tom Tillotson, owner of a company that makes and sells surgical gloves.

Murphy showed the glove maker the gloves found near the crime scene and then showed him a number of different gloves found in Greineder's car and household. Before he asked Tillotson to compare the first set, he recalled Wednesday's setback. On Wednesday, Murphy expected the glove seller to testify that he had sold more than 60,000 pairs of the killer's gloves in the area over the past decade, but when presented with one of the killer's gloves for comparison, the witness said it wasn't the type he sold, and stated "mine's a better glove."

So Thursday, Murphy began by asking Tillotson, "Are they better or worse than the gloves in the bag?"

"Absolutely better," Tillotson replied.

Tillotson noted that the surgical gloves found in an EZ-Loaf pan in the forest were made of latex. "This is a very standard latex examination glove," he said. "Fifty different manufacturers could have made this glove."

The glove maker told the jury that each type of glove — including one from a tackle box in Greineder's car and from a box in his house — didn't match the type found in the forest. On cross-examination, however, Grundy noted that Tillotson did not examine gloves from Greineder's laboratory, implying that the doctor could very well have used other types of gloves than were in evidence.

Marc Taylor, a forensic scientist and one-time TV actor, also took the stand on behalf of the defense to advance the DNA "transfer" theory after Murphy fought to have Taylor's testimony allowed during voir dire hearings Wednesday.

Taylor, who played a medical examiner in the hit TV series "Quincy" from 1977 to 1983 and now runs his own forensics lab, told the jury how he signed onto the television show as a technical consultant and eventually became a producer and actor. He also explained that he thought the defense's transfer theory — that the doctor's DNA got on the towel, onto his wife's face when she had a nosebleed, then onto the glove and knife of the unknown assailant during the attack — was valid because he had performed a series of experiments that replicated such a transfer, though with only one "transfer" episode.

Prosecuting attorney Grundy attacked the Taylor's scientific merit. "You don't consider yourself an authority on the transfer of DNA do you?" he asked.

"I'm not sure who would be considered an authority," Taylor replied.

Grundy also criticized the transfer experiments Taylor explained to the jury. "Have they been published?" he asked.

"No," said Taylor.

"Have they been peer reviewed?"

"No," admitted Taylor.

Grundy asserted that, given Taylor's reasoning, "just about virtually anything is possible, isn't it?"

"I wouldn't say that," the scientist shot back.

But later, Grundy was successful in getting the scientist to admit that because DNA transfer is possible doesn't mean it's probable. "I don't think there's enough data to call it a probability," Taylor said.

The trial, which is being broadcast by Court TV, was to continue Friday at 8:30 a.m.