The U.S. government is expected to show test results Friday in Roger Clemens's perjury trial that link the former major league pitcher's DNA to a used needle and other medical waste that also tested positive for steroids.

Roger Clemens's genetic makeup helped make him one of the most successful pitchers in baseball history. Now prosecutors hope that Clemens's own DNA will help them convict him of a federal crime.

The government is expected to show test results Friday in Clemens's perjury trial that link the former pitcher's DNA to a used needle and other medical waste that also tested positive for steroids. Forensic scientist Alan Keel, who spent the last half-hour of testimony Thursday giving jurors a brief genetics lesson, returns to the stand to testify about the genetic tests he did for the government.

Chief prosecution witness Brian McNamee, Clemens's former strength and conditioning coach, testified earlier that he saved the needle and other materials from an alleged steroids injection of Clemens in 2001 and then turned them over to federal authorities in 2008. On Thursday, two experts testified that the materials tested positive for steroids.

Clemens is on trial for allegedly lying to Congress in 2008 when he denied using performance-enhancing drugs.

While it will be a significant moment when the government places both the steroids and Clemens's DNA on the drug needle, the defence indicated early on it wouldn't contest that the needle had both on it.

But Clemens's lawyer Rusty Hardin said in his opening statement that the defence will contend that McNamee put the steroids in the needle after injecting Clemens and that the coach in fact had used the needle to inject Clemens with vitamin B12.

Clemens has maintained for years that he received B12 shots and the local anesthetic lidocaine but not performance-enhancing drugs.

No trace of B12

Another defence lawyer, Michael Attanasio, tried to push that theory by posing a hypothetical question to one of the government's expert witnesses, Jeremy Price, who once worked at the nonprofit Anti-Doping Research laboratory. Price testified Thursday that he found steroids but no trace of B12 or lidocaine on the saved items. So Attanasio asked: If he had a syringe with B12, used it, cleaned it out and then put in steroids, would the test reveal the B12? Price said he didn't know.

Thursday's main headliner was former major league outfielder-first baseman David Segui, who recalled that in a 2001 phone conversation, McNamee "mentioned that he had kept darts to get his wife off his back."

Prosecutors hope that will bolster McNamee's testimony that he saved the medical waste to allay his wife's fears he would take all the blame if the drug use was discovered.

"He mentioned that the relationship between Brian and Roger had put stress on his married life. … Coming and going … leaving at the drop of a hat to go train," Segui said.

Segui, who has acknowledged using performance-enhancing drugs during his 15-year career with seven teams, wasn't allowed to say that "darts" means "needles." Despite a couple of spirited appeals by prosecutor Gilberto Guerrero, U.S. District Judge Reggie Walton ruled that the jurors would have to make that assumption themselves -- unless McNamee were to return to the stand to explain.

When Guerrero made a second run at the judge, Walton chided the prosecutor.

"I'm not the court of appeals," the judge said. "I made my ruling. So I don't reverse myself."

Segui, who became friends with McNamee when they met in Toronto during the 1999 season, also wasn't allowed to relate a second, similar "darts" conversation because he couldn't remember when it happened.

Segui: 'I don't log my life'

"I'm not good with dates. I don't log my life," said Segui, appearing with a shaved head, dark suit, white shirt and black tie. He was certain it happened before the publication of the 2007 Mitchell Report on drug use in sports, but that wasn't good enough for Walton.

"You are to disregard the question," the judge told the jurors, who had been ushered from the courtroom as the lawyers debated the matter.

But at least one juror apparently submitted a written question about that conversation.

"I've legally ruled that is not before you," Walton lectured in an irritated tone as he pointed toward the jury.

Clemens's lawyers have implied that McNamee conjured up the evidence after becoming the subject of a federal investigation in 2007. Segui's testimony was allowed by the judge because it appears to contradict that argument, a significant victory for the prosecution.

Another witness, former McNamee workout client Anthony Corso, is expected to give similar testimony about conversations in 2002 and 2005 with McNamee. That should come Friday, when the government is expected to wrap up its case.

Segui, whom the government portrayed as a reluctant witness, said under cross-examination that McNamee didn't attach a specific name to the "darts" but that he knew McNamee was training Clemens at the time.

"All I know is what I was told over the phone," said Segui, raising his hands as if to pantomime pushing away. "I didn't see it, didn't ask to see it."

Segui was a colourful witness. When asked whether McNamee was a cutting-edge coach, Segui said, "Yeah, for baseball. Baseball's prehistoric. They're still doing jumping jacks."