WASHINGTON -- A needle stored with a beer can appeared to contain an extremely tiny amount of Roger Clemens' DNA, which turned out to be good news and bad news for both sides in the perjury trial of the seven-time Cy Young Award winner.

A forensic scientist on Friday linked Clemens to cotton balls and a syringe needle saved from an alleged steroids injection 11 years ago. His testimony, laced with statistics and probabilities, was one of the last pieces of the government's case in its effort to prove that the pitcher lied to Congress in 2008 when he denied using performance-enhancing substances.

Under cross-examination, Clemens' lawyer tried to poke holes in the physical evidence. He got the expert to acknowledge there were "hundreds of thousands" of white males in the United States who could be a match for the scant amount of DNA found on the needle, and that it's "conceivable" the cotton balls could have been contaminated by beer and saliva.

Prosecutors had hoped to wrap up their case heading into the long holiday weekend as the trial reached the end of its sixth week, but the DNA expert's testimony took much longer than expected. U.S. District Judge Reggie Walton then ended the session a half-hour early when one of the jurors learned that her mother had died.

The judge said he doesn't expect the juror, a woman who works in law enforcement with the local public transportation authority, to return. Two jurors have previously been dismissed for sleeping, and another departure would leave only one alternate in a trial expected to last at least two more weeks.

The government's key witness, longtime Clemens strength coach Brian McNamee, says he injected Clemens with steroids in 1998, 2000 and 2001 and with human growth hormone in 2000. He said he kept the needle and other waste from a 2001 injection and stored it in and around a beer can in a FedEx box in his home for more than six years before turning it over to federal investigators.

Alan Keel of Forensic Science Associates told jurors that the DNA found on two cotton balls was "unique to one person who has ever lived on the planet" -- Clemens. He said that one of the cotton balls had a random match possibility of one in 15.4 trillion for Clemens' DNA, and the other was one in 173 trillion, when compared to the population of white people in the U.S.

But the needle was not as conclusive. Keel was able to detect only six to 12 cells for testing when he examined it. A drop of blood, by comparison, contains up to 30,000 cells.

The match: one in 449 for Clemens.

"That means that Mr. Clemens is the likely source of that biology," Keel said.

Knowing that the defense would attempt to undermine the integrity of the evidence, prosecutor Courtney Saleski asked: "Is there any way to fake this?"

"No," said Keel, shaking his head. "If this were contrived, I would expect to obtain much more biological material."

In other words, it would have been extremely difficult for anyone, including McNamee, to purposely contaminate the needle because it contained such a minute amount of human residue.

During cross-examination, Clemens lawyer Michael Attanasio attacked the findings in several ways. He pointed out that Keel was being paid by the government. He pointed out that Keel didn't test all of the items available. He pointed out that the DNA had degraded over time. He noted that 449 was a "far, far smaller number" than the other numbers in the trillions, and it therefore can't be said with uncontested certainty that the DNA on the needle belongs to Clemens.

Attanasio got Keel to agree that the Clemens blood found on one cotton ball appeared to be from the aftermath of an injection, but that the Clemens puss on the other cotton ball "is not from an immediate injection site." The lawyer also suggested the blood on the cotton ball might not have come from an injection at all: "Is it not at all uncommon for a pitcher to have a little blood blister at the end of his finger when he was pitching?"

Attanasio further implied that residue beer and saliva inside the can could have soaked the cotton balls. Keel said that was "conceivable" and "not implausible," but he added that the appearance of the cotton balls would have reflected the contamination.

"You would have a big, diffuse mess," Keel said.

Saleski picked up on that point in her follow-up questioning.

"Did you see any evidence that these cotton balls were exposed to a bunch of beer?" she asked the witness.

"Not really, no," Keel said.

Keel also re-emphasized his opinion that the minute sample of DNA on the needle could not have been manipulated or put there "by design."

"It would be virtually impossible," Keel said.

The needle naturally caught the attention of the jurors, who submitted multiple questions for the judge to ask the witness about the one-in-449 ratio.

"There's the rub," said Keel as he explained again that the results were compatible with Clemens -- but couldn't be considered a conclusive match.

Clemens' lawyers have maintained all along that a beer can is no way to store evidence. During the questioning of Keel, the government decided to emphasize that point, too, inferring that if McNamee had truly intended to keep the needle and cotton balls with the intention of implicating Clemens, he would have found a more sterile place for it. McNamee has said he kept the evidence to placate his wife, who was concerned he would take the fall if his involvement in performance-enhancing drugs ever came to light, and that he had no plans to make it public.

Keel also found a gauze pad and tissue that matched McNamee's DNA to an even greater probability than the Clemens matches. McNamee has said he would sometimes accidently cut himself while opening small glass containers of steroids before injecting Clemens.

The gauze pad match was 1 in 1.8 quintillion people for white Americans.

A quintillion has 18 zeroes in it.