Roger Clemens is on accused of lying to Congress over his alleged steroid use.

AP

WASHINGTON (AP) -- A forensic scientist testified Friday that two cotton balls and a syringe needle allegedly saved after a steroids injection tested positive for Roger Clemens' DNA, a key moment as the government tries to prove the former pitcher used performance-enhancing drugs.

Alan Keel told jurors that the DNA on both cotton ball matches were "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 had one in 173 trillion, when comparing to the population of white people in the U.S.

The needle was not as conclusive, because Keel was only able to get a handful of cells. That match was one in 449.

Brian McNamee, Clemens' former strength coach, earlier testified he collected the medical waste after injecting the pitcher with steroids in 2001, and turned them over to federal authorities in 2008. Earlier government witnesses testified that steroids were found on the medical waste.

Clemens is accused of lying to Congress in 2008 when he denied using performance-enhancing drugs.

Prosecutors hope that Clemens' DNA -- which helped make him one of the most successful pitchers in baseball history -- will help them convict him of a federal crime.

While Keel's testimony was a milestone moment for the government, the defense indicated early on it wouldn't contest that the needle had both steroids and Clemens' DNA on it. But Clemens' lawyer Rusty Hardin said in his opening statement that the defense 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.

Prosecutor Courtney Saleski tried to pre-empt that by asking Keel if there was any way to "fake this."

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Expert: DNA on evidence matches Clemens'