WASHINGTON (AP) – Roger Clemens’ chief accuser is expected to testify Monday against the former pitcher, a make-or-break moment for the prosecution as it seeks to convict Clemens of perjury.

Brian McNamee, Clemens’ former strength coach who said he injected the seven-time Cy Young Award winner with steroids and human growth hormone in 1998, 2000 and 2001, takes the stand as the trial moves into its fifth week. Clemens is accused of lying when he told Congress in 2008 he never used those drugs.

A lot rides on McNamee’s testimony, especially after the government’s other key witness, pitcher Andy Pettitte, wavered in his appearance two weeks ago. Pettitte, a former Clemens teammate, testified that Clemens said he had used HGH — only to say under cross-examination he might have misunderstood their conversation.

U.S. District Judge Reggie Walton is expected to take up a defense motion Monday morning to strike that testimony. The government has argued that jurors should be allowed to decide for themselves how much weight to give Pettitte’s testimony.

The judge is also expected to rule on a defense subpoena for records from the ongoing contentious divorce proceedings of McNamee and his estranged wife. Clemens’ lawyers want the records about McNamee’s past to attack his credibility. Prosecutors want this material kept out of the trial.

McNamee says he saved items that he used while injecting Clemens with performance-enhancing drugs, including gauze, tissues, syringes, cotton balls and needles. Prosecutors say they have evidence that some of the materials tested positive for the drugs as well as Clemens’ DNA.

Clemens’ lawyers have said they will contend that the items saved by McNamee have been tainted and contaminated because they were stored so haphazardly. They refer to the collection as a “mixed-up hodgepodge of garbage.”

Clemens insists that McNamee injected him with vitamin B12 and the anesthetic lidocaine.

Monday will be a reunion of sorts for the two men. In 2008, when they testified together before a congressional committee, McNamee said he injected Clemens with steroids and HGH, and Clemens denied it.

Two weeks later, the chairman and ranking member of the committee asked the Justice Department to investigate whether Clemens committed perjury at the hearing, and two years after that, Clemens was indicted by a grand jury.

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