bryant mcnamee, the former trainer for the seven-time cy young winner claims that he injected clemens with steroids on more than one occasion. danielle nottingham has more from the trial in northwest washington. >> reporter: roger clemens entered the u.s. federal courthouse for what's expected to be a key day of testimony in the pitching great's perjury trial. taking the stand former clemens trainer ryan mcnamee. his 2008 congressional testimony set the stage for charges that clemens lied about using steroids. >> i injected them into the body of roger clemens at his direction. >> reporter: clemens contends he received vitamin shots, not steroids. the government needs mcnamee to make its case, especially after the other key prosecution witness wavered on the stand. two weeks ago clemens' former teammate andy pettitte backed away from earlier statements that he witnessed clemens taking steroids. the defense is also calling into question the handling of vials and syringes mcnamee saved. the prosecution claims the items have tested positive for both steroids and clemens' d.n.a. began yell