The Roger Clemens perjury retrial is in recess.

The government's star witness took the stand today and testified about the life-changing moment when, he says, he first gave Clemens a "booty shot" of steroids.

Speaking softly, nervously and in detail, Brian McNamee told the jury that he injected one of baseball's most successful pitchers with steroids about eight to 10 times when they were with the Toronto Blue Jays in 1998.

McNamee said "I knew what I was doing was illegal," adding "I wish to God I could take it back."

When McNamee returns to the stand Tuesday, he's expected to testify that he injected Clemens with performance-enhancing drugs in 2000 and 2001. The prosecution is also expected to head off, as best it can, an upcoming cross-examination that's expected to attack McNamee's integrity. The defense wants to paint McNamee as a serial liar out for personal gain.

Clemens is charged with lying to Congress when he testified in 2008 that he had never used steroids or human growth hormone.