Now prosecutors hope that Clemens' own DNA will help them convict him of a federal crime.
The government is expected to show test results Friday in Clemens' perjury trial that link the former pitcher's DNA to a used needle and other medical waste that also tested positive for steroids.
Forensic scientist Alan Keel returns to the stand to testify about the genetic tests he did for prosecutors.
Chief prosecution witness Brian McNamee, Clemens' former strength and conditioning coach, has testified that he saved the needle and other materials from an alleged steroids injection of Clemens in 2001.
Clemens is accused of lying to Congress in 2008 when he denied using performance-enhancing drugs.