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"However, on July 20, 1990, with the help of the American Civil liberties Union (ACLU), North's convictions were vacated, after the appeals court found that witnesses in his trial might have been impermissibly affected by his immunized congressional testimony

As North had been granted limited immunity for his congressional testimony, the law prohibited a prosecutor from using that testimony as part of a criminal case against him. To prepare for the expected defense challenge that North's testimony had been used, the prosecution team had—before North's congressional testimony had been given—listed and isolated all of its evidence."

Source, Wickipedia