After the government's witness testifies, the court must, upon motion of the defendant, order the government to produce any statement of the witness in the government's possession relating to the subject matter as to which the witness testified. The court's denial of such a motion by a defendant is reversible error, although the court need not order the disclosure "sua sponte". The usual remedy for failure of the government to produce the documents is a mistrial and dismissal of criminal charges against the defendant.