Mo. Rev. Stat. § 540.300: Grand jurors required to testify, when.

Members of the grand jury may be required by any court to testify whether the testimony of a witness examined before such jury is consistent with or different from the evidence given by such witness before such court. They may also be required to disclose the testimony given before them by any person, upon a complaint against such person for perjury, or upon his trial for such offense.