Mo. Rev. Stat. § 540.050: Disqualification of grand juror — new juror summoned.

If any person is summoned as a grand juror who is not qualified as required by law, he may be challenged and discharged if such challenge is verified according to law or by his own oath. In such case, and also in case of nonattendance of any grand juror after he shall have been qualified, or in case any grand juror is excused by the court from further service for any cause, the court shall cause another grand juror to be summoned and sworn.