Mo. Rev. Stat. § 491.050: Convicts competent witnesses — convictions and certain pleas may be proved to affect credibility.

Any person who has been convicted of a crime is, notwithstanding, a competent witness; however, any prior criminal convictions may be proved to affect his credibility in a civil or criminal case and, further, any prior pleas of guilty, pleas of nolo contendere, and findings of guilty may be proved to affect his credibility in a criminal case. Such proof may be either by the record or by his own cross-examination, upon which he must answer any question relevant to that inquiry, and the party cross-examining shall not be concluded by his answer.