Mo. Rev. Stat. § 324.039
There shall be established in each board within the division of professional registration, including the division itself when empowered with licensing authority, which was on August 28, 1998, required or authorized to revoke a license for failure to submit an application for renewal, failure to provide information required for renewal or nonpayment of the required renewal fee, a classification for a licensee who desires to remove himself or herself from participating in the licensing system of the board or division. This classification shall be distinguished from revocation of a license and from surrender of a license pursuant to an agreement between the board or division and the licensee filed with and approved by the administrative hearing commission. This classification shall not be available to a licensee during the time there is an investigation of the licensee or the licensee's practices or during the pendency of a disciplinary complaint filed with the administrative hearing commission. Each board within the division or the division when empowered with licensing authority shall establish by rule qualifications for such classification and procedures for a licensee to request an inactive license as provided in this section. Notwithstanding any other law to the contrary, no board within the division or the division shall be required to revoke a license when the licensee qualifies for the classification authorized by this section, as provided by rule. An inactive license authorized by this section shall be subject to the same requirements for reinstatement or restoration as a lapsed, expired, or revoked license due to failure to renew the license. This section shall not affect those boards which are otherwise authorized to classify a license as inactive.
§ 324.039, RSMo