Mo. Rev. Stat. § 144.746: Extension of time for filing for refund or proposed assessment by agreement — requirements.

The director of revenue and a taxpayer may agree in writing to extend the periods prescribed in sections 144.190 and 144.220, within which a refund claim may be filed or a proposed assessment may be served and mailed. Such an agreement must be made before the expiration of such periods and may be extended by subsequent agreements at any time before the expiration of the period previously agreed upon.