Mo. Rev. Stat. § 145.601
If the amount of a taxpayer's federal taxable estate, adjusted taxable gifts, or credit for state death taxes reported on his federal estate tax return is changed or corrected by the United States Internal Revenue Service or other competent authority, the taxpayer shall report the change or correction within ninety days after the final determination of the change or correction or as otherwise required by the director of revenue. Each report shall state whether and wherein the determination is believed to be erroneous. Any taxpayer filing an amended federal estate tax return shall also file within ninety days thereafter an amended return under sections 145.011 to 145.995 and shall give such information as the director of revenue may require. The imposition of an additional tax by Internal Revenue Code Section 2032A shall constitute a change. The director of revenue may by regulation prescribe exceptions to the requirements of this section as he deems appropriate.
§ 145.601, RSMo
Effective 1/1/1981