Mo. Rev. Stat. § 404.460
404.460 Future transfers of property revocable by donor, exception.
404.460. Future transfers of property revocable by donor, exception. — A provision in a will, trust, power of appointment, benefit plan, life or endowment insurance policy, annuity or other contract, or a pay or transfer on death direction, for a transfer of property to be made in the future to a custodian for a beneficiary, made by a donor in the manner prescribed in subdivision (4) of subsection 1 of section 404.540 , remains revocable by the donor until the property becomes transferable in accordance with the terms of the governing instrument to the beneficiary or a personal custodian for the beneficiary under sections 404.400 to 404.650 , unless expressly made irrevocable at the time provision for the gift is made or subsequently.
(L. 1986 S.B. 651 § 3 subsec. 3)