Mo. Rev. Stat. § 356.121: Proxies and voting trusts, valid when.

No proxy for voting shares of a professional corporation shall be valid unless it shall be given to a qualified person. A voting trust with respect to shares of a professional corporation shall not be valid unless all the trustees and beneficiaries thereof are qualified persons; except that, a voting trust may be validly continued for a period of six months after the death of a deceased beneficiary or for a period of six months after a beneficiary has become a disqualified person.