Mo. Rev. Stat. § 442.260: Proof of execution of instruments.

The proof of the execution of any instrument in writing, conveying real estate, or whereby any real estate may be affected in law or equity, shall be:

(1) By the testimony of a subscribing witness; or

(2) When all the subscribing witnesses are dead or cannot be had, by evidence of the handwriting of the party, and of at least one subscribing witness, given by at least two credible witnesses to each signature.