Mo. Rev. Stat. § 492.570: Depositions, legally taken, may be read, when.

Depositions taken and certified in conformity to sections 492.420 to 492.560, or duly certified copies of the record of such depositions, may be used and read as evidence in any cause or judicial proceeding to which they relate, in favor of any parties thereto, or any or either of them, or his or their personal representatives, heirs or assigns, or their legal representatives

(1) If the deponent is dead;

(2) If he be unable to give testimony, by reason of mental incapacity;

(3) If he be rendered incompetent, by judgment of law;

(4) If he be removed, so that his testimony cannot be obtained.