Mo. Rev. Stat. § 115.161: Registration of persons unable to write.

If any person applying to register in person pursuant to this subchapter is unable to write, the election official shall fill in the registration cards, and the applicant shall sign by mark, witnessed by the election official. The election official shall note the applicant's height in feet and inches, weight, color of eyes and other distinguishing features under the title "Remarks" on the registration cards.