Mo. Rev. Stat. § 116.060

Any registered voter of the state of Missouri may sign initiative and referendum petitions. However, each page of an initiative or referendum petition shall contain signatures of voters from only one county. Each petition page filed with the secretary of state shall have the county where the signers are registered designated in the upper right-hand corner of such page. Signatures of voters from counties other than the one designated by the circulator in the upper right-hand corner on a given page shall not be counted as valid.