Mo. Rev. Stat. § 417.200: Fictitious names to be registered.

That every name under which any person shall do or transact any business in this state, other than the true name of such person, is hereby declared to be a fictitious name, and it shall be unlawful for any person to engage in or transact any business in this state under a fictitious name without first registering same with the secretary of state as herein required.