Abstract: There is provided a process for industrial production of non-naturally occurring proteins composed of less than 20 amino acids, wherein the proteins retain their original functions while being capable of site-specific modification or immobilization, or having new functions not found in nature in addition to the original functions of the proteins. Specifically there are provided a process for producing a functional non-naturally occurring protein having a specific amino acid type(s) replaced with a natural amino acid(s) other than the amino acid type(s).