A method for producing a mutant gene encoding a functional gene product comprising: (a) introducing one or more mutations into a gene to form a mutated gene; (b) providing the mutated gene to a host microorganism; (c) culturing the host microorganism containing the mutated gene under conditions which allow expression of the mutant gene product; and (d) selecting a host microorganism capable of producing a functional gene production from the mutated gene.
