Qualitative, trial-and-error methods designed to increase the flux to desirable biotechnological products have led to new technologies and vast improvements in existing ones. However, these methods now appear in many cases to have approached their limit. In addition, there is a strong feeling in industry that much of the recent boom in academic knowledge of biochemistry and molecular biology passes biotechnology by, simply because one cannot evaluate the implications of molecular kinetics for the functioning of the producer organisms as a whole. New methods, or more rational methods, are called for. One, aimed at increasing only the concentration of a single metabolite by site-directed mutagenesis is developed here.