The heterologous expression of proteins is without doubt one of the most fascinating applications of the recombinant DNA technique. Despite clear successes many attempts to produce a certain protein in a heterologous host cell have met with technical difficulties. Secretion from cells has been used as a solution to overcome the intracellular formation of inactive protein. Microorganisms with a history of use in the fermentation industry exhibit clear advantages over the frequently used Escherichia coli as host cells for secreted products. Interleukin-3, chymosin and phytase are examples of commercial products that are produced efficiently with the aid of industrial microorganisms.