More then 20 years have passed now since the first recombinant protein producing microorganisms have been developed. In the meanwhile, numerous proteins have been produced in bacteria, yeasts and filamentous fungi, as weIl as higher eukaryotic cells, and even entire plants and animals. Many recombinant proteins are on the market today, and some of them reached substantial market volumes. On the first sight one would expect the technology - including the physiology of the host strains - to be optimised in detail after a 20 year's period of development. However, several constraints have limited...