Initially, reports of the expression of a heterologous protein in a host organism yielded the academic satisfaction of being able to genetically manipulate an organism. However, the past decade has witnessed rapid advances in the development of economically viable production systems for recombinant proteins of medical and industrial importance in prokaryotes, lower-and higher-eukaryote cell culture, and in transgenic higher eukaryotes.

As a laboratory-scale research tool, Escherichia coli has become the host organism of choice for the expression of many heterologous genes, primarily because of the ease with which it may be manipulated genetically. Large-scale biotechnological production of recombinant proteins, where product quality and process economics must also be considered, requires additional factors to be taken into account. Few organisms, however, can satisfy all of the criteria (for example, high growth rate, relatively simple nutritional requirements, genetic stability, ease of product purification, and appropriate post-translational modificatio~ to retain activity and comply with legislation), and this has led to the development of a wide range of expression systems, host organisms and processing procedures.