**2. Optimizing protein expression: rational control of cell condition**

There is no doubt that among the myriads of systems available for heterologous protein expression, the Gram-negative bacterium *Escherichia coli* remains one of the most popular owing to its relative simplicity, its inexpensive and fast high-density cultivation, its well-known genetics, and the large number of cloning vectors and mutant host strains that are commonly available. Though not every gene can be efficiently and fully expressed in this system, much progress has been made to improve the performance and versatility of this workhorse microbe. One of the most sought after outcomes is the overexpression of high quality target proteins, however difficulties such as stimulated protease activity and reduced growth rate, as pointed out decades ago, often arise accompanying overexpression [5–7]. In this section, a brief review of the general background pertaining to *E. coli* protein overexpression is presented.
