The production of proteins is one of the main applications of genetic engineering in biotechnology. Even though standard cloning procedures are now routine and a large variety of host-vector systems for gene expression are available, difficulties are encountered when theoretical strategies are put into practice, so gene expression is still quite empirical. E. coli remains an important host system for the industrial production of proteins from cloned genes, and considerable lore has accumulated since the pioneering gene expression experiments. The extensive knowledge about E. coli’s physiology and genetics accounts for its preferential use as a host for gene expression. The inability of this organism to exert certain posttranslational modifications of proteins that lead to correct folding and activity represents its major drawback as a production organism.