Escherichia coli is the most commonly used and best characterized organism for overexpressing foreign and nonforeign proteins. The use of E. coli confers several immediate advantages to the user: rapid and high-level expression as a result of the speed of cell growth to high density; low complexity and low cost of growth media; and the ability to target proteins to the desired subcellular location (1 ). In addition, a number of mutant host strains are available that can improve recombinant protein expression. For example, the use of host strains with mutations in cytoplasmic protease genes decreases protein degradation. Hosts that encode tRNAs for codons seldom found in E. coli can successfully express some otherwise difficult to express proteins that contain these rare codons (see Chapter 3 ).