There are many choices when selecting a method for expressing a recombinant protein. In general, common expression systems used in life sciences research are microbial (bacteria or yeast), higher eukaryotic (insect or mammalian cells) or cell-free (in vitro protein expression). Because of their speed, low cost, and ability to be easily genetically modified, microbes are often a first choice for in vivo expression of a target protein. For expression in bacteria, E. coli has been the most widely utilized used host for almost 25 years. Similarly, expression in various yeasts has been common since the mid-1980’s. Finally, production of proteins in vitro in cell-free systems is often used in situations where a target protein is toxic to a host or introduction of non-native amino acids into the protein’s structure is desired.