Clinical use of antibodies or antibody fragments is dependent on highly effective production systems. Antibody fragments like single-chain Fv or Fab have been successfully expressed in microorganisms for more than a decade, and the most commonly used host has been Escherichia coli . The ease of genetic manipulation in E. coli has made it the first choice for expression of antibody fragments, and by fusion of the protein to a signal sequence, antibody fragments are secreted to the periplasm of the bacteria, allowing correct folding and recovery of functionally active proteins. Finally, E. coli can be grown to high cell densities in an inexpensive medium allowing high volumetric yield of antibody fragments.