The use of antibody (Ab) molecules and their fragments in research, diagnosis, and therapy has prompted the development of methods to improve their affinity and stability to increase their expression levels and to change or improve their specificity. This is easier to carry out on Ab fragments (scFvs or Fabs) expressed in Escherichia coli than on a complete Ab molecule expressed in B cells. Several methods can be used in E. coli to generate mutations: chemical mutagenesis, use of mutagenic strains of bacteria, incorporation of degenerate oligonucleotides, DNA shuffling, or error-prone polymerase chain reaction (PCR).