The antibodies selected from many of the immune repertoires are often directly useful for ELISA. In some instances, their affinity is not sufficient, such as for many diagnostic applications or some applications in immunotherapy. Affinity maturation might be bypassed by the construction of multivalent molecules, however, there will be occasions in which affinity maturation of the selected antibodies is required. With phage display, as in the immune system, antibodies can be affinity-matured in a stepwise fashion, by incorporating mutations and selecting variant cells with decreasing amount of antigen. Various procedures for introducing diversity in the antibody genes have been described, ranging from more or less random strategies, to strategies targeting the CDR region of the antibody of mutagenesis.