Abstract: The present invention relates generally to mutagenesis of target genes that takes advantage of the natural mutagenic capabilities of B cells, and enhances those capabilities by bringing the process of diversification under control. The invention provides a method for rapidly and inducibly generating point mutations and other types of diversification in expressed genes, such as antibody genes. This method can be coupled with selection to identify B cell clones that produce, for example, antibodies of high affinity or specificity. The diversification process can be modulated, accelerated, halted, switched between methods of mutagenesis and the like. The modulation of diversification in accordance with the invention is both inducible and reversible. The invention provides a means of rapid and feasible development of a repertoire of variant immunoglobulins and other polypeptides.