Current polymerase chain reaction (PCR) innovations provide powerful tools for the cloning of previously unknown genes as well as characterization of their functions. Examples of the latter used include PCR-mediated in vitro mutagenesis and recombination of the cloned genes. PCR approaches have become the method of choice to generate arrays of predefined mutations or recombination within the gene of interest. In the postgenomic era, these mutants or recombinants are highly desirable for the study of functional genomics, gene expression, protein structure-function relationships, protein-protein interactions, protein engineering, and in vitro evolution of enzymes.