Protein engineering based on the use of recombinant DNA technology to change their amino acid sequences to obtain novel or desirable functions. Rational design, which is the most classical method in protein engineering, is an effective approach only when the structure and mechanism of the protein is well-known. In many cases, however, the structure information is quite limited and the mechanism frequently remains unclear. Directed evolution which involves random mutagenesis and selection for the interested protein is a more practical approach. There are many kinds of methods to introduce random or region-specific mutagenesis, such as saturation mutagenesis and DNA shuffling.