The present invention provides compositions and improved methods for multi-site directed mutagenesis and DNA shuffling. The present compositions and methods provide increased mutation frequency and increased number of transformants which allow one to sequence only a few clones in order to identify the correct mutants and to obtain the desired mutant by screening large number of transformants in a short time. Moreover, the inclusion of FEN-1, PEF and optimized buffer and cycling conditions provided in the present invention should also facilitate random mutagenized library construction and the mutagenesis of large or difficult templates.