Abstract: A method for modifying the specificity or efficiency of an enzyme, while retaining its catalytic activity, is disclosed. The method is characterized by selecting an enzyme, the tertiary structure of which is substantially known or deduced; identifying a single specificity or efficiency-related region of the enzyme; identifying or constructing unique restriction sites bounding the identified region in the DNA coding therefor; generating a DNA sequence which corresponds to at least a portion of the identified region, except that the nucleotides of at least one codon are randomized, using the generated DNA sequence to replace the original such sequence; expressing the DNA including the generated DNA sequence; and selecting for a desired modification so that the DNA coding therefor may be isolated; the randomized DNA being generated by means of a PCR assembly method. Enzyme generated using this method, and having enhanced specificity or efficiency, are also disclosed.