Site-directed mutagenesis is an important procedure in studies of gene expression and protein structure/function relationships. Three basic methods for this purpose use (i) single-stranded DNA (8,9), (ii) the polymerase chain reaction (PCR) (6) and (iii) doublestranded plasmid (5). The advantages and shortcomings of these methods have been discussed (5,6). Limitations include the availability of restriction sites for subcloning, the instability of large insertions in Ml3 vectors, the low fidelity ofthe Taq DNA polymerase and the low mutant yields with the double-stranded plasmid method (5).
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