A method for effecting site-directed mutagenesis characterized by involving the step of effecting PCR by using a double-stranded DNA vector having one or more amber codons carrying a DNA fragment inserted thereinto and serving as the target in the site-directed mutation and at least two types of selective primers; and a site-directed mutagenesis kit for effecting this method characterized by containing primers for the reversion of the amber codons. The method and the kit are useful in more conveniently and quickly effecting the site-directed mutagenesis in the fields of genetic engineering and protein engineering. By using the method and the kit, it is possible to obtain a gene wherein mutagenesis has been efficiently effected at the desired site merely by transforming a PCR product obtained by PCR into a host.
