Yeast cells are mutagenized to obtain desirable mutants. Mutagenesis is mediated
by a defective mismatch repair system which can be enhanced using conventional
exogenously applied mutagens. Yeast cells with the defective mismatch repair system
are hypermutable, but after selection of desired mutant yeast strains, they can
be rendered genetically stable by restoring the mismatch repair system to proper functionality.
