Most pyrG mutants were isolated following either ultraviolet
(UV) or chemical mutagenesis. Mutations induced in these
ways are often unstable (i.e., they revert to the wild type)
andmay includemultiple, undesirablemutations. In contrast,
mutants generated by targeted gene replacement are stable as
the targeted gene is removed fromthe genome. It is, however,
more difficult to mutate multiple genes this way