The power of transposon mutagenesis evolves from its ability to mark the location of a genetic change. In comparison, the locations and nature of genetic changes caused by chemical or irradiational mutagenesis are difficult to determine or characterize. Separation of transposons on their vectors and transposition mutations in the target replicon may be difficult since they present the same phenotype—usually antibiotic resistance. Transposon mutagenesis is a useful tool for marking and locating loci involved in bacterial-plant interaction and environmental survival. Extension from an annealed mutagenic primer incorporating a-phosphorothioate-modified deoxycytidine triphosphate protects that strand from digestion by certain restriction endonucleases while allowing nicking in the nonphosphorothioated strand. The power of marker exchange mutagenesis is its ability to select specific genes for mutation. In contrast, chemical or irradiational mutagenesis likely produces more than one mutation per genome, making it difficult to ascertain if one or more mutations is responsible for observed phenotypic changes.