The gene in question is inactivated by
insertional mutation; a
transposon is used which inserts itself into the gene sequence. When that gene is
transcribed and
translated into a protein, the insertion of the transposon affects the protein structure and (in theory) prevents it from functioning. In STM, mutants are created by random transposon insertion and each transposon contains a different 'tag' sequence that uniquely identifies it. If an insertional mutant bacterium exhibits a
phenotype of interest, such as susceptibility to an
antibiotic it was previously
resistant to, its genome can be sequenced and searched (using a computer) for any of the tags used in the experiment. When a tag is located, the gene that it disrupts is also thus located (it will reside somewhere between a start and stop
codon which mark the boundaries of the gene). STM can be used to discover which genes are critical to a pathogen's virulence by injecting a 'pool' of different random mutants into an
animal model (e.g. a mouse infection model) and observing which of the mutants survive and proliferate in the host. Those mutant pathogens that ''don't'' survive in the host must have an inactivated gene, required for virulence. Hence, this is an example of a negative selection method. ==References==