Signature-tagged mutagenesis (STM) is a genetic technique used to study gene function.
Using STM, the function of the product of a particular gene can be inferred by disabling it and observing the effect on the organism.
The original and most common use of STM is to discover which genes in a pathogen are involved in virulence in its host, to aid the development of new medical therapies/drugs.
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.
Those mutant pathogens that don't survive in the host must have an inactivated gene, required for virulence.