Figure 1. Genetic footprinting. Shown are two neighboring genes from the whole genome. Gene A encodes a virulence factor; gene B does not. Neither gene is essential. After transposon mutagenesis, multiple insertions (vertical triangles) are obtained in each gene (only two are shown). Polymerase chain reaction (PCR) primers (horizontal arrows) to the start of the gene and the transposon are used to amplify the sequences between insertion and start of gene. After electrophoresis, a characteristic set of bands is seen for each gene, corresponding to the location of insertions. Mutants of genes A and B grew under permissive conditions (Ap, Bp). Only mutants of gene B (Bn) grew under nonpermissive (animal model) conditions.