Shigella pathogenicity is a multi-genic phenomenon involving the participation of genes on both the 230 kilobase virulence plasmid and the chromosome. A key feature of the regulation of Shigella virulence is its response to growth temperature. Genes in the virulence regulon are fully expressed at 37 degrees C, the normal temperature of Shigella's mammalian host, and the regulon is repressed at lower temperatures. Virulence gene expression is regulated in both a positive and a negative fashion by several plasmid-encoded activators and at least one chromosomally encoded repressor. The use of a variety of molecular tools including gene fusions, cloning, complementation, DNA sequencing and mRNA analysis, has provided a more complete understanding of how various, unlinked genetic loci contribute in a co-ordinated fashion to the pathogenic phenotype expressed by Shigella.