A pathogenicity island present only in highly pathogenic strains of Yersinia (Y. enterocolitica 1B, Y. pseudotuberculosis I and Y. pestis) has been identified on the chromosome of Yersinia spp. and has been designated High-Pathogenicity Island (HPI). The Yersinia HPI carries a cluster of genes involved in the biosynthesis, transport and regulation of the siderophore yersiniabactin. The major function of this island is thus to acquire iron molecules essential for in vivo bacterial growth and dissemination. The presence of an integrase gene and att sites homologous to those of phage P4, together with a G + C content much higher than the chromosomal background, suggests that the HPI is of foreign origin and has been acquired by chromosomal integration of a phage. The HPI can excise from the chromosome of Y. pseudotuberculosis and is found inserted into any of the three copies of the asn tRNA loci present in this species. A unique characteristic of the HPI is its wide distribution in various enterobacteria. Although first identified in Yersinia spp., it has subsequently been detected in other genera such as E. coli, Klebsiella and Citrobacter.