To date, only one PAI has been well characterized on the chromosome of the three pathogenic Yersinia, Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica (1): the socalled High-Pathogenicity Island (HPI) ranges from 36 to 43 kb in size (according to the species in question) and bears genes involved in the biosynthesis, transport and regulation of the yersiniabactin siderophore (see Chapter 14). We discovered a 98,058-base pair (bp) segment that is inserted in the chromosome of the HPI-positive Y. pseudotuberculosis 0:1 strain 32777 between the Y. pestis CO92 YPO0339 and YPO0341 homolog genes (Parkhill et al., 2001). The segment is associated with a phenylalanine (phe)-specific tRNA gene, a frequent target for PAI integration (Hacker and Kaper, 2000). The 3′ end of the phe tRNA-encoding gene contains a 54-nucleotide motif that is directly repeated (DR) at the opposite extremity of the 98-kb segment, DR sequences being another feature common to PAIs (Hacker and Kaper, 2000). This large chromosomal segment (called YAPI, for Yersinia Adhesion Pathogenicity Island) is missing in the region separating YPO0339 and YPO0341 in the fully-sequenced genome of serotype O:1 strain 32953 (HPI+).