The remarkable repetitive elements called CRISPRs (clustered regularly interspaced short palindromic repeats) consist of repeats interspaced with non-repetitive elements or 'spacers'. CRISPRs are present in both archaea and bacteria, in association with genes involved in DNA recombination and repair. In the Yersinia pestis genome, three such elements are found at three distinct loci, one of them being highly polymorphic. The authors have sequenced a total of 109 alleles of the three Y. pestis CRISPRs and they describe 29 new spacers, most being specific to one isolate. In nine strains of Yersinia pseudotuberculosis, 132 spacers were found, of which only three are common to Y. pestis isolates. In Y. pestis of the Orientalis biovar investigated in detail here, deletion of motifs is observed but it appears that addition of new motifs to a common ancestral element is the most frequent event. This takes place at the three different loci, although at a higher rate in one of the loci, and the addition of new motifs is polarized. Interestingly, the most recently acquired spacers were found to have a homologue at another locus in the genome, the majority of these inside an inactive prophage. This is believed to be the first time that the origin of the spacers in CRISPR elements has been explained. The CRISPR structure provides a new and robust identification tool.