The 49 rRNA-free atypical regions vary in size from 1.5 to 135 kb and make up 13% of the total genome (Table 1) . About 50% of atypical regions are less than (or around) 6 kb long. Distances of outlier from first potential donor often fall within the intra-genomic range ( However, in some instances, the outlier-to-donor distance is too great to consider the 'closest' species as potential donor. In contrast, unusual small values deserve a specific attention. In particular, the very small distance between bacteriophage SPBc2 and '2150751-2285750' atypical region (d = 2) allows to spot the part of B.subtilis genome where bacteriophage SPBc2 is incorporated (12, 47) . Other regions in the genome are also found similar (in terms of signature) to bacteriophage SPBc2. Most of them correspond to bacteriophages, imbedded in B.subtilis genome, whose free forms are not sequenced (12, 47) . Observed similarities with SPBC2 are, however, expected since signatures of phages usually share some characteristics with the species they infect (48) . The SPBc2 sequence is the only foreign sequence identified in B.subtilis, using homology as criterion (BlastN, with parameters set to default). In fact, Blast analysis of B.subtilis outliers leads to contrasted results. Besides SPBc2 and 7 out of 9 prophages imbedded in the genome, the only atypical regions identified are those containing the 30 rRNA genes coded in B.subtilis genome. The only few genes that are homologous to parts of atypical regions are found in species belonging to the Bacillus genus. It is interesting to note that no house-keeping genes (except rRNA) are detected in atypical regions. In fact, a great number of genes in atypical regions (except bacteriophage genes and rRNA) have no known function.