Genes from a given species are very similar to one another with respect to base composition, codon biases and short oligonucleotide composition (15, 16, (22) (23) (24) . As a general rule, usage of oligonucleotides varies less along genomes than among genomes (24) (25) (26) (27) . In addition, it has been observed that transferred DNA retains (at least for some time) characteristics from its species of origin (8, 14) . These particularities are used alone or in conjunction to detect DNA transfers between species (8, 12, 13) . Transferred DNA is consequently detected on the basis of some of its singularities with respect to the sequence characteristics of the recipient species. However, these techniques suffer several drawbacks and weaknesses (28) (29) (30) that led us to consider generalizing the above approach for the screening of atypical regions in sequences. In fact, the genomic signature that accounts for all possible biases in DNA sequences has been shown to be speciesspecific (26, 27, 31, 32) . The signature is approximately invariant along the genome in such a way that the species of origin of DNA segments as small as 1 kb could be identified with a surprisingly high efficiency by means of their signatures (25, 27) . As a consequence, the sequence signature may be most often (at least in bacteria) considered a valuable estimation of the genomic signature. Assuming that (i) transferred DNA fragments exhibit signature of the species they come from and (ii) recipient and donor signatures are different, the screening of local variations of signature along genomes is expected to reveal regions of interest where HTs might be located. In addition, the status of HT is strongly suggested if the signatures of these regions of interest are found close to the signature of other species.