It would be interesting to localize the region the transfer may come from when the complete genome of the donor is available. However, homology (at the DNA level) is not a pertinent criterion for the comparison of sequences as soon as amelioration has taken place (8, 14) . In fact, homology is sometimes weak, e.g. between genes of Escherichia and Salmonella although these species have 'recently' diverged (34) . It is clear that a more powerful search for the origin of putative HTs would have to embody models of amelioration [such as the one designed by Lawrence and Ochman (8) ].