Supporting evidence for the directionality of the genetic exchange of viral and cellular organisms relies on a detailed phylogenetic analysis. The topology of the reconstructed tree is used to support the most parsimonious scenario (see Materials and Methods). The simplified illustrations in Figure 1A do not address the more complicated, realistic instances in which different viruses carry sequences that resemble various organisms. An additional criterion used in supporting the occurrence of sequence acquisition by viruses is the presence of a sequence resemblance in the known host. The origin of viruses is probably preceding the cellular life [8, 10] . Thus, the ancient events in which viral sequences were incorporated into an ancestor eukaryote cannot be traced by their sequence similarity. Still, a conserved functional or structural similarity could expose such early events [33] . In this study, we have not attempted to date the horizontal transfer event. Furthermore, we will not discuss the events of genetic material exchange (see discussion in [34] ), but limit our study to the acquisition of coding sequences in viruses and metazoa.