Discriminating whether a sequence has originated from the virus or the host is not straightforward. We generated for each cluster a phylogenetic dendogram and analyzed the connectivity of the viral protein in view of its neighboring sequences. Often the analyzed cluster is too small. In such cases, we expanded the cluster to the relaxed UniRef50 classification. We applied additional criteria in support of a virus having acquired protein sequences from the host: (i) The tested sequence appears in several organisms ($2) on the same evolutionary branch (as in Figure 1A , right); (ii) The tested sequence is not associated with viral contamination. Most analyzed cross-taxa clusters derive from the contamination by viral proteins following integration of the virus to a mammalian genome. Several instances were contaminated by the extensive use of viral vectors as vehicles in variety of molecular manipulations (e.g., Adenoviruses, Table 1 ).