There are about one million viral proteins in the UniProt database (990,049, August 2010) that represent about 66,000 viral strains. This is a highly redundant resource and about half of it composed of medically relevant strains including Hepatitis B viruses (HBV) and Human immunodeficiency virus (HIV). We took advantage of a reliable source of UniRef [35] that unifies sequences according to their identity level along the sequence length. We used UniRef90 classification (see Materials and Methods). There are .165,000 UniRef90 clusters that contain at least one viral protein ( Figure 1B) . However, from this set, we only considered 262 instances that contain at least two proteins, where one of them must be a eukaryote ( Figure 1B) . Of the 5,482 cross-taxa clusters that contain sequences from viruses and cellular organisms, 95% are sequences of bacteriophages and plasmids confined to the bacteria [36] . We will not further discuss the events that are confined to bacteria and archaea.