Inspecting the UniRef90 clusters that contain proteins from viruses and metazoa (187 clusters) shows a wide variation in the distribution of protein lengths (Supportive data Table S2 ). Viruses tend to reduce their production load by deleting and reducing the unessential genetic material [44] . While this length reduction is an absolute necessity for most viruses, some giant viruses (e.g., Mimivirus, Chlorovirus, and Cafeteria roenbergensis virus) include ,1000 proteins [24, 25] . The evolution origin of proteins from the Giant viruses remains unknown [45] . Still, 12% of the Acanthamoeba polyphaga mimivirus (APMV) proteins constitute a large number of host related sequences. The average length of this subset of the Mimivirus proteins (523 amino acids) is similar to the length of their homologous sequences.