Several statistical observations regarding the sequences among the cross-taxa families of viruses and multicellular organisms can be made: (i) The average length of the metazoan proteins is 507 amino acids, while the average length for the viral proteins in these families is only 396 amino acids (P-value of ,1.0e-17 by the KS-test, Figure 5A ). (ii) For 73% of all families, the viral proteins are shorter than the length of the average metazoan proteins in the family (P-value,1.0e-13 by the Hypergeometric test). (iii) In 67% of the families, the number of Pfam domain appearances (including several repeats of the same domain or different ones, Figure 5B ) is smaller in the viral proteins relative to the metazoan proteins in the family (P-value,1.0e-40 by the KS test). (iv) In 62% of the families, the number of different Pfam domains is higher in the metazoan proteins relative to the viral proteins. (v) For the discussed families, the median number of Pfam domains is 1.06 while, for the metazoan proteins, this value is 1.7 (Pvalue,1.0e-32 by KS test).