Among the 667 analyzed Pfam families, in 103 of them, the metazoan proteins contain at least 3 Pfam domains. In 85% of this set (88 families), the viral proteins are shorter ( Figure 5B, virus) . Remarkably, the average length of these 103 metazoan proteins families is 912 amino acids relative to 503 amino acids for the viral proteins that belong to these families. Similarly, in this set of multidomain proteins the viral proteins have an average of 2.9 domains, while the metazoan proteins have 4.6 domains on average (paired t-test, p-value of 1.0e-11). This shows that the tendency to reduce the protein length and the number of domains is stronger when the number of Pfam occurrence in the original host protein is higher.