Through selective mutation, protein domains have been the tools of evolution to create an enormous and diverse assembly of proteins from likely an initially relatively limited set of domains. The combined data in GenBank and other databases now covers over 200.000 species with at least 50 complete genomes and this greatly facilitates genome comparisons [57] [58] [59] . Following such extensive comparisons, currently > 1700 domain superfamilies are recognized in the recent release of the Structural Classification of Proteins (SCOP) [60] and it has become clear that many proteins consist of more than one domain [17, 61, 62] . Indeed, it has been estimated that at least 70% of the domains is duplicated in prokaryotes, whereas this number may even be higher in eukaryotes, likely reaching up to 90% [35] .