Interacting domain pairs shared by several organisms. The histogram shows the number of DDIs shared by three, four, and all five organisms. White bars represent DDIs that are used also in E. coli and black bars represent DDIs common only to the eukaryotes in our study. Twenty-seven DDIs were shared by all five organisms. E: E. coli. Y: yeast (S. cerevisae). W: worm (C. elegans). F: fly (D. melanogaster). H: human. DDI, domain-domain interaction.