We assembled direct protein-protein interactions from BioGRID, IntAct, MINT, and HPRD (see Materials and Methods for details).

Low-throughput interactions are those interactions that have supporting evidence in publications that report fewer than one hundred interactions. (The trends are not sensitive to this choice of cut-off, see Table S1 in Text S1.) The S. cerevisiae network is more complete than those of the other species: a much higher fraction of S. cerevisiae proteins have protein-protein interaction data, and each protein is involved in more interactions. The approximate number of proteins only considers one protein isoform per gene. (We report the number of unique STRING identifiers; see Materials and Methods.)