PGL and number of protein-protein interactions for yeast proteins. Yeast proteins were binned into four classes according to the PGL values for the corresponding KOGs. The average number of interactions was calculated for each class. For KOGs with multiple yeast paralogs, the sum of interactions for all paralogs was used, with the rationale that this is the natural integral measure of the interactivity of the proteins in the given KOG, under the assumption that all paralogs in a KOG have evolved via relatively recent, lineage-specific duplications.