Table 1. Protein interaction networks.

Species		Interactions
	Genes	Physical	Genetic	Other	Total	Total interactions
						per gene
S. cerevisiae	6,328	55,767	104,926	17,674	178,367	28.19
H. sapiens	28,383	43,412	71	20,992	64,475	2.27
D. melanogaster	14,321	19,088	2,118	17,265	38,471	2.69
S. pombe	4,958	1,943	9,665	804	12,412	2.5
C. elegans	20,184	5,483	1,785	4,208	11,476	0.57
M. musculus	24,865	3,513	3	2,596	6,112	0.25
A. thaliana	26,496	5,048	67	937	6,052	0.23
P. falciparum	5,503	2,215	0	4	2,219	0.4
R. norvegicus	24,770	804	0	867	1,671	0.07
D. rerio	24,352	173	11	13	197	0.01

Overview of protein interaction networks extracted from the database iRefIndex [33] for the ten eukaryotic model species with the largest protein interaction networks. Besides the total number of protein interactions, the number of physical, genetic and interactions with an unknown interaction type is given. Only the interaction network of S. cerevisiae, H. sapiens, D. melanogaster and S. pombe have more than 2 interaction per gene (S. cerevisiae peaks with 28.19).