complex. In fact, it is a formidable task to identify protein complexes in an interactome, given that a network on its own does not directly reveal the presence of a stable complex. Studied interactomes Viral interactomes Viral protein interactomes consist of interactions among viral or phage proteins. They were among the first interactome projects as their genomes are small and all proteins can be analyzed with limited resources. Viral interactomes are connected to their host interactomes, forming virus-host interaction networks. Some published virus interactomes include Bacteriophage Escherichia coli bacteriophage lambda Escherichia coli bacteriophage T7 Streptococcus pneumoniae bacteriophage Dp-1 Streptococcus pneumoniae bacteriophage Cp-1 The lambda and VZV interactomes are not only relevant for the biology of these viruses but also for technical reasons: they were the first interactomes that were mapped with multiple Y2H vectors, proving an improved strategy to investigate interactomes more completely than previous attempts have shown. Human (mammalian) viruses Human varicella zoster virus (VZV) Chandipura virus Epstein-Barr virus (EBV) Hepatitis C virus (HPC), Human-HCV interactions Hepatitis E virus (HEV) Herpes simplex virus 1 (HSV-1) Kaposi's sarcoma-associated herpesvirus (KSHV) Murine cytomegalovirus (mCMV) Bacterial interactomes Relatively few bacteria have been comprehensively studied for their protein–protein interactions. However, none of these interactomes are complete in the sense that they captured all interactions. In fact, it has been estimated that none of them covers more than 20% or 30% of all interactions, primarily because most of these studies have only employed a single method, all of which discover only a subset of interactions. Among the published bacterial interactomes (including partial ones) are The E. coli and Mycoplasma interactomes have been analyzed using large-scale protein complex affinity purification and mass spectrometry (AP/MS), hence it is not easily possible to infer direct interactions. The others have used extensive yeast two-hybrid (Y2H) screens. The Mycobacterium tuberculosis interactome has been analyzed using a bacterial two-hybrid screen (B2H). Note that numerous additional interactomes have been predicted using computational methods (see section above). Eukaryotic interactomes There have been several efforts to map eukaryotic interactomes through HTP methods. While no biological interactomes have been fully characterized, over 90% of proteins in Saccharomyces cerevisiae have been screened and their interactions characterized, making it the best-characterized interactome. Species whose interactomes have been studied in some detail include Schizosaccharomyces pombe Caenorhabditis elegans Drosophila melanogaster Homo sapiens Recently, the pathogen-host interactomes of Hepatitis C Virus/Human (2008), Epstein Barr virus/Human (2008), Influenza virus/Human (2009)