In contrast to a genome, the interactome has been more difficult to define. Some authors have argued32 that the "True Interactome" should be defined as all possible interactions encoded by a genome-i.e., the set of all pairwise protein interactions that occur in at least one biological condition or cell type. The assumption is that every true interaction will be detectable by some assay, and that given enough independent measurements, most of the interactome could be detected. Many assay types have been described for detecting proteinprotein interactions, a few of which have been adapted to large-scale screening1,32-34. On the other hand, some interactions may be immeasurable by any available assay, or will not arise in the conditions surveyed. Therefore, we use the term "Mappable Interactome" for the subset of true pairwise interactions that are reproducibly detectable by any given assay method or combination of methods.