We first simulated a "Basic serial" strategy, in which all pairs of proteins are tested for interaction sequentially. Under this formulation, achieving a saturation of 95% required eight comprehensive screens, in which each protein pair was tested by eight independent assays, equivalent to ~7×108 pairwise tests assuming a total of 13,600 protein-encoding genes in fly37 (Table 1 and Fig. 2a) . Moreover, 93% of all observed interactions in this network were false positives (including 99% of interactions observed exactly once and 21% of interactions observed twice- Fig. 2b ). The false-positives predominate because, although the 0.2% FPR seems low, the number of non-interacting protein pairs is far in excess of the number of true interactions.