Adult flies and cells have also been used to screen for pathogenencoded factors that contribute to virulence. One particularly elegant screen for bacterial virulence factors was carried out for P. aeruginosa, an opportunistic human pathogen that can cause serious disease. Over 4000 transposon insertion mutants of the bacterium were screened by injecting them into the adult fly hemolymph and measuring percent lethality. This resulted in the identification of 15 different bacterial loci that contributed significantly to virulence (Kim et al., 2008) . The authors examined the basis of virulence for one of these genes, hudR. hudR encodes a transcription factor that represses expression of the neighboring gene hudA, which is involved in ubiquinone biosynthesis. On the basis of their genetic analysis, the authors hypothesized that the decreased virulence of hudR mutants resulted from overexpression of hudA. They confirmed this hypothesis by showing that overexpression of hudA in a hudR-mutant background resulted in attenuated virulence of P. aeruginosa in flies and that hudA hudR double mutants had normal virulence.