Quorum sensing is a regulatory system used by bacteria for controlling gene expression in response to increasing cell density. This regulatory process manifests itself with a variety of phenotypes including biofilm formation and virulence factor production. Coordinated gene expression is achieved by the production, release, and detection of small diffusible signal molecules called autoinducers. Autoinducers containing a phenylacetanoyl group in the acyl chain display a wide range of inhibitory and activating effects on various R protein transcriptional regulators. N-phenylacetyl-L-Homoserine lactone is a small diffusible signaling molecule involved in quorum sensing in the model symbiont V. fischeri and the pathogens A. tumefaciens and P. aeruginosa. It has been reported to antagonize TraR signaling in A. tumefaciens, LasR signaling in E. coli, and LuxR signaling in V. fischeri.