In quorum sensing genes of Gammaproteobacteria, which includes "Pseudomonas aeruginosa" and "Escherichia coli", the LuxI/LuxR genes form a functional pair, with LuxI as the auto-inducer synthase and LuxR as the receptor. Gamma Proteobacteria are unique in possessing quorum sensing genes, which, although functionally similar to the LuxI/LuxR genes, have a markedly divergent sequence. This family of quorum-sensing homologs may have arisen in the gamma Proteobacteria ancestor, although the cause of their extreme sequence divergence yet maintenance of functional similarity has yet to be explained. In addition, species that employ multiple discrete quorum sensing systems are almost all members of the gamma Proteobacteria, and evidence of horizontal transfer of quorum sensing genes is most evident in this class.