One primary system that controls bioluminescence through regulation of the "lux" operon is quorum sensing, a conserved system across many microbial species that regulates gene expression in response to bacterial concentration. Quorum sensing functions through the production of an autoinducer, usually a small organic molecule, by individual cells. As cell populations increase, levels of autoinducers increase, and specific proteins that regulate transcription of genes bind to these autoinducers and alter gene expression. This system allows microbial cells to "communicate" amongst each other and coordinate behaviors like luminescence, which require large amounts of cells to produce an effect.