Aims to collect known and conserved operons in multiple species. All the known operons are derived from the literature and from publicly available database including operon information. We will collect as many operons (transcriptional units) as possible from as many organisms as possible. As for the concept of reference operons, our system provides candidates of operons based on the orthologous genes conserved in the genomes. This database integrates both known literature-based operons as well as predicted well-conserved operons, to provide a useful system for bioinformatics researchers and experimental biologists.