Most of the times, regulatory elements occur upstream of operons. However, there is a good number of regulatory elements (promoter and binding sites) located inside a promoter, defining a different transcription unit.An important aspect to keep in mind in order to avoid misunderstandings in the content of a database, is the fact that our current understanding and characterization of different genes, operons and regulatory mechanisms, is quite variable. For some genes, their mechanisms are very well described, whereas in other cases there is no regulation defined for a given promoter, or a promoter has been characterized upstream of a poorly characterized operon or transcription unit. Our definitions and conventions affect not only the way well-characterized systems are described, but also