It was thought that polycistronic transcription is a characteristic of bacteria and archaea, where many of the genes are clustered in operons composed of two to more than ten genes. By contrast, the genes of eukaryotes are generally considered to be monocistronic, each with its own promoter at the 5' end and a transcription terminator at the 3' end; however, it has recently become clear that not all eukaryotic genes are transcribed monocistronically. Numerous instances of polycistronic transcription in eukaryotes, from protists to chordates, have been reported. These can be divided into two broad types. Dicistronic transcription units specify a messenger RNA (mRNA) encoding two separate genes that is transported to the cytoplasm and translated in that form. Presumably, internal ribosome entry sites (IRES), or some form of translational re-initiation following the stop codon, are responsible for allowing translation of the downstream gene. In the other type, the initial transcript is processed by 3' end cleavage and trans-splicing to create monocistronic mRNAs that are transported to the cytoplasm and translated. Like bacterial operons, eukaryotic operons often result in co-expression of functionally related proteins.