Internal transcribed spacer (ITS) refers to the spacer DNA situated between the small-subunit ribosomal RNA (rRNA) and large-subunit rRNA genes in the chromosome or the corresponding transcribed region in the polycistronic rRNA precursor transcript. In bacteria and archaea, ITS is located between the 16S and 23S rRNA genes. On the other hand, there are two ITS's in eukaryotes; ITS1 is located between 18S and 5.8S rRNA genes, while ITS2 is between 5.8S and 25S (in plants, or 28S in animals) rRNA genes. ITS1 corresponds to the ITS in bacteria and archaea, while ITS2 originated as an insertion that interrupted the ancestral 23S rRNA gene.