text: Because of their sequence similarity and operon structure, many two-component systems – particularly histidine kinases – are relatively easy to identify through bioinformatics analysis. (By contrast, eukaryotic kinases are typically easily identified, but they are not easily paired with their substrates.) A database of prokaryotic two-component systems called P2CS has been compiled to document and classify known examples, and in some cases to make predictions about the cognates of "orphan" histidine kinase or response regulator proteins that are genetically unlinked to a partner.