Relationships among biosystems are identified during the NCBI data processing procedures, and then presented as "Related BioSystems" in Links pop-up menus on search results pages as well as in the folder tabs shown in an individual biosystem record.

Types of related biosystems include:

• Linked BioSystems - the source database has explicitly stated the biosystems are connected in some way



• Similar BioSystems - share at least one identical protein sequence from the same source organism. In other words, the biosystems include a protein that is in the same protein identity group (PIG) and has the same Taxonomy ID (TaxID). An example is shown in the illustration below.



• Biosystems that have an evolutionary relationship, such as an Organism Specific BioSystem that is an instance of a canonical, Conserved BioSystem. (For example, human arachidonic acid metabolism (bsid82991) is an organism-specific instance of the conserved biosystem for arachidonic acid metabolism (bsid366). The conserved biosystem record, in turn, links back to the human and other organism-specific instances of the biosystem, reflecting the relationships specified by the source database, KEGG.)



• Biosystems that have a functional relationship with each other, either as a Superset of smaller biological processes or a Subset of, or small reaction within, a larger biological process. (For example, "Phase 1 - Functionalization of compounds" (bsid105699) is a superset biosystem that includes "COX reactions" (bsid105711) as one of its subset biosystems, reflecting the hierarchical organization of those biosystems specified by the source database, Reactome.)