Ontologies and relationships used in computable definitions. (a) Ovals indicate the OBO Foundry ontologies whose classes are used in computable definitions with the CL. Arrows indicate the relationships used with a given ontology. The Protein Ontology and the cellular component of the Gene Ontology both use the 6 relationships describing expression. (b) Graphical representation of how cross product classes are used to define a natural killer cell type.