GeXKB ontologies. The illustration shows the layout of the nested GeXKB ontologies (GeXO, ReXO and ReTO).The blue nodes represent the upper level ontology (ULO), the common root of the three ontologies. The black and red edges depict ‘is_a’ and ‘part_of’ relations, respectively. The three ontologies cover an increasingly wide domain. Each GO sub-domain term (e.g. GO:0010467; denoting ‘gene expression’) and its descendants are linked to the ULO as a subclass of ‘Biological Process’ represented by the ‘dotted edges’.