The Mondo Disease Ontology (Mondo) aims to harmonize disease definitions across the world. It is a semi-automatically constructed ontology that merges in multiple disease resources to yield a coherent merged ontology. Original versions of Mondo were constructed entirely automatically and used the IDs of source databases and ontologies. Later, additional manually curated cross-ontology axioms were added, and a native Mondo ID system was used to avoid confusion with source databases.
A key feature of Mondo is that it goes beyond loose database cross-references (xrefs). It curated precise 1:1 equivalence axioms connecting to other resources, validated by OWL reasoning. This means it is safe to propagate across these from Online Mendelian Inheritance in Man (OMIM), ORDO (Orphanet), Experimental Factor Ontology (EFO), Disease Ontology (DOID) and the neoplasm branch of National Cancer Institute Thesaurus (NCIt).