The Human Disease Ontology Project develops and maintains the Disease Ontology (DO), an open-access ontology of human diseases, phenotypes, and related terms. The DO provides structured terminology to facilitate biomedical research and clinical care. It is funded by the NIH/NHGRI and maintained by researchers at the University of Maryland School of Medicine. The DO contains over 12,000 disease terms, is integrated with other biomedical ontologies, and sees increasing use by researchers around the world.