RGD uses the Human Disease Ontology (DO, https://disease-ontology.org/) for disease curation across species. RGD automatically downloads each new release of the ontology on a monthly basis. Some additional terms which are required for RGD's curation purposes but are not currently covered in the official version of DO have been added. As corresponding terms are added to DO, these custom terms are retired and the DO terms substituted in existing annotations and subsequently used for curation.
A multiple sclerosis that is characterized by a worsening of disability, rather than by relapses followed by recovery, which progressively worsens until a steady progression of disability replaces the cycles of attacks. (DO)