Table 1. Size of biomedical ontologies and seven thesauri.

Biomedical Ontologies	Disease concepts	Disease relations
International Classification of Diseases, Clinical Modification (ICD9-CM)	6,011	5,904
Canonical Clinical Problem Statement System (CCPSS)	3,500	12,112
Systematized Nomenclature of Medicine, Clinical Terms (SNOMED-CT)	30,760	62,146
Medical Subject Headings 2009 (MeSH)	3,776	2,605

*Note: WordNet is subdivided into synonymous sets (synsets) rather than being organized by headwords. We extracted all possible synonym pairs, which explains why WordNet contains so many headwords.